The Fruit Growers News

Peach trees are pruned annually to maintain tree shape and size, to improve light distribution throughout the canopy, and to partially adjust crop load. Even when properly pruned, peach trees usually set more fruit than they can adequately size. Peach fruit size is linearly and negatively related to the number of fruit per tree.

In the Eastern states, peach growers remove excess fruit within 50 days after bloom to produce large fruit that bring premium prices. Although post-bloom hand-thinning is one of the most expensive practices in peach production, it is cost effective. Maximum fruit size is obtained when thinning is accomplished before or during bloom. Therefore, many peach growers remove about half the blossoms during bloom and complete the thinning operation about 50 days after bloom. Because fruit thinning is expensive, and labor requirements are high, new methods that are less expensive and labor intensive for adjusting crop load are needed. Below is a discussion of our work during the last eight years where we evaluated different methods of pruning to adjust crop load on peaches.

Thinning improves winter hardiness

Not only will proper fruit thinning improve flower bud formation, but the flower buds are also more tolerant of low winter temperatures. Ross Byers and I conducted some experiments where crop load was adjusted in various ways and flower buds were counted the following spring. In one experiment trees were thinned by hand at 38 or 68 days after full bloom (DAFB), or they were bloom thinned by applying ammonium thiosulfate (ATS) during bloom (Table 1). There are several chemicals available for bloom thinning peaches, but they perform so inconsistently that they are not widely used. ATS was used in this experiment to provide an early thinning treatment.

An ideal crop density would be about eight fruit/cm2, bloom thinning over thinned the trees and trees that were hand thinned at 68 DAFB were severely over-cropped. The number of buds per meter of shoot length increased as crop density was reduced. Not only did early thinning and heavy thinning increase the number of flower buds, but the percentage of flower buds surviving the winter was also increased.

In another experiment, a range of crop densities was established by hand thinning at about 45 DAFB. For Redhaven and Cresthaven the number of flower buds per meter of shoot length and the number of live flower buds were negatively related to crop density the previous year. Results from these two experiments indicate that adjusting the crop load improves not only flower bud formation, bud the winter survival of those buds. Thinning is most effective when it is performed early, so we try do have our hand thinning completed by pit hardening which occurs about 50 DAFB.

Retain high-quality shoots

While pruning it is important to remember that long, thick, non-branched fruiting shoots produce the largest fruit. This is at least partly because such shoots produce a lot of leaf surface to support fruit growth in the immediate vicinity of the fruit. Therefore, all one-year-old shoots less than 12” long should be removed while dormant pruning.

In 1992 we compared two pruning treatments on Redhaven trees. Some trees were pruned to retain only short shoots less than 12” long. Other trees were pruned to retain only shoots longer than 12”. At harvest all fruit were harvested, weighed, counted and separated into four sizes with a chain sizer, and crop value was calculated using prices received for Appalachian peaches in 1992.

Normally fruit weight is linearly and negatively related to number of fruit per tree. In this experiment, trees with long shoots had the most fruit and the highest yield; they unexpectedly also had the largest fruit. The combination of more fruit and larger fruit resulted in a greater crop value for trees with long shoots. These results verified our earlier observations where we found that fruit size was positively related to shoot length on trees with a mixture of shoot lengths.

Heading shoots to reduce thinning costs

In 1995 an experiment was performed with Cresthaven trees to investigate the possibility of reducing the number of blossoms per tree by heading fruiting shoots at varying severities. Trees were first pruned to retain only shoots that were longer than 12”. All the shoots on a tree were then headed by 0, 50, 75, 87.5, or 93.75%. Results for this experiment are presented in Table 2.

Heading shoots by 50% had little effect on fruit set, fruit thinned per tree, or fruit weight. However, heading by 75% or more greatly reduced fruit set and fruit thinned per tree, but also reduced yield and fruit weight. The experiment was repeated in 1996 with Cresthaven to compare trees with non-headed shoots to trees with all fruiting shoots headed by 50%. Heading shoots reduced fruit set by 24%, the number of fruit thinned per tree by 42%, and thinning time by about 14 minutes per tree (47%). Heading also reduced yield by 10 pounds per tree (6%), but fruit weight and crop value was similar for the two treatments.

Summary

Based on these experiments plus two others, it seems that heading shoots by 50% can reduce thinning costs by 10% to 50%, without reducing yield or fruit size. Shoot heading may save growers about $50 to $200 per acre in thinning costs.

Shoot Removal

Another way to reduce the number of flower buds per tree is to remove more shoots per tree while pruning. In 1997 Norman’ trees were pruned to retain 73, 110, 146, or 220 shoots per tree. We had planned to thin the trees 45 days after bloom to leave 6, 4, 3,or 2 fruit per shoot, respectively. Therefore, all trees would have 440 fruit, but on differing numbers of shoots. Unfortunately, there was a frost during bloom, so the crop was reduced to below the target level. Data are presented in Table 3.

Fruit set, fruit harvested, and yield were positively related to number of shoots per tree, whereas fruit thinned per tree and fruit weight were negatively related to number of shoots per tree. Severe pruning reduced thinning time by about six minutes per tree, but crop value was reduced $2.64 per tree. Therefore, in a year when the crop is reduced by frost, a grower may lose about $2 per tree ($300/acre) by severe pruning. The experiment was repeated in 1998 and the crop was not reduced by frost.

Fruit set per tree and fruit thinned per tree increased as the number of shoots per tree increased. Although we attempted to thin all trees to a crop of 440 fruit per tree, the trees with the fewest shoots had the most fruit and the highest yields. Despite the fact that trees with fewest shoots had the most fruit, they also had the largest fruit and the highest crop value. Compared to leaving 146 or more shoots per tree, pruning to retain only 73 shoots per tree resulted in an $11 per tree increase in crop value. When reduced costs for thinning are added to the increased crop value, severe pruning increased net profits by about $2000 per acre.

To verify these results, the experiment was repeated in1999 with a slightly broader range of numbers of shoots per tree, and the crop was thinned to 500 fruit per tree. The results are presented in Table 4.

Fruit set and number of fruit thinned per tree increased with increasing number of shoots per tree. Fruit harvested per tree was similar for all treatments, but yield and fruit weight declined with increasing number of shoots per tree. The two treatments with the most shoots had an average crop value of $49.30 per tree, whereas the two treatments with the fewest shoots had an average crop value of $67.70. Thus, when thinning costs and crop value are considered together, severe pruning increased profit by about $20 per tree or about $2,700 per acre.

Adjusting the Crop

Consider adjusting the crop to a specific number of fruit per acre. Growers often thin trees to a certain distance between fruit. Generally, small-fruited varieties are thinned to eight inches between fruit, medium-size varieties are thinned to six inches, and large-fruited varieties are thinned to four to five inches between fruit. Results from our experiments and experiments performed in California indicate that the distance between fruit is irrelevant, because fruit size is related to the number of fruit per tree or, more accurately, the number of fruit per acre.

Large trees can support more fruit than small trees and tree size increases as the number of trees per acre decreases. Additionally, trees that are severely pruned require less severe thinning than trees that are lightly pruned. Therefore, peach trees should be thinned to a certain number of fruit per acre. The number of fruit per tree must be adjusted for the natural size of the variety and the availability of irrigation. An ideal crop load may be about 50,000 fruit per acre for small varieties, 57,000 fruit per acre for medium-size varieties, and 62,000 fruit per acre for large varieties. If irrigation is not available, and it appears that trees may be drought stressed, these numbers should probably be adjusted down by about 20%. These numbers should be considered “conditional” until they can be verified with grower observations or research data.