Stella Otto: The Backyard Fruit Gardener

Today's Tip: Mow vegetation short around the base of fruit trees and bushes to reduce winter habitat for rodents.

Thinning Apples for the Time-Challenged Home Orchardist

Maturing too many blossoms can stress a fruit tree needlessly.

It is very stress­ful for the tree to mature all of it’s blos­soms to poten­tial, full-sized fruit.

If you have an over­abun­dance of apple fruitlets, you can thin excess fruit by hand. It is pre­cise, but time consuming.

Another way to thin excess fruit is with the use of Sevin™ (car­baryl) insec­ti­cide. It is an insec­ti­cide that also mim­ics the nat­ural plant hor­mone, auxin, that reg­u­lates plant growth and devel­op­ment. When applied between bloom and 30 days post-bloom this mate­r­ial can cause a per­cent­age of the devel­op­ing apples to drop from the trees. The results are not fully pred­i­ca­ble. Lots of fac­tors influ­ence the effect, but it is a tech­nique to con­sider if you have a lot of fruit and lit­tle time to thin by hand.


Fac­tors that influ­ence fruit-thinning results

Some fac­tors that influ­ence the amount of fruit thin­ning that occurs include:

  • Stress level of the tree — heavy crop load, min­i­mal mois­ture, high temperatures.
  • Tem­per­a­ture — Thin­ning effects are great­est at tem­per­a­tures above 70 degrees F. Over-thinning can occur if tem­per­a­tures exceed 80 degrees F.
  • Fruit size — Smaller fruit tend to drop read­ily, fruit over 16 mm becomes dif­fi­cult to thin
  • Amount of devel­op­ing fruit — If pol­li­na­tion was good and the crop load is heavy, thin­ning may be eas­ier. When pol­li­na­tion was poor and fewer fruit were fer­til­ized and develop, chem­i­cal thin­ning may have less effect.
  • Vari­ety — Some vari­eties are nat­u­rally harder to thin: Golden Deli­cious, in par­tic­u­lar, as well as Empire, Macoun, and Spy.
  • Tree age — Trees less than 5 years old drop fruit more eas­ily and may overthin
A “help­ful hand”
The king bloom

The king bloom is usu­ally first to open and pro­duces the largest apple of the cluster.

The ideal time to thin apple fruitlets is when they are between 12mm and 16mm in diam­e­ter. This is usu­ally one to three weeks after bloom. A handy size ref­er­ence is your own hand. On an aver­age hand, 12 mm = the tip of you pinky, 16mm = the tip of your pointer fin­ger. Ide­ally you would like to keep the “king” bloom, that first and usu­ally largest fruit­let, in the clus­ter and remove the rest.


Use respon­si­bly

Car­baryl is an insec­ti­cide that is toxic to bees. Do not use it when blos­soms are present on trees and ground cover plants, or dur­ing active bee flight.
Car­baryl is also toxic to desir­able preda­tor mites. Restrict­ing use to early in the sea­son and direct­ing it toward the tops of trees will do the least dam­age to ben­e­fi­cial mites. Con­sider this in your over­all tree man­age­ment strat­egy.
Use car­baryl as a thin­ner only on apples, it can cause deformed pears when applied within a month of bloom.
It does not act as a thin­ner on stone fruit.

If addi­tional fruit thin­ning is needed, you can come back and hand-thin in the weeks ahead. At least you will have less hand work remaining.