Garden Sage - Saving it for hummers; spider mites devastate; mildew wilts bush
Answers to your gardening questions from an expert in Southern Arizona.
- By Peter L. Warren Arizona Daily Star
- Updated
Q: How do you keep bees out of the hummingbird feeders effectively enough that the little hummers have a chance to feed? Despite their aggressive nature, it seems that our hummers don’t want to challenge the bees. What to do?
A: Bees are seeking nectar and water for the same reasons as the hummingbirds and they both prefer actual plant nectar to sugar water if they can get it. I recommend providing a variety of flowering plants for the bees to distract them from your feeders. Hummingbirds prefer flowers that are tubular in shape but bees can’t access the nectar in these because they lack the long tongues of hummingbirds. Bees would more likely be found on plants that have a wide-open bloom with a place for them to land and access the nectar and pollen up close. Sunflowers are an example of the shape they prefer.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Questions and photos may be emailed to tucsongardensage@gmail.com.
- By Peter L. Warren Arizona Daily Star
- Updated
Q: My tomatoes show an unknown leaf disease that has slowly spread over 35 percent of all tomato plants. Based on my observations, it started approximately four weeks ago within the center of one plant and has slowly advanced. Also, there is a very fine spider like web covering some of the browned leaf areas. I am familiar with “leaf curl” problems, but this is a new one for me.
A: From the photos and your description, I suspect spider mites. The symptom that is most noticeable is the color change on the leaves as a result of their feeding. Their tiny mouthparts pierce the leaves and remove sap. Just one mite feeding wouldn’t be noticeable but after weeks of feeding by a number of them, the change in leaf color becomes more noticeable. The fine spider-like webbing is a sign as well. Spider mites, just like many of their spider relatives, produce webbing where they live. This is also something that may go unnoticed with a small number of mites but becomes apparent with a significant population. Identifying mites often requires a 10X magnifier and it’s good to make sure you have live ones before spending time and money treating for them. Sometimes we only see the damage after they moved on. Spider mites thrive in our arid climate and can be a problem on a variety of plants. They aren’t all bad though, since some mites are predatory and feed on the pest mites. When they become a problem there are a few tactics you can use to manage them. Sprays of water, horticultural oil, or insecticidal soap are usually effective in reducing their numbers. Mites often hang out on the underside of leaves so sprays should be directed to contact them there. Make sure your plants are properly irrigated because stressed-out plants are less tolerant of their feeding damage. Spider mites sometimes become a problem after spraying insecticides. The reason is that their natural predators are often victims of insecticides and the mites are less susceptible. That is why less toxic chemicals, such as oils and soaps, are recommended over stronger products to spare the predators.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Questions and photos may be emailed to tucsongardensage@gmail.com.
- By Peter L. Warren Arizona Daily Star
- Updated
Q: I have two well-established lilac bushes that always did well with minimal care until about two years ago. I used to water them for 2 to 3 minutes every day, but stopped doing that at one point. When I realized that they were getting stressed I started watering them again — once a week for 20 to 30 minutes. That was a year ago. But they haven’t recovered. They’ve had mildew on the leaves for the last year or more. They grow well, but most of the leaves get mildew and wither up. Now I’m watering them every day for 2 to 3 minutes again. Am I overwatering now that it’s November? What can I do to help them recover at this point? The house is about 50 years old, we’ve been here 14 years, and so the lilacs could be quite old.
A: Non-native shrubs such as lilacs do best when watered infrequently and deeply. The recommended schedule is once every seven to 14 days in the summer to a depth of 24 to 36 inches. In the spring and fall you can reduce this to once every 10 to 21 days and in the winter every 14 to 21 days. Powdery mildew can be a problem in certain situations when the climate is suitable. Like any successful disease story, you need a susceptible host, the presence of the disease pathogen, and a suitable environment. If you were planting a new lilac I would encourage you to select a mildew resistant variety. To prevent an infestation, the first thing to do is remove any diseased leaves from the ground or on the plant. This will prevent the disease from overwintering nearby and spreading to new growth. Second, you can selectively prune your lilacs to provide space between the branches for air circulation and sunlight penetration. Dense areas will favor the infection. There are also fungicides you can use to spray the mildewed leaves. Choose a fungicide specifically labeled for mildew and spray according to instructions. Typically, they recommend beginning to spray when new leaves are emerging.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Questions and photos may be emailed to tucsongardensage@gmail.com.
- By Peter L. Warren Special to the Arizona Daily Star
- Updated
Q: We have a question about how to control an insect that is destroying our otherwise healthy palo verde tree. I’m enclosing a photo of the tree and three photos of what this insect is doing. In the latter you can see the little bored holes it uses to get inside a branch, what the branch looks like when it is done and the third shows some brown spots that may, or may not be related to the insect. Our friends think something similar might be destroying their mesquite trees, although they have never seen the attackers. Any thoughts?
