Franz Niederholzer, UCCE Farm Advisor, Colusa and Sutter/Yuba Counties
Dani Lightle, former UCCE Orchards Advisor, Glenn, Butte & Tehama Counties

When needed, shaker thinning can improve a grower’s bottom line this year and next year. Careful, timely thinning increases A & B size fruit production, limits small, lower value prunes in the bin at harvest and improves return bloom next year.

We strongly recommend checking fruit load (prunes/tree) every year. If you need to thin, we recommend thinning.  If you don’t need to thin, don’t thin. To check fruit number from 2-3 trees per orchard at or just before reference date, which usually falls between April 20^th and May 10^th. Reference date occurs when 80 to 90% of the fruit have a visible endosperm (see Figure 1), which is approximately one week after the pit tip begins to harden. The endosperm, a clear gel-like glob, the beginning of the developing seed, will be found in the seed cavity on the blossom end of the prune (Figure 1) and is solid enough to be removed with a knife point.

To check fruit number per tree and decide whether or not to thin, follow theses steps. Estimate the number of fruit per tree needed to produce your desired crop, determine the number of fruit on 3 representative trees, at or just before reference date, and, using those numbers, decide if you need to thin. Calculate how much fruit needs to come off if thinning is needed. Finally, shake ASAP after reference date if thinning is needed. Below we walk through the math, step by step. Or, skip doing the calculations by hand and use the prune thinning calculator on phone, tablet or laptop.

1. Estimate the targeted tonnage from a given block by considering orchard history, age, etc. Let’s assume a target of 3 tons/ac, and shoot for 55 dry count/lb in an orchard spaced 16’ x 18’ (151 trees/acre). From there, calculate a targeted number of fruit per tree:

(Dry pounds per ac x Dry count per lb) ÷ Trees per ac = Target number fruit per tree (at harvest)

6,000 lbs/ac × 55 count/lb ÷ 151 trees/acre = 2,185 fruit/tree (target)

2. Determine the actual number of fruit in a sample tree and compare that number to the target of 2,185 fruit (from step 1). Ideally, repeat this procedure on 3 representative trees to ensure accuracy. Place a tarp under the tree and mechanically shake off as much fruit as possible, then hand strip any remaining fruit. Collect all the sound fruit and weigh them (for easy math, let’s assume it weighs 100 lbs). Take a 1-lb subsample of the fruit and count how many sound fruit are in a pound (assume 90 fruit/lb). Don’t count fruit that looks like it wouldn’t have stayed on the tree until harvest – these fruit are light green or otherwise look slightly “off” compared to the strong fruit. Then use those numbers to determine the total number of fruit per tree:

Total tree fruit weight x Number of prunes per lb = Total number of fruit per tree

100lbs × 90 fruit/lb = 9,000 fruit/tree (actual)

3. Decide if you need to thin. Subtract the target number of fruit (at harvest) from the number of fruit on the tree now (reference date). In this example, there is roughly 4 times the number of fruit on the tree than desired to hit the target of 55 dry count/lb. You don’t want to simply remove all those extra fruit, because you need to account for natural fruit drop and variability in fruit per tree across the orchard. Estimates of natural fruit drop range from 10% to 40%. Selecting the appropriate drop percentage should account for orchard history, as well as your own risk threshold. Based on years of experience, many growers prefer to leave approximately 50% more fruit on the tree after mechanical thinning than they want remaining on the tree at harvest [to avoid the risk of over thinning across an orchard ]:

Target number prunes per tree x 1.5 (= 50% fruit drop buffer) = Adjusted number fruit per tree

2,185 × 1.5 = 3,278 fruit/tree (adjusted target)

4. Calculate how many fruit to remove by subtracting the adjusted target number from the actual number of prunes on the tree:

Actual fruit per tree – Adjusted target fruit per tree = Number fruit to remove

9,000 fruit/tree – 3,278 fruit/tree = 5,722 fruit/tree to remove

5. Shaker thin (if needed). Use harvest machinery (shaker) to remove the approximately 5,700 excess fruit. Shake a tree for one second, and following the steps above, calculate how many fruit were removed. If needed, increase the shaking time until the desired numbers are removed. Typical shaking time is 2 to 4 seconds; avoid shaking for longer than 6 to 7 seconds to prevent unnecessary tree damage. Once you’ve calibrated your shaking time, go through and thin the block. If you are thinning for more than a week, check fruit per tree and green fruit per pound every few days to make sure that your shake time doesn’t need to be adjusted down as fruit grow.

Reminder: Check out the free prune thinning calculator on your phone, tablet or laptop.