Q: I have a question regarding a Mexican fence post cactus we have planted in our landscaping.ย We remodeled our patio two years ago and had this cactus transplanted to a different location in the patio. In the last several months, the plant has a brownish tint and we are wondering what is the cause and what we should do about it.
The cactus still feels firm; the soil was treated for grubs in the summer; the plant is on light irrigationโsame schedule as prior to transplanting. We also lightly fertilize all our plants in the spring.
We are located in the Catalina foothills in the Northeast part of Tucson. Do you think it is getting too much or too little water?
A:ย The Mexican fencepost cactus (Pachycereus marginatus) is a tough plant but there are always a few things that could go wrong.
When transplanting cacti it is best to orient them in the same direction as they were originally planted. Plants develop a tolerance for the amount of sun they receive on each side and facing them in a new direction may cause them to become sunburned.
The brown color of your cacti looks like sunburn so this is a potential cause of the discoloration. The cacti may recover from this, time will tell.
Another potential problem is rot caused by a fungus that may have entered through a wound in the transplant process. Because you arenโt able to detect any softening of the exterior this is only a possibility.
Once again, time will tell. Another potential problem is the soil where it is planted. Cacti do best in well-drained soil and suffer from overwatering if the soil holds its water too well. Irrigation for cacti should be light, as in once every two weeks in the summer and every three weeks in the spring and fall. No irrigation should be applied in the winter season or during the monsoon season.
Overall, it would be good to keep an eye on the situation to see if the damaged area is spreading, remaining the same, or getting better. These cacti donโt have a common grub problem so you might discontinue treating for them.
Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Email questions and photos toย tucsongardensage@gmail.com