A: Your tree is being attacked by the palo verde twig borer, a sporadic pest that seems to prefer pencil-sized branches to set up housekeeping. They hollow out the area near the holes in the branches, which become brittle and snap off on a windy day.
I see this damage around Tucson most years but not in very many places at once and not always in the same areas. There is not a good way to manage them because their occurrence is hard to predict and fortunately the damage isn’t long lasting or critical. I wouldn’t worry and I would try to think of it as nature’s way of pruning your palo verde.
Your neighbors might be referring to the mesquite twig girdler, which is also a beetle, but from a different family. Its MO is to girdle the twigs from the outside but the end result is similar, twigs fall off the tree. These are not a big deal either and another one of nature’s interesting ways of trimming trees.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Questions may be emailed to
- By Peter L. Warren Special to the Arizona Daily Star
- Updated
Q: We have a large ruby red grapefruit tree that is about 12 years old. We have always had nice crops of pink grapefruit until last year. Last year’s crop had smaller fruits and the flesh was yellow instead of pink. This year, the fruits look like they will be even smaller. We have consistently fertilized the tree three times a year, and it is on drip irrigation. The foliage is nice and dark green. What are we doing wrong?
A: Smaller fruit is often a symptom of insufficient irrigation. You said the tree is on a drip system but not how much water it is receiving. The recommendation for citrus trees is a deep soak to a depth of 36 inches every 7 to 10 days in the summer, every 10 to 14 days in the fall and spring, and 14 to 21 days in the winter. If you are already irrigating in this way, please let me know so we can puzzle it out further.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Email questions to tucsongardensage@gmail.com
- By Peter L. Warren Special to the Arizona Daily Star
- Updated
Q: Yesterday I noticed the leaves were gone on our Desert Willow. As I got close I saw the same type of worm/caterpillar that were found on tomato plants back in Pennsylvania. There were four or five about five or six-inches long and two smaller ones. What are they? I removed them and have not found any more on any other plants.
A: Your insect is the rustic sphinx moth caterpillar. They are common across the U.S. and are not considered to be a pest insect worth managing. They will eat a few leaves but there usually aren’t enough of them to defoliate a tree. Picking them off the plants is often enough and as you saw they are only around for a short time before they crawl off to pupate. They are closely related to the tomato hornworm you remember from back in PA but they are a different species and feed on different plants.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Questions may be emailed to
- By Peter L. Warren Special to the Arizona Daily Star
- Updated
Q: What is this? It’s growing at the base of our palo verde.
A: That is a Ganoderma fungus and unfortunately it’s not a good sign. The part you see is the fruiting body of the fungus and the rest is inside the tree. It begins as a root rot that infects the tree from a wound of some sort. The fungus is found in the soil. Once it is in the tree there is nothing you can do to treat the problem. A tree that is rotten on the inside is a liability. The tree may live a bit longer but its days are numbered. I can’t see the whole tree from your photo so it would be good to have an arborist examine the tree in person to determine the gravity of the situation. If this tree is of significant size and location that it could harm people or property nearby if it fell, you should look into having the tree taken down soon. Consulting a couple certified arborists would be wise to get multiple estimates on the potential cost and timing for this operation.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Questions may be emailed to
- By Peter L. Warren Special to the Arizona Daily Star
- Updated
Q: Our olive tree was planted about three years ago and most recently has developed a growth on the trunk and throughout other areas of the tree. We are concerned that this might kill the tree and spread to other plants. Would appreciate your input on what growth might be and what we can do the properly treat it.
A: The growths you are seeing are called epicormic sprouts. These are often seen as a response to stress in a tree. In healthy trees, these buds remain dormant under the bark, held in check by growth regulators produced in the tree’s crown. If the crown has been damaged or pruned heavily, the tree may respond by producing these shoots to expand the amount of leaves for photosynthesis. Other causes may include a sudden change in environmental conditions and disease. If you want to remove these sprouts, it’s best not to remove them all at once, causing further stress, so remove no more than one-third at a time. The tree would likely produce more of them anyway. If you know of any changes in the tree or surrounding area, such as recent pruning or damage, that would help narrow down why the tree is responding in this way and possibly offer a way to mitigate a recurrence.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Email questions to tucsongardensage@gmail.com
- By Peter L. Warren Special to the Arizona Daily Star
- Updated
Q: I have two Aleppo pine trees in my front yard. The massive amount of brown needles occurred rapidly and I am not sure why. Do you have any ideas what may have gone wrong?
A: Your trees are suffering from long-term drought. Ideally, they would receive irrigation every two weeks during the summer to a depth of 36 inches, every three weeks in the spring and fall, and once a month in the winter. Because this tree species originates in the Mediterranean region where rainfall is primarily a winter event, the monthly soaks in the cold part of the year are important.
