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Sunday, March 6, 2016

Stopping or Suppressing Flowering and Fruiting of Ornamentals

Q. Is there anything to stop or suppress flowering of a Palo Verde tree? My wife is allergic to the pollen and unless I can mitigate its effects, I guess it's gone. 

A. You are hitting on a very important topic as we convert our landscapes into water conserving desert landscapes. Before the conversion of landscapes from traditional to desert landscapes, the three biggest problem trees for allergy sufferers were mulberry, pine and olive.
Olive flowers just before they begin to open
Female mulberries produce only fruit. Male mulberries produce only pollen. Years ago fruitless (male) mulberry was planted because it was a great shade tree for the desert, water for landscapes was abundant and the male tree produced only pollen. Female mulberry trees made a mess on cars and sidewalks and birds who ate the fruit dropped it everywhere.
Pine getting ready to release pollen from the male flowers
Pine trees and olives were different. They also produced allergenic pollen but pollen and fruit were produced on the same tree; male and female trees were not separate from each other. Fruitless (and flowerless) olive was discovered, promoted and adopted as the only olive tree which could be planted in major urban centers like Las Vegas because it produced no flowers and hence no pollen.
Mulberry flowers getting ready to release pollen
Palo Verde produces allergenic pollen on flowers that are both male and female in the same flower. Comparing them to the above 3 trees, they are more similar to olive than pine or mulberry. There are no flowerless Palo Verde, unlike the olive.
Olive fruit can sometimes be prevented by spraying the tree with a product like Florel Fruit Eliminator. (contains ethephon). This product also controls the fruiting of several ornamental plants (and sometimes the flowers) when applied just before bloom. Depending on your Palo Verde, flowering will probably be in April or May in our Las Vegas climate. You can read the label below

I hesitate to recommend this product because if it is applied at the wrong dose it can cause leaf drop or possibly damage the tree. There is a recommended rate for several ornamental trees including olive but Palo Verde is not listed on the label. Use it at your own risk but if you are thinking about removing the tree anyway it might be worth a shot. I am not sure who sells it in Las Vegas but you cannot purchase it online.

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