Protecting cactus in cold weather

The photo which was taken at the Pima County Cooperative Extension shows a Trichocereus Hybrid with Styrofoam cups on top to help with the cold days and nights. Photo by James S. Wood / Arizona Daily Star 

Editor's note: This story was originally published in December 2020.

If you've ever seen columnar cacti wearing Styrofoam cups, that's just the sign of a responsible plant owner. 

Although many plants don't need to be covered to protect against the freezes Tucson does get, some do need a little extra help come 32 degrees. 

To cover or not to cover. That's the question we asked Bryce Beamish, the general services manager at Tohono Chul, a botanical garden on the northwest side with 49 acres of desert landscaping. 

"Especially for newcomers here, it's easy to be surprised by how cold it gets at night," Beamish says. "And it's not just cold, but there's also no humidity. Cold, dry nights can wreak havoc on anything not adapted to this environment. And we always tell people to plant as many native things as they can." 

1. Know what you have

Before you can decide whether you need to cover a plant, you need to know what it is. Beamish says the most important thing is to know the plants you have so you can do a bit more research. 

Native plants, for example, usually don't need to be covered — another benefit to opting for native plants. 

"You'll see sheets over prickly pear or saguaros in paper bags, and that's just a simple misunderstanding," Beamish says. "One thing I always tell people is our native plants are generally adapted to survive our winters." 

It's some of the other plants that need that extra warmth. 

Tucson Botanical Gardens also opts for styrofoam cups.

2. What to cover and how

Beamish says columnar cacti are some of the most common Tucson landscaping plants that need some extra TLC during the winter. 

"The reason why we put Styrofoam cups on cacti is the most sensitive part of the cacti is where new growth comes in right at the top," he says. 

If you're out of Styrofoam cups, get creative. A Santa hat will do. You're really just looking for anything insulated. 

Citrus trees also require some additional warmth when temperatures plunge. More established trees can handle a freeze better than newer plants. The easiest fix? If your tree is in a pot, bring it in. Otherwise, you have a few options. 

"The method to preserve it in a freeze is to give warmth to the trunk," Beamish says. "One of the easiest ways to do that is to wrap the actual trunk of the tree." 

You can use paper or cloth — this is where extra bed sheets come in handy — just not plastic. That's because plastic will freeze if it gets wet. 

"The most important thing is you get that wrap to touch the ground because that's where you'll get heat from," he says. "That's where the heat is, and you want to transfer heat into the wrap to get it to hold on to the trunk of the tree." 

You can also make a tent over your tree, but again, you want your tent to make contact with the ground so that it holds heat. Incandescent Christmas lights can also help keep things toasty. 

Succulents will also need to be moved inside or covered. You can also move potted plants near a wall with lots of sun exposure. The radiation of heat from the wall plus any coverings you add should add that extra bit of warmth. 

3. When to get out the sheets

"The gauge I use for when to start covering things here at the park is when I see any forecast that goes close to 30 (degrees) for a couple of days," Beamish says. "If I see it forecasts anything below 30, and I check multiple forecasts constantly this time of year, that is when I start pulling out cloths and wraps for the trees." 

It's also important to continue watering your plants throughout the winter, but to be aware of the forecast. Beamish says he might back off on watering only if the temperatures remain extremely low for several days, but you don't want to stop watering altogether during the winter.

And you definitely want to make sure you don't water the plants' foliage. 

"There are certain plants that won't take a ton of damage if water freezes in the ground, but if water freezes on the foliage, that can wipe it out," he says. 

4. Don't prune

If your plants do take some frost damage, don't immediately get out your clippers. Yes, it looks brown and dead and probably isn't the look you were going for in your backyard, but it's important to leave those brittle bits in place until early spring when you see the forecast begin to rise again, Beamish says. 

"That's when you want to trim back the frost-damaged stuff. The reason for that is the dead leaves, and even the vines, will hold in heat," he says. "And a lot of times, unless we have a bad cold snap, we're just trying to gain an advantage of a couple of degrees of difference." 

5. Give your plants some sun

While tenting certain plants can help them get through winter, you also don't want to leave them covered all winter. 

The leaves still need sunlight, so in the winter months you have to resign yourself to an on-again, off-again relationship between your plants and their covers. 

When the temperature rises again, remove the cover, usually during the warmer parts of the day, he says. 

If you wrapped a trunk, Beamish says you can likely leave that a bit longer, since the leaves are still exposed. 

"It's important to remember we don't usually just get one freeze here," he says. "We tend to get a couple." 

For additional help managing your garden during the winter, check out this resource page from the University of Arizona Cooperative Extension.

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