the best way to answer these questions is: picture the apples fully grown. It does take vigor away from growth...but do not whack out about it. Let the "fruitlets" drop before thinning.

I always thin my apples early, starting at about half inch diameter. That's the only way you can be assured of return bloom the following year. I've never had an apple tree go biennial that way. Is it more work, ya probably but I like fruit every year possible.

I have had Bosc pear go biennial. In fact it always has the few years it's cropped. All my pears and apples froze out this year so I'll have to thin everything hard and early next year.

My guess on your 15 foot tree is to leave about 100 apples. It might crop more than that OK depending on canopy size. I leave one fruit about every 3-4 clusters. That's with a heavy bloom which you get by thinning early.

Here's a few references which attempt to quantify potential cropload. Keep in mind that all of these articles are for high-density plantings, so I'm not sure how the numbers will vary for a home orchard.

FruitGrowersNews article on Tall Spindle:
"âÂÂVarieties differ in their biennial bearing tendency, and this must be incorporated into crop loads allowed on young trees,â he said. âÂÂFor annual cropping varieties like Gala, we recommend crop loads of 15 to 20 apples per tree in the second year, 50 to 60 apples per tree in the third year and 100 apples per tree in the fourth year. For slow-growing and biennial bearing varieties like Honeycrisp, crop loads should be half that used with Gala.âÂÂ"

Last year, my best trees were at about half their number in the 2nd leaf and several dropped all their apples. This year, I can see 50-60 being reasonable in 3rd leaf. Given that you had only 4 last year, you may want to aim for 50-60, rather than the 100 they list for 4th year trees.

UMASS and Cornell recommend 3-6 fruit per square CM in cross sectional area. That would work out to 3.14*((2.5/2)*2.54)^2 = 95-190 for your tree.

This same formula would give me 40-70 per tree (on the 4 largest trees). I had been shooting for 30-50 (depending on tree), based on the the blooms and how much space I had planned to leave between apples (I'll try for 8-12"). Yes, I counted on each tree and wrote down my predictions :)

Thanks, this is very helpful. I was thinking 50 or so made sense.

So when is best to thin? When the fruits are 1/2 inch? And how does one decide which apples to leave? Is it selective or random?

A rule I try to follow: you should not be able to touch adjacent apples or pears with the same wide-opened hand. In other words, about 9" between fruit.

I find that very hard to do. I thin down to one fruit per cluster at first, before the June fruit drop, and then go back and try to be more disciplined after the drop. Apples are pea-sized right now, pears are about as big as a bean.

It's best to thin early, very early if you want fruit every year. I've never had excessive drop after thinning early. In fact there is usually very little drop. To me that's the only reason to not thin early. But in other climates and circumstances, like insect damage, there might be a reason to wait.

One way to thin more is to bag the fruit. Thinning is easy when the alternative is to bag. It also lets you know how many apples are left on the tree.