Re: Thinning to prevent biannual bearing?
I have thinned apples as late as July 4th on a farm in order to get decent sized apples.
But thinning apples to help with a biennial pattern is a different story......and generally the EARLIER you thin the better your results will be to help settle the tree into a more regular bearing pattern. The earlier you thin, the more effectively you can "communicate" to the auxins in the tree to produce bearing fruit for the next year by thinning it now.
If you had a light crop last year then maybe this year it would be good to do some thinning as you anticipate a big crop this year but a poor crop the next year if you have a biennial pattern......the earlier you thin the better. Some people even thin the blossoms now....but that is risky if you don't know which blossoms will bear fruit....so if you do that, be conservative. Most wait until they see the fruit and then thin the fruit out to 6 to 8 inches as soon as the fruit is visible.
Some will wait until after June drop and ensure that they know which fruit remains on the tree, but that is too late to effectively control the biennial pattern.
The Good Fruit Grower recently had an article about an "Equifruit Disc" that will be used to scientifically gauge just how much fruit to leave on any given limb.......it is my belief that super-hobby enthusiasts will start to work in this direction in the future, but for right now that is too tedious....and the usual rule is one fruit per cluster (select out the bad ones) or 6 to 8 inches between fruit. http://read.dmtmag.com/i/57720/15
Final Note: Seattle will reach a sunny 70 degrees on Sunday. I will make a "bet" that this year both Portland and Seattle will finally get a good fruit set year after two bad years in a row where we have had wet and cool Springs. Therefore, if I am right, this year would be a good year to thin on some of the trees.