Spray Olives Twice for Good Fruit Control

Q. We have seven large olive trees on our property.  In the past, we used two different local tree services to spray our trees to minimize the amount of olives.  One company told us we needed two applications in early Spring, and the other said we only needed 1.  Both came at a very high cost and results left much to be desired.  Every day we had to sweep up hundreds of olives. Do you have any suggestions as to what might work, and the timing of when the trees should be sprayed?

Olive flowers

A.  The chemicals used for preventing olive fruit from forming work if the timing is right. The timing depends on when the flowers open.  If the right chemical is used, it is applied at the right concentration and applied correctly, you should prevent 98% of the fruits from forming.  Two sprays are more effective than one spray.

Olive flowers hanging from the tree

            Nearly all the chemicals now being used need to be applied when flowers are open to get satisfactory results.  If flowers are open, pollen is released so applying these chemicals has little to do with preventing allergy problems.  Its purpose is to prevent fruit from forming.

            Here are why two applications work better than one.  First, flowers must be open when the spraying is done.  This is because the spray must enter flowers to be effective.  It is not effective if it lands on the outside of the flower; when the flower is closed.

            Secondly, flowers do not open all at the same time.  Flowers exposed to full sun open first.  These are primarily on the south and west sides of the tree.  Flowers on the north and east sides open later.  Flowers that are not in full sun, scattered throughout the inside of the tree, also open later. 

            For all the flowers to open may take up to 10 days.  The length of time needed for all the flowers to open depends on the weather.  If it is cold, all the flowers take longer to open.  If it is hot, all the flowers finish opening much sooner. 

            For best control of the fruit, two sprays should be applied; the first when 20% of the flowers have opened and the second when 80% of the flowers have opened.  One spray prevents fruit from forming but two sprays prevent MORE of the fruit from forming.  Never expect 100% control.

            Applicators have a very limited amount of time to get all their customers trees sprayed.  If this spray “window” is missed it results in very poor control of the fruit. 

            I should mention there is one chemical which can be applied which prevents flowers AND fruit from forming.  This chemical is normally applied from February until March, before flowering.  It is hard to get but there may be a few applicators still using it.