For chemical thinning of apples, the best time for post-bloom thinning is around 10-12 mm fruit diameter. With warm temperatures fruit can grow up to 1mm per day, so in northern areas we are right in the middle of chemical thinning time. Often at thinning time we talk about temperatures needing to be warm enough for thinners to work. This year we have the opposite problem – how warm is too warm?
Let’s think for a moment how chemical thinners work. Natural fruit drop is due to a shortfall in carbohydrates in the tree – leaf area is limited which limits photosynthesis (the supply of carbohydrates) and developing fruitlets are all demanding carbohydrates, therefore there’s an imbalance of supply vs demand. Post-bloom thinners work by reducing the supply (NAA, Sevin) or increasing the demand (6-BA) for carbohydrates. With higher temperatures, respiration is increased so there is already a higher demand for carbohydrates. Therefore, it seems to me that 6-BA MIGHT be a slightly safer way to go with higher temperatures. Dr. Jim Schupp at Penn. State University knows more about thinning than anyone I know. Jim says “There is no “safe” thinner at high temperatures”. Tread carefully.
In more southern areas where fruit size is larger, the window for chemical thinning is pretty much closed. Once fruit diameter is above about 15 mm, really the only materials likely to be effective are Sevin and ethephon. When applied in very warm conditions, ethephon can easily overthin and could be a good option in situations where you want to drop all fruit off the tree. Such a situation might include very young trees that you don’t want to carry any fruit on yet.