Does this information seem to be accurate?
Fruit set occurs after the petals have fallen off, and the remaining ovary begins to swell. That’s pretty simple. But if we can’t see the flowers, how do we know when initiation happens? Thankfully flower initiation is dictated by day length, which is quite reliable, and generally occurs around June 20 in this region. Fruit thinning should be done before then or next year’s harvest will be compromised. A good rule of thumb is to thin the tree when the fruits are about ½” in diameter, or about the size of a dime.
http://blog-yard-garden-news.extension.umn.edu/2009/05/simple-steps-to-productive-apple-trees.html
I think you know that my background in this stuff is close to non-existent. I read the page in your link. I really don’t know anything about when flower bud initiation begins, but part of that doesn’t quite make sense to me. I doubt that flower initiation is dictated by the total number of hours of daylight because by this time of year Minneapolis has about an hour longer day-length than we do here. We will not, at any time of year get to the number of hours of daylight as they will have in late June, but apple trees still make flowers here. The date they gave, June 20th, is generally the summer solstice, though. So, I think maybe what they were indicating in a mixed up way, is that the changeover from increasing day length to decreasing is what triggers initiation.
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