Thinning Peach Trees

By Lydia Balogh (OMAFRA Summer Student) and Kathryn Carter (OMAFRA)

As most growers know, thinning is an essential orchard practice for peaches. Well thinned peach trees have many benefits, including larger fruit size/value, hastening ripening and increasing antioxidants in fruit. Properly thinned peaches have improved finish and colouring that will be more uniform throughout the crop, improving marketability.  In addition, thinning fruit helps to reduce limb breakage and maintain the tree structure. Thinning results in increased fruit size, which can help make orchards more profitable. Although thinning is labour-intensive and can be expensive, the benefit is that fruit that can sell at premium prices and improve orchard profits.

Thinning can be done before, during or after bloom. However, pre-bloom or blossom thinning has the greatest benefits to fruit size since less energy is spent on unharvested fruit. Delaying thinning until post-bloom thinning provides an opportunity to evaluate fruit set and avoid over thinning.

Thinning by Pruning

Pruning peaches is one of the first techniques in helping to reduce crop load while removing excess or poor-quality wood. Removing limbs in the low center area of the tree will increase light levels and improve fruit size and quality. Prune to remove all fruiting shoots shorter than 20 cm (8 inches) in length as these limbs tend to produce smaller fruit. Crop load can also be managed by making heading cuts to shorten fruiting wood (limbs that grew last year). Be careful not to cut limbs shorter than 31 cm (12 inches). Pruning usually starts just before bloom but may take longer to complete than usual at some farms this year, due to issues with accessing labour. For orchards that are struggling to get pruning complete due to issues with labour, consider doing a quick prune, then following up with thinning, and summer pruning to fine-tune the canopy as required.

Evaluating Crop Loads Prior to Fruit Post-Bloom Thinning

Evaluating fruit set shortly after bloom provides the opportunity to estimate how much fruit thinning will need to be done. This evaluation is especially important in orchards where frost damage is an issue.  

Look at 10 to 20 typical limbs in a peach block, to get a rough estimate of the potential crop load. There is still a large variation in fruit size on some trees and it is likely there will be more fruit drop, so keep this in mind when estimating crop load. Keep in mind that these estimates will not take into consideration the impacts of future weather conditions (i.e. drought/rainfall) that will impact fruit sizing. Mild conditions following bloom increase fruit cell division resulting in increased fruit size.

Determining How Much to Thin

When thinning, consider fruit load as well as fruit spacing. Optimum fruit load depends on cultivar, tree vigour, tree age and health, and orchard management practices such as tree spacing, irrigation and pruning. Peach growers tend to space the fruit 15-20 cm apart. Using a larger spacing between fruit for early season or hard to size varieties. However, the distance between fruit is less critical, provided clusters are broken up, and fruit is separated with enough limb space to support their growth. Always keep the largest fruit on a limb, even if they are clustered. Tree crop load (kg per tree or number of fruit per tree) is really the most important consideration. As an example, a yield of 18-27 tonnes/ha (8-12 tons/acre) in an orchard with a tree density of 490 trees per ha (200 trees per acre) requires about 200-300 fruit per tree, if you assume an average fruit size of 4-7 fruit per kg (2-3 fruit per lb). With current emphasis on increased fruit size for better marketability and trends towards higher tree densities, fruit loads of 175-200 fruit per tree may be more suitable, at least for cultivars that normally produce large fruit.

If working with a crew, thin several trees in advance and mark these with coloured ribbons to serve as examples (Shane, 2006).

Post Bloom Thinning

Hand thinning can be started post-bloom about two to three weeks after bloom by removing fruitlets. At that early stage, running a hand along the bottom of a fruiting limb can quickly remove half or more of the fruit. As the fruit grow, it takes more time and effort to remove the fruit. At this time, rubber-tipped poles, padded bats and plastic bats can be used to strike limbs to remove excess peaches more quickly.

The earlier the thinning is completed, the greater the benefit to fruit size. Avoid thinning during pit hardening as it can cause split pits. Always thin early ripening cultivars first to ensure good fruit sizing.  Some cultivars (early ripening cultivars or those with a tendency of split pits) may need to be thinned twice. Thinning more gradually helps to reduce excessive thinning that can result in rapid fruit swell and contributes to split pits.

With early-season cultivars, thinning prior to pit hardening provides more benefits to fruit size and quality, especially in a dry growing season.  For mid- to late-season cultivars, thinning is often initiated after June drop (mid-June) when it can be determined which fruits will abort naturally.  Varieties that are hard to size, such as Redhaven, should be more severely thinned. Easier to size varieties such as Vivid can carry a heavier crop load. Thin small pit varieties that are less prone to pit splitting first (Glowingstar and Sun fire), and delay thinning large pit varieties that are more prone to split pits later (Redhaven) (Shane, 2018).

Avoid excessively thinning as it may help to contribute to split pits.Evaluate the crop level a few days after thinning to make sure that the fruit numbers per tree are optimal to produce the best quality fruit possible. Follow-up hand thinning might be needed to maximize the benefits of thinning.Documenting strength of bloom, weather conditions both during bloom and following fruit set can provide useful information on the effectiveness of thinning.  

Conclusion

Overall, peach thinning is an essential practice for growers to produce the best quality peaches possible. Thinning increases fruit size and quality while reducing limb breakage and extending the life of the orchard. Although hand-thinning may be labour intensive, it is necessary for premium, high quality and large sized peaches and gives growers the best return on their crop.

References

Environmental and orchard bases of peach fruit quality. Scientia Horticulturae. Volume 235, 17 May 2018, Pages 307-322. Ioannis S. Minas, Georgia Tanou, Athanassios Molassioti. https://doi.org/10.1016/j.scienta.2018.01.028

Effects of crop load and time of thinning on the incidence of split pits, fruit yield, fruit quality, and leaf mineral contents in ‘Andross’ peach. The Journal of Horticultural Science and Biotechnology. Volume 84, 4 April 2009, Pages 505-509. P. D. Drogoudi, C. G. Tsipouridis, G. Pantelidis.https://doi-org.proxy1.lib.uwo.ca/10.1080/14620316.2009.11512556

Peach Fruit Weight, Yield, and Crop Value Are Affected by Number of Fruiting Shoots per Tree. HortScience. Volume 38 Issue 4, July 2003, Pages 512-514. Richard P. Marini.https://doi.org/10.21273/HORTSCI.38.4.512

Fruit thinning of peach trees. Plant Growth Regulation. Volume 31, May 2000, pages 113-119. Guglielmo Costa, Giannina Vizzotto.https://doi-org.proxy1.lib.uwo.ca/10.1023/A:1006387605042http://www.omafra.gov.on.ca/english/crops/hort/thinning.htm#peaches

Thinking through strategies for peach crop thinning. June 13, 2006. Bill Shane. Michigan State University Extension. https://www.canr.msu.edu/news/thinking_through_strategies_for_peach_crop_thinning

Peach and nectarine varieties and observations on their susceptibility to cold injury and bacterial spot. 2018. Bill Shane. Michigan State University Extension. Peach and Nectarine Varieties and Observations on their Susceptibilityto Cold Injury and Bacterial Spot