While it appears the browning of the needles happened suddenly, these trees have been slowly reacting to less than average rainfall for many years by gradually dropping needles and branches. According to climatologists, we are in a long-term drought and trees such as yours that did fine for many years on annual precipitation are now reaching a point where they can no longer survive on rainfall alone.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Questions may be emailed to
- By Peter L. Warren Special to the Arizona Daily Star
- Updated
Q: I have a sick orange tree. One side is doing somewhat well, but the other side is dying, and the leaves are curled up. There is no sign of insect infestation. I have to water the tree every four to five days, otherwise the leaves begin to shrivel. I think the tree is about 6 years old but could be more. Originally it was on a drip system, but I stopped that about a year ago and use a sprinkler spray.
A: The tree is old enough to be established in its current location. The sprinkler you are using is likely not delivering enough water and not deep enough. I recommend you adjust your irrigation schedule to deliver enough water to reach a depth of 36 inches at the drip line once every two weeks. You can still use the sprinkler but it will not be as efficient as a drip system. More is lost to evaporation when it is airborne. You can check the depth the water is reaching by using a soil probe. The simplest kind is a three-foot piece of rebar with a handle that you can push into the ground after irrigation. It will meet resistance when you hit dry soil.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Questions may be emailed to tucsongardensage@gmail.com
- By Peter L. Warren Special to the Arizona Daily Star
- Updated
Q: When field mice eat the top tips of sweet potatoes (while still in the soil), are the tubers safe for home consumption?
A: While the practical me says just wash it off really well, I checked with our food safety microbiologist, and she says no way. The chewed part may not be the only part of the plant that the rodent touched. The problem is that the rodent may have touched its own feces or other rodent feces and if that gets anywhere on the produce, and it is not washed properly (with an actual sanitizer), it could harbor foodborne pathogens. My advice would be to put those veggies in the compost pile.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Questions may be emailed to tucsongardensage@gmail.com
- By Peter L. Warren Special to the Arizona Daily Star
- Updated
Q: What kind of cactus is this (pictured above, left), what are the fruits called, and are they edible?
A: It’s called Cereus repandus, the Peruvian apple cactus. It’s a South American species as you probably guessed from the name. It does well in our area except when we have a frost or freeze. The growing tips are damaged by below-freezing temperatures and need to be pruned as a result. In ideal locations protected from cold temperatures this plant may grow quite large. The fruit are edible and you may find birds, insects, and desert tortoises eating them as they ripen and fall to the ground.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Questions may be emailed to tucsongardensage@gmail.com
- By Peter L. Warren Special to the Arizona Daily Star
- Updated
Q: What is this pretty little weed plant (above, right)?
A: Your plant is called broom snakeweed (Gutierrezia sarothrae). It’s a native plant and regarded as a weed, especially where livestock graze, because it is poisonous.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Questions may be emailed to tucsongardensage@gmail.com
- By Peter L. Warren Special to the Arizona Daily Star
- Updated
Q: This is not a question about plants, but I thought you might know the answer. We enjoy sitting on the patio but haven’t been able to for several weeks. The problem is we’re out there for just a few minutes, and then our ankles and feet start to itch like crazy. I have sprayed the deck, but that hasn’t seemed to help.
A: Mosquitoes are fairly common ankle biters this time of year. While there are landscape sprays you can use for short-term effect, I recommend you use an insect repellent on your legs to see if that stops or at least mitigates the problem. The most effective repellents are those that contain the active ingredient DEET.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Questions may be emailed to tucsongardensage@gmail.com
- By Peter L. Warren Special to the Arizona Daily Star
- Updated
Q: I have a love/hate relationship with this “weed.” For about 10 years I cut it off at the ground. It came up as one puny stalk with a reddish stem and then got other stems growing next to it and I’ve seen it in other places where it volunteers near drip emitters. I always wanted a real tree in that corner but couldn’t afford it. About three years ago I thought: maybe if I let it grow and tied it up, it would get a strong enough stem to become a tree, sort of… so I did that. Looking it up on line, I thought it looked very like an African Willow. So now it is strong enough to support itself, but I really don’t feel like giving it a tree status. What the heck is it? Its perseverance has finally got my grudging admiration. It won the battle.
A: Your tree is called African sumac (Rhus lancea) and it is a very common plant in our area. They can grow to a height and width of 30 to 40 feet. Once they become large trees, some people like them for the shade they provide. They are tough and do well with little care and may be used as street trees or in places where little else will grow well. Unfortunately, they are invasive and can spread easily in urban areas. Their seeds will grow wherever there is available water. As you know, persistence is required to keep them down once they start growing. Some people report allergies to the pollen so that is another concern. It’s hard to say no to a free tree but this is one that you might consider removing given the associated problems it brings.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Email questions to tucsongardensage@gmail.com
- By Peter L. Warren Special to the Arizona Daily Star
- Updated
Q: I’ve got a previously healthy stand of Mexican fence post cactus that has been infested with a white scale pest of some kind. Can you tell me what it is — and how I might get rid of it?
