Q: One of my triple totem pole plants has a vertical crack. I wonder if this could be from overwatering? I followed instructions when I planted it in May and did not water it for six weeks. Then we had quite a bit of rain this summer. Any tips would be greatly appreciated.
A: Yes, this is most likely due to overwatering. The totem pole cactus, Lophocereus schotti, is a typical desert plant that is native to Baja California and Mexico and well adapted to the conditions in Arizona. We recommend watering cacti every 10 to 14 days in the summer, every 21 to 28 days in the spring and fall, and not at all during the winter. The soil should be sandy and well drained as well.
Q: Our olive tree is 11 years old and has started shedding leaves since early spring. We did some deep watering to supplement the drip system but it has not stopped the shedding. Is there a “cure” you can advise us on stopping the shedding of leaves?
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A: Leaf drop can be attributed to several things and watering is one of them. For a well-adapted nonnative tree like the olive (Olea europaea), we recommend watering infrequently and deeply rather than a regular drip. So for the winter, every 14 to 21 days down to 24 to 36 inches is best. In the spring and fall you can increase the frequency to 10 to 21 days and in the summer bump it up even more to seven to 10 days. In all cases, accounting for recent rains and snow is important.
If you see other symptoms in addition to leaf drop, please let me know and we can consider other problems. It is always good to step back and look at the big picture. For example, have there been any disturbances to the landscape around the tree or did someone apply herbicides in the area of the root zone? Trees often die of many causes and it is good to consider all the possibilities when diagnosing.
Q: My large palo verde has developed “witches broom.” Is this a major problem? It is quite extensive and I don’t care for the look, but am not sure how to deal with it. Also a root from the tree has heaved my sidewalk and I wonder about cutting the root and fixing the sidewalk. Hope you can give me some suggestions.
A: Palo verde trees are susceptible to witches broom. The cause has been attributed to feeding damage from eriophyid mites for many years, although there is ongoing research to determine if other factors may play a role. When it comes to the health of the tree it is a minor problem although aesthetically it is a major problem. You can prune out the brooms, but they may grow back. Treating for mites when they are found is a tactic to reduce the incidence of the brooms, but can be expensive if you have a lot of trees and it is hard on the natural enemies of the mites and other pests. Getting used to the look of nature’s abnormalities is difficult for many people, but as my friend Carl Olson (University of Arizona department of entomology), says, “This isn’t Disneyland.”
Peter L. Warren is the Urban Horticulture Agent for the Pima County Cooperative Extension and the University of Arizona. Questions may be emailed to plwarren@cals.arizona.edu