Thinking About Thinning Jon Clements MSU Extension Berrien County Horticulture Agent 1737 Hillandale Rd. Benton Harbor, MI 49022 clementj@msue.msu.edu

Your apple thinning spray(s) are arguably one of the most important sprays of the year. Unfortunately, they are also the most difficult to get right. A number of important variables—including bloom and pollination conditions, tree and orchard factors, and weather during and after application—impact the results of your chemical thinning spray(s).

The astute grower must consider the impact of these factors when ‘thinking about thinning.’ What exactly needs to be considered? In his ‘Apple Thinning Guide,’ MSU Extension District Fruit Agent Phil Schwallier discusses them in detail. Here, I will review them briefly, the point being to get you to start ‘thinking about thinning’ in more than just terms of ‘ppm’s of NAA’ and ‘10 mm fruitlet size.’

Bloom and pollination conditions

A very heavy or ‘snowball’ bloom has the potential for setting a large crop. But it’s also likely (all other things being equal) to exhibit the most ‘response’ to a thinning application, because there is great competition amongst the developing fruitlets. Likewise, a short bloom period will make thinning ‘easier,’ vs. an extended bloom period, wherein good cross pollination is more likely.

Weather during bloom equates to pollination conditions (assuming bees are in place at king bloom) and low temperatures (i.e., frost). Good bee activity and warm temperatures are going to result in fruit set that is ‘harder’ to thin. Cool, wet, cloudy weather, and/or frost injury will make the response to thinning sprays greater and somewhat more unpredictable.

Tree, orchard, and management factors

The condition of the orchard and it’s recent management history have an impact on chemical thinning efficacy. For example, a heavy crop the previous season will require treading lightly when applying chemical thinning sprays this season. On the other hand, a light crop the previous year predisposes the tree to produce a heavy crop, now requiring a more aggressive thinning program.

Trees that are adequately nourished and healthy will be less likely to respond to thinning applications, whereas trees with low nitrogen levels (as well as other nutrients) will generally lose fruitlets much easier. And some varieties, such as ‘McIntosh’, are just plain easier to thin (more responsive) compared to more difficult to thin varieties such as ‘Golden Delicious.’

Finally the grower must consider the impact of their thinning spray decisions —including concentration, wetting agents, timing, and spray water pH—as they all have an impact on thinning efficacy. (Hint: concentration and timing are likely to have the biggest effect!)

Weather during and after thinning application

Considerable research has looked at the impact of weather during and after a thinning application on thinning efficacy. The conclusion? Indeed, it has a big effect! In fact, obtaining the best weather forecast for the two to three days during and following your spray may be your best clue as to how well your chemical thinning application will work.

To start, chemical thinners are acting as plant growth regulators—their absorption and uptake by the plant is a prerequisite for them to work. Slow drying conditions during application favor uptake and absorption; quick drying will mean less thinning chemical will be absorbed. Similarly, warm temperatures during and shortly after application will favor uptake and increase chemical thinning response because plant cells are more active.

For the few days following thinning application, also consider that warm sunny weather is less stressful for the tree. Therefore, fruitlets will be less likely to succumb to your thinning spray. But, if it is cloudy and cool, watch out! – fruitlets are apt to ‘rain-off’ a little faster than you might like. Put another way, ideal conditions for a strong thinning response are warm, humid weather during and immediately following application, followed by a period of cool, cloudy weather for several days. Expect a weak thinning response when thinner(s) are applied under cool, fast drying conditions, followed by a period of nice, sunny weather. This is VERY IMPORTANT and often overlooked! You should be basing your chemical thinning application timing and concentration on weather factors during and preceding application. A good weather forecast is a prerequisite for a successful thinning application!!!

Bottom Line

What’s the bottom line? Here is a ‘minimalist’ (but thoughtful!) approach to take when attempting to put together a thinning program—you can fine tune and adjust as appropriate:

• pay very close attention to conditions during bloom, the point being to estimate the potential for fruit set, be it light, moderate, or heavy. Consider the past cropping history, vigor of trees, and cultivar too.
• if a moderate to heavy fruit set is anticipated, begin thinning immediately at petal fall. Use 5 ppm NAA initially. Apply when temperatures are warm (> 70 F.) and drying is slow, if possible. This application will also promote return bloom for next year. Follow-up with one or two NAA sprays at rates of 5-10 ppm during the 8-15 mm. fruitlet size stage. Sevin XLR at one quart per acre may be substituted for the later NAA applications. Or, if particularly aggressive thinning is desired, add one pint of Sevin to the later NAA application(s) too.
• alternate approaches to the moderate to heavy fruit set scenario include: NAD at 25 ppm plus Sevin XLR at one quart per acre shortly after petal fall (do not use on Delicious); evaluate thinning efficacy of this application, and follow-up with NAA, Sevin, or a combination of the two as appropriate. Another aggressive thinning approach includes using Sevin XLR plus Accel (both at one quart per acre), at a timing centered on approximately 10 mm. fruitlet size. Do not worry about fruitlet size so much as applying the thinning spray when weather conditions are good, as discussed above. Note: the Sevin/Accel combination has proven ‘hot’ when applied to some cultivars under ideal thinning conditions, so be careful if using this combination.
• if a light fruit set is likely, a mild thinning response is obviously desirable. Use a lower rate of NAA (app. 5 ppm), or a full rate of Sevin or Accel (do not combine!) at a fruitlet size centered around 10 mm. Again, apply the thinning spray when weather conditions during and after the application are favorable for the kind of response desired.

Finally, by all means, get a copy of Phil Schwallier’s publication, the ‘Apple Thinning Guide.’ (Available from Great Lakes Publishing, 616-887-9008.) It is the most comprehensive discourse on apple fruit thinning written. Schwallier takes you through all the considerations and steps necessary for a successful thinning job, in very understandable terms—and in far greater detail than I have outlined above! It is absolutely a ‘must-have’ for any serious apple grower’s library.