A:Your scale insect was identified as Diaspis echinocacti. They are found on a variety of cacti and other plants. The recommended management strategy for scale insects is a combination of integrated pest management tactics including natural enemies, physical removal, and applications of horticultural oil or insecticide. Because their outer covering protects scale insects, the ideal time to spray them is during the time in their life cycle when the young have recently emerged from their eggs and are crawling about unprotected. The timing is not always easy to determine but you have options to figure it out. Late winter is a good time to start watching. Carefully examine the infested plant to see if crawlers are present. A magnifying glass or loupe will come in handy when looking for these tiny insects. Alternatively, place double-sided tape around the cacti in affected areas of the plant when crawlers are expected. The tiny insect bodies get stuck in the adhesive. Once you see them you will have a general idea when they emerge. The next trick is to notice what else is happening nearby with your other plants. For example, what is recently blooming in your landscape? Year to year variation in insect development can be estimated by the use of phenology (the known influences of weather variations on plant and animal life cycles). By watching known phenological indicator plants you can accurately predict when particular scale insect eggs will hatch in your area in following years and thus you will be better prepared to manage the scale insects.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Email questions to tucsongardensage@gmail.com
- By Peter L. Warren Special to the Arizona Daily Star
- Updated
Q: How can I get rid of the squirrels that are destroying my backyard? They have three huge (apparently connecting) holes around the large palo verde tree. They keep the gravel all messed up with their digging. Of course, my cat (who is an indoor cat) loves watching them from the door. The chipmunks have many holes around a large cholla on the bank outside the front wall, so I assume that is where they are coming from.
A: Round-tailed ground squirrels are not likely something you can eradicate. As you noticed, they can migrate from outside a wall by digging or climbing if it’s not made of slippery material. The best strategy is an integrated approach that includes systematic destruction of their habitat, trapping to reduce the population, and protecting the plants you like best with some sort of fencing. Since they are fairly small, using hardware cloth with smaller holes that can be buried a few inches into the soil may help. Since they climb, this means you also have to protect from that likelihood with either a cover or lining your barrier with flashing or some other material on which they cannot get traction. You can contain them but you can’t stop them.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Questions may be emailed to tucsongardensage@gmail.com
- By Peter L. Warren Special to the Arizona Daily Star
- Updated
Q: My father has a tangelo tree that will sometimes have a second bloom in the summer. This year on one of the branches there are two grapefruit! Usually I would think if there is another fruit it would come from a branch that sprouted from below the graft but this did not. He picked and cut one open and the flesh was a very light orange and tasted like a bland tangelo.
A: The tangelo is a hybrid citrus created by crossing a tangerine with a pomelo or grapefruit. Occasionally hybrids or part of them revert back to one of the original parents by producing plant parts such as foliage, flowers, or fruit that more resemble the parent plants. These growths are sometimes referred to as a ‘sport.’ So the odd fruit is likely a sport but probably something other than a pure grapefruit. Hopefully, they taste good. If not, you can prune that branch out of the tree or just let it be.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Questions may be emailed to tucsongardensage@gmail.com
- By Peter L. Warren Special to the Arizona Daily Star
- Updated
Q: I have heard from a number of people that mistletoe does not kill a tree. That might be true, but it weakens the tree so much that anything that comes along after the infestation of mistletoe will kill the tree—like our drought. I think people want to hear that it doesn’t kill the tree so they don’t have to act on ridding the tree of it. Or maybe people don’t care that much about their trees. Even the Rillito River Path trees have a lot of it in them, and the city pays to plant and water the trees.
A: There are many species of mistletoe and they are parasitic plants. In general, the species we are familiar with in the low elevations of the Sonoran Desert attach to their host plant to absorb water and nutrients. The healthier the host plant, the healthier the parasitic mistletoe that depends on it. If the host plant is suffering from other problems, such as drought, the mistletoe can take enough water and nutrients to weaken the host and that can lead to the death of the host. A common species in our area is called mesquite mistletoe (Phoradendron californicum). This is a native plant that has a place in our desert ecology in that it provides food and shelter for some animals. The birds that eat the seeds will spread mistletoe by pooping them out in other trees along with a dollop of their own brand of fertilizer. It would be impossible to eradicate mistletoe from our environment because it of the way it is spread by birds. It would also be hard to generalize about the relationship between people and mistletoe. I am fairly certain there are a few different reasons why mistletoe is allowed to thrive on landscape trees. Depending on the person, these may include the knowledge that it is food for birds, that it’s not always easy or affordable to cut it out of every tree, and that there is an endless supply of this plant in the open desert from which it can spread. I am familiar with the city’s lack of resources and I expect they are doing the best they can with limited staff and funds to manage trees and other plants on public property.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Questions may be emailed to tucsongardensage@gmail.com
- By Peter L. Warren Arizona Daily Star
Q: How do you keep bees out of the hummingbird feeders effectively enough that the little hummers have a chance to feed? Despite their aggressive nature, it seems that our hummers don’t want to challenge the bees. What to do?
