The 2020 growing season has had a challenging start with delays in availability to seasonal foreign workers and quarantine requirements. In addition, growers may be forced to train new, inexperienced workers due to ongoing issues with accessing labour due to COVID-19. Stone fruit production is highly labour dependent, with growers relying heavily on Seasonal Agricultural Workers for pruning, thinning and harvesting.
Pruning is critical to maintaining tree health and ensuring a productive orchard. Pruning plays a critical role in:
- Developing a strong frame for supporting high yielding crops.
- Balancing vegetative growth and fruit production.
- Maintaining tree height and spread.
- Opening canopy for light and pesticides.
- Removing broken or diseased limbs.
- Maintaining fruiting wood.
- Partially adjusting the crop load before bloom.
Peaches bear on first year wood so pruning is important to ensure good sunlight penetration for the development of fruit buds for next years crop. Research has shown that the critical time for flower bud development is mid June to early-August for flower bud formation. Only 3 weeks of shade (20% full sun) during this period will reduce the flower bud density. In comparison a six week period of heavy shade (20% full sun) from mid-August to early October didn’t affect the flower density the following spring.
Sunlight also plays an important role in the colouring and ripening of fruit. Research has shown that forty-five percent full sun during the six weeks before the harvest is required for high-quality fruit. Fruit developing in the shade during the final swell will be small and soft, with poor red color and soluble solids (Marini et al., 1991).
Reduced pruning in peach orchards can result in increased vegetative growth, shading, and blind wood. Reduced pruning can have a negative impact on the health of the tree, tolerance of cold temperatures, as well as fruit bud production and yields.
Some strategies to try to assist with pruning when labour is not available or delayed include:
- Prioritizing the work-Prune your best blocks first and leave more marginal blocks until later. Prioritize blocks based on market needs and economics.
- Orchard updating-Consider removing older orchards that are less efficient and productive.
- Selective pruning -When going through the orchard make large cuts first, then follow up with fine tuning as time permits.
- Hedging-Mechanical hedgers can be a useful tool in reducing ladder work during pruning.
- Team work-Use the buddy system so that inexperienced workers can work with more experienced workers, which may help to improve their speed.
- Managing vigour-Reduce Nitrogen where necessary to minimize vigourous growth.
- Summer pruning -Can be a useful tool for increasing light penetration in orchards where minimal pruning was conducted early in the season due to labour issues. Summer pruning before late July helps increase light penetration and enhances flower bud development. Summer pruning about two weeks before harvest can increase light penetration and improve red fruit color development without adversely affecting fruit size and sugar levels (provided leaf removal near the fruit is minimized). Mowing/hedging trees during summer pruning can enhance light penetration and red color development, but also reduce leaves near the fruit decreasing fruit size and sugar levels.
Fruit thinning affects fruit size, yields and vegetative growth. If left unthinned, heavy crop loads can result in broken limbs and an abundance of poorly sized, low quality fruit (Marini and Reighard, 2008). Peaches are usually hand-thinned around 40-50 days after bloom following June drop to increase fruit size. Hand thinning is very labourious and expensive with estimates ranging between $C738/acre (OMAFRA, 2016). With reduced labