Crop Load Adjustment (Thinning Prunes)

The crop load (number of fruit per tree) has a large influence on the fruit size, sugar accumulation, drying ratio, and prices received. The goal is to set a good crop this year, without producing too many small fruit or depleting the tree of so many resources that it cannot also set a good crop next year. Other reasons you may consider adjusting the crop load is to reduce limb breakage, mitigate alternate bearing tendencies, and reduce damage associated with potassium deficiency (defoliation, bark sunburn and Cytospora infection).

The earlier the thinning is done, the greater effect it will have on final fruit size at harvest. However, if you try to shake too early, you may damage the trees without getting the desired number of fruit removed. Thinning should occur roughly around the same time as ‘reference date’. Prune reference date is the point at which 80-90% of the fruit have a visible endosperm. The endosperm, a clear gel-like glob, will be found in the seed on the blossom end of the prune (Figure 1) and is solid enough to be removed with a knife point. Typically, the reference date occurs in late April or early May, approximately one week after the pit tip begins to harden.

Figure 1. Extraction of the endosperm on a developing prune

Figure 1. Extraction of the endosperm on a developing prune

You should estimate the number of fruit per tree needed to produce your desired crop, and compare that to your actual numbers of fruit to ensure the trees aren’t carrying too much crop. These numbers will then be used to help determine how much fruit to remove from the tree if thinning is needed. Below I walk through the math, step by step.

First, calculate a targeted tonnage from a given block by considering orchard history, age, etc. Let’s assume a target of 4 tons/ac, and shoot for 60 dry count/lb. From there, we calculate a targeted number of fruit per tree:

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

Next, you need to estimate the actual number of fruit per tree and compare that number to the target of 3,200 fruit. Ideally, you would repeat this procedure on 3 trees to ensure accuracy. Place a tarp under a tree and mechanically shake off as much fruit as possible, then hand strip any remaining fruit. Collect all the sound fruit from the tarp and weigh them (we’ll assume for this example math it weighs 100 lbs). Take a 1-lb subsample of the fruit and count how many sound fruit are in a pound (we’ll assume 90 fruit/lb). Don’t count fruit that look like it will not stay on the tree; these fruit are light green or otherwise look slightly “off” compared to the strong fruit that will “make” a fruit. Then use those numbers to estimate the total number of fruit per tree:

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

In this case, you have approximately 2.8 times the number of fruit on the tree as desired to hit the target of 60 dry count/lb, and you may consider thinning the orchard. You don’t want to simply remove all those fruit though, because you need to account for natural fruit drop. Estimates of natural fruit drop range from 10%-40% – again, this is an area where you need to account for orchard history, as well as your own risk threshold.

It is much safer to under thin than to over thin! Because of that risk, many growers prefer to leave approximately 50% more fruit on the tree than the target amount. This means that we need 50% more fruit on the tree after mechanical thinning than we want remaining on the tree at harvest:

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

And finally, you can 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

From here, you can use harvest machinery to remove the approximately 2,600 fruit desired. Shake a tree for one second, and following the steps above, calculate how many fruit were removed. Adjust the shaking time until the desired numbers are removed. Typical shaking time is 2-4 seconds at full throttle; avoid shaking for longer than 6-7 seconds to prevent unnecessary damage. Once you’ve calibrated your shaking time, go through and thin the block.

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