A: Bees are seeking nectar and water for the same reasons as the hummingbirds and they both prefer actual plant nectar to sugar water if they can get it. I recommend providing a variety of flowering plants for the bees to distract them from your feeders. Hummingbirds prefer flowers that are tubular in shape but bees can’t access the nectar in these because they lack the long tongues of hummingbirds. Bees would more likely be found on plants that have a wide-open bloom with a place for them to land and access the nectar and pollen up close. Sunflowers are an example of the shape they prefer.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Questions and photos may be emailed to tucsongardensage@gmail.com.
- By Peter L. Warren Arizona Daily Star
Q: My tomatoes show an unknown leaf disease that has slowly spread over 35 percent of all tomato plants. Based on my observations, it started approximately four weeks ago within the center of one plant and has slowly advanced. Also, there is a very fine spider like web covering some of the browned leaf areas. I am familiar with “leaf curl” problems, but this is a new one for me.
A: From the photos and your description, I suspect spider mites. The symptom that is most noticeable is the color change on the leaves as a result of their feeding. Their tiny mouthparts pierce the leaves and remove sap. Just one mite feeding wouldn’t be noticeable but after weeks of feeding by a number of them, the change in leaf color becomes more noticeable. The fine spider-like webbing is a sign as well. Spider mites, just like many of their spider relatives, produce webbing where they live. This is also something that may go unnoticed with a small number of mites but becomes apparent with a significant population. Identifying mites often requires a 10X magnifier and it’s good to make sure you have live ones before spending time and money treating for them. Sometimes we only see the damage after they moved on. Spider mites thrive in our arid climate and can be a problem on a variety of plants. They aren’t all bad though, since some mites are predatory and feed on the pest mites. When they become a problem there are a few tactics you can use to manage them. Sprays of water, horticultural oil, or insecticidal soap are usually effective in reducing their numbers. Mites often hang out on the underside of leaves so sprays should be directed to contact them there. Make sure your plants are properly irrigated because stressed-out plants are less tolerant of their feeding damage. Spider mites sometimes become a problem after spraying insecticides. The reason is that their natural predators are often victims of insecticides and the mites are less susceptible. That is why less toxic chemicals, such as oils and soaps, are recommended over stronger products to spare the predators.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Questions and photos may be emailed to tucsongardensage@gmail.com.
- By Peter L. Warren Arizona Daily Star
Q: I have two well-established lilac bushes that always did well with minimal care until about two years ago. I used to water them for 2 to 3 minutes every day, but stopped doing that at one point. When I realized that they were getting stressed I started watering them again — once a week for 20 to 30 minutes. That was a year ago. But they haven’t recovered. They’ve had mildew on the leaves for the last year or more. They grow well, but most of the leaves get mildew and wither up. Now I’m watering them every day for 2 to 3 minutes again. Am I overwatering now that it’s November? What can I do to help them recover at this point? The house is about 50 years old, we’ve been here 14 years, and so the lilacs could be quite old.
A: Non-native shrubs such as lilacs do best when watered infrequently and deeply. The recommended schedule is once every seven to 14 days in the summer to a depth of 24 to 36 inches. In the spring and fall you can reduce this to once every 10 to 21 days and in the winter every 14 to 21 days. Powdery mildew can be a problem in certain situations when the climate is suitable. Like any successful disease story, you need a susceptible host, the presence of the disease pathogen, and a suitable environment. If you were planting a new lilac I would encourage you to select a mildew resistant variety. To prevent an infestation, the first thing to do is remove any diseased leaves from the ground or on the plant. This will prevent the disease from overwintering nearby and spreading to new growth. Second, you can selectively prune your lilacs to provide space between the branches for air circulation and sunlight penetration. Dense areas will favor the infection. There are also fungicides you can use to spray the mildewed leaves. Choose a fungicide specifically labeled for mildew and spray according to instructions. Typically, they recommend beginning to spray when new leaves are emerging.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Questions and photos may be emailed to tucsongardensage@gmail.com.
- By Peter L. Warren Special to the Arizona Daily Star
Q: We have a question about how to control an insect that is destroying our otherwise healthy palo verde tree. I’m enclosing a photo of the tree and three photos of what this insect is doing. In the latter you can see the little bored holes it uses to get inside a branch, what the branch looks like when it is done and the third shows some brown spots that may, or may not be related to the insect. Our friends think something similar might be destroying their mesquite trees, although they have never seen the attackers. Any thoughts?
A: Your tree is being attacked by the palo verde twig borer, a sporadic pest that seems to prefer pencil-sized branches to set up housekeeping. They hollow out the area near the holes in the branches, which become brittle and snap off on a windy day.
I see this damage around Tucson most years but not in very many places at once and not always in the same areas. There is not a good way to manage them because their occurrence is hard to predict and fortunately the damage isn’t long lasting or critical. I wouldn’t worry and I would try to think of it as nature’s way of pruning your palo verde.
Your neighbors might be referring to the mesquite twig girdler, which is also a beetle, but from a different family. Its MO is to girdle the twigs from the outside but the end result is similar, twigs fall off the tree. These are not a big deal either and another one of nature’s interesting ways of trimming trees.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Questions may be emailed to
- By Peter L. Warren Special to the Arizona Daily Star
Q: We have a large ruby red grapefruit tree that is about 12 years old. We have always had nice crops of pink grapefruit until last year. Last year’s crop had smaller fruits and the flesh was yellow instead of pink. This year, the fruits look like they will be even smaller. We have consistently fertilized the tree three times a year, and it is on drip irrigation. The foliage is nice and dark green. What are we doing wrong?
A: Smaller fruit is often a symptom of insufficient irrigation. You said the tree is on a drip system but not how much water it is receiving. The recommendation for citrus trees is a deep soak to a depth of 36 inches every 7 to 10 days in the summer, every 10 to 14 days in the fall and spring, and 14 to 21 days in the winter. If you are already irrigating in this way, please let me know so we can puzzle it out further.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Email questions to tucsongardensage@gmail.com
- By Peter L. Warren Special to the Arizona Daily Star
Q: Yesterday I noticed the leaves were gone on our Desert Willow. As I got close I saw the same type of worm/caterpillar that were found on tomato plants back in Pennsylvania. There were four or five about five or six-inches long and two smaller ones. What are they? I removed them and have not found any more on any other plants.
A: Your insect is the rustic sphinx moth caterpillar. They are common across the U.S. and are not considered to be a pest insect worth managing. They will eat a few leaves but there usually aren’t enough of them to defoliate a tree. Picking them off the plants is often enough and as you saw they are only around for a short time before they crawl off to pupate. They are closely related to the tomato hornworm you remember from back in PA but they are a different species and feed on different plants.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Questions may be emailed to
- By Peter L. Warren Special to the Arizona Daily Star
Q: What is this? It’s growing at the base of our palo verde.
A: That is a Ganoderma fungus and unfortunately it’s not a good sign. The part you see is the fruiting body of the fungus and the rest is inside the tree. It begins as a root rot that infects the tree from a wound of some sort. The fungus is found in the soil. Once it is in the tree there is nothing you can do to treat the problem. A tree that is rotten on the inside is a liability. The tree may live a bit longer but its days are numbered. I can’t see the whole tree from your photo so it would be good to have an arborist examine the tree in person to determine the gravity of the situation. If this tree is of significant size and location that it could harm people or property nearby if it fell, you should look into having the tree taken down soon. Consulting a couple certified arborists would be wise to get multiple estimates on the potential cost and timing for this operation.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Questions may be emailed to
- By Peter L. Warren Special to the Arizona Daily Star
Q: Our olive tree was planted about three years ago and most recently has developed a growth on the trunk and throughout other areas of the tree. We are concerned that this might kill the tree and spread to other plants. Would appreciate your input on what growth might be and what we can do the properly treat it.
A: The growths you are seeing are called epicormic sprouts. These are often seen as a response to stress in a tree. In healthy trees, these buds remain dormant under the bark, held in check by growth regulators produced in the tree’s crown. If the crown has been damaged or pruned heavily, the tree may respond by producing these shoots to expand the amount of leaves for photosynthesis. Other causes may include a sudden change in environmental conditions and disease. If you want to remove these sprouts, it’s best not to remove them all at once, causing further stress, so remove no more than one-third at a time. The tree would likely produce more of them anyway. If you know of any changes in the tree or surrounding area, such as recent pruning or damage, that would help narrow down why the tree is responding in this way and possibly offer a way to mitigate a recurrence.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Email questions to tucsongardensage@gmail.com
- By Peter L. Warren Special to the Arizona Daily Star
Q: I have two Aleppo pine trees in my front yard. The massive amount of brown needles occurred rapidly and I am not sure why. Do you have any ideas what may have gone wrong?
A: Your trees are suffering from long-term drought. Ideally, they would receive irrigation every two weeks during the summer to a depth of 36 inches, every three weeks in the spring and fall, and once a month in the winter. Because this tree species originates in the Mediterranean region where rainfall is primarily a winter event, the monthly soaks in the cold part of the year are important.
While it appears the browning of the needles happened suddenly, these trees have been slowly reacting to less than average rainfall for many years by gradually dropping needles and branches. According to climatologists, we are in a long-term drought and trees such as yours that did fine for many years on annual precipitation are now reaching a point where they can no longer survive on rainfall alone.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Questions may be emailed to
- By Peter L. Warren Special to the Arizona Daily Star
Q: I have a sick orange tree. One side is doing somewhat well, but the other side is dying, and the leaves are curled up. There is no sign of insect infestation. I have to water the tree every four to five days, otherwise the leaves begin to shrivel. I think the tree is about 6 years old but could be more. Originally it was on a drip system, but I stopped that about a year ago and use a sprinkler spray.
A: The tree is old enough to be established in its current location. The sprinkler you are using is likely not delivering enough water and not deep enough. I recommend you adjust your irrigation schedule to deliver enough water to reach a depth of 36 inches at the drip line once every two weeks. You can still use the sprinkler but it will not be as efficient as a drip system. More is lost to evaporation when it is airborne. You can check the depth the water is reaching by using a soil probe. The simplest kind is a three-foot piece of rebar with a handle that you can push into the ground after irrigation. It will meet resistance when you hit dry soil.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Questions may be emailed to tucsongardensage@gmail.com
- By Peter L. Warren Special to the Arizona Daily Star
Q: When field mice eat the top tips of sweet potatoes (while still in the soil), are the tubers safe for home consumption?
A: While the practical me says just wash it off really well, I checked with our food safety microbiologist, and she says no way. The chewed part may not be the only part of the plant that the rodent touched. The problem is that the rodent may have touched its own feces or other rodent feces and if that gets anywhere on the produce, and it is not washed properly (with an actual sanitizer), it could harbor foodborne pathogens. My advice would be to put those veggies in the compost pile.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Questions may be emailed to tucsongardensage@gmail.com
- By Peter L. Warren Special to the Arizona Daily Star
Q: What kind of cactus is this (pictured above, left), what are the fruits called, and are they edible?
A: It’s called Cereus repandus, the Peruvian apple cactus. It’s a South American species as you probably guessed from the name. It does well in our area except when we have a frost or freeze. The growing tips are damaged by below-freezing temperatures and need to be pruned as a result. In ideal locations protected from cold temperatures this plant may grow quite large. The fruit are edible and you may find birds, insects, and desert tortoises eating them as they ripen and fall to the ground.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Questions may be emailed to tucsongardensage@gmail.com
- By Peter L. Warren Special to the Arizona Daily Star
Q: What is this pretty little weed plant (above, right)?
A: Your plant is called broom snakeweed (Gutierrezia sarothrae). It’s a native plant and regarded as a weed, especially where livestock graze, because it is poisonous.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Questions may be emailed to tucsongardensage@gmail.com
- By Peter L. Warren Special to the Arizona Daily Star
Q: This is not a question about plants, but I thought you might know the answer. We enjoy sitting on the patio but haven’t been able to for several weeks. The problem is we’re out there for just a few minutes, and then our ankles and feet start to itch like crazy. I have sprayed the deck, but that hasn’t seemed to help.
A: Mosquitoes are fairly common ankle biters this time of year. While there are landscape sprays you can use for short-term effect, I recommend you use an insect repellent on your legs to see if that stops or at least mitigates the problem. The most effective repellents are those that contain the active ingredient DEET.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Questions may be emailed to tucsongardensage@gmail.com
- By Peter L. Warren Special to the Arizona Daily Star
Q: I have a love/hate relationship with this “weed.” For about 10 years I cut it off at the ground. It came up as one puny stalk with a reddish stem and then got other stems growing next to it and I’ve seen it in other places where it volunteers near drip emitters. I always wanted a real tree in that corner but couldn’t afford it. About three years ago I thought: maybe if I let it grow and tied it up, it would get a strong enough stem to become a tree, sort of… so I did that. Looking it up on line, I thought it looked very like an African Willow. So now it is strong enough to support itself, but I really don’t feel like giving it a tree status. What the heck is it? Its perseverance has finally got my grudging admiration. It won the battle.
A: Your tree is called African sumac (Rhus lancea) and it is a very common plant in our area. They can grow to a height and width of 30 to 40 feet. Once they become large trees, some people like them for the shade they provide. They are tough and do well with little care and may be used as street trees or in places where little else will grow well. Unfortunately, they are invasive and can spread easily in urban areas. Their seeds will grow wherever there is available water. As you know, persistence is required to keep them down once they start growing. Some people report allergies to the pollen so that is another concern. It’s hard to say no to a free tree but this is one that you might consider removing given the associated problems it brings.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Email questions to tucsongardensage@gmail.com
- By Peter L. Warren Special to the Arizona Daily Star
Q: I’ve got a previously healthy stand of Mexican fence post cactus that has been infested with a white scale pest of some kind. Can you tell me what it is — and how I might get rid of it?
A:Your scale insect was identified as Diaspis echinocacti. They are found on a variety of cacti and other plants. The recommended management strategy for scale insects is a combination of integrated pest management tactics including natural enemies, physical removal, and applications of horticultural oil or insecticide. Because their outer covering protects scale insects, the ideal time to spray them is during the time in their life cycle when the young have recently emerged from their eggs and are crawling about unprotected. The timing is not always easy to determine but you have options to figure it out. Late winter is a good time to start watching. Carefully examine the infested plant to see if crawlers are present. A magnifying glass or loupe will come in handy when looking for these tiny insects. Alternatively, place double-sided tape around the cacti in affected areas of the plant when crawlers are expected. The tiny insect bodies get stuck in the adhesive. Once you see them you will have a general idea when they emerge. The next trick is to notice what else is happening nearby with your other plants. For example, what is recently blooming in your landscape? Year to year variation in insect development can be estimated by the use of phenology (the known influences of weather variations on plant and animal life cycles). By watching known phenological indicator plants you can accurately predict when particular scale insect eggs will hatch in your area in following years and thus you will be better prepared to manage the scale insects.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Email questions to tucsongardensage@gmail.com
- By Peter L. Warren Special to the Arizona Daily Star
Q: How can I get rid of the squirrels that are destroying my backyard? They have three huge (apparently connecting) holes around the large palo verde tree. They keep the gravel all messed up with their digging. Of course, my cat (who is an indoor cat) loves watching them from the door. The chipmunks have many holes around a large cholla on the bank outside the front wall, so I assume that is where they are coming from.
A: Round-tailed ground squirrels are not likely something you can eradicate. As you noticed, they can migrate from outside a wall by digging or climbing if it’s not made of slippery material. The best strategy is an integrated approach that includes systematic destruction of their habitat, trapping to reduce the population, and protecting the plants you like best with some sort of fencing. Since they are fairly small, using hardware cloth with smaller holes that can be buried a few inches into the soil may help. Since they climb, this means you also have to protect from that likelihood with either a cover or lining your barrier with flashing or some other material on which they cannot get traction. You can contain them but you can’t stop them.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Questions may be emailed to tucsongardensage@gmail.com
- By Peter L. Warren Special to the Arizona Daily Star
Q: My father has a tangelo tree that will sometimes have a second bloom in the summer. This year on one of the branches there are two grapefruit! Usually I would think if there is another fruit it would come from a branch that sprouted from below the graft but this did not. He picked and cut one open and the flesh was a very light orange and tasted like a bland tangelo.
A: The tangelo is a hybrid citrus created by crossing a tangerine with a pomelo or grapefruit. Occasionally hybrids or part of them revert back to one of the original parents by producing plant parts such as foliage, flowers, or fruit that more resemble the parent plants. These growths are sometimes referred to as a ‘sport.’ So the odd fruit is likely a sport but probably something other than a pure grapefruit. Hopefully, they taste good. If not, you can prune that branch out of the tree or just let it be.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Questions may be emailed to tucsongardensage@gmail.com
- By Peter L. Warren Special to the Arizona Daily Star
Q: I have heard from a number of people that mistletoe does not kill a tree. That might be true, but it weakens the tree so much that anything that comes along after the infestation of mistletoe will kill the tree—like our drought. I think people want to hear that it doesn’t kill the tree so they don’t have to act on ridding the tree of it. Or maybe people don’t care that much about their trees. Even the Rillito River Path trees have a lot of it in them, and the city pays to plant and water the trees.
A: There are many species of mistletoe and they are parasitic plants. In general, the species we are familiar with in the low elevations of the Sonoran Desert attach to their host plant to absorb water and nutrients. The healthier the host plant, the healthier the parasitic mistletoe that depends on it. If the host plant is suffering from other problems, such as drought, the mistletoe can take enough water and nutrients to weaken the host and that can lead to the death of the host. A common species in our area is called mesquite mistletoe (Phoradendron californicum). This is a native plant that has a place in our desert ecology in that it provides food and shelter for some animals. The birds that eat the seeds will spread mistletoe by pooping them out in other trees along with a dollop of their own brand of fertilizer. It would be impossible to eradicate mistletoe from our environment because it of the way it is spread by birds. It would also be hard to generalize about the relationship between people and mistletoe. I am fairly certain there are a few different reasons why mistletoe is allowed to thrive on landscape trees. Depending on the person, these may include the knowledge that it is food for birds, that it’s not always easy or affordable to cut it out of every tree, and that there is an endless supply of this plant in the open desert from which it can spread. I am familiar with the city’s lack of resources and I expect they are doing the best they can with limited staff and funds to manage trees and other plants on public property.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Questions may be emailed to tucsongardensage@gmail.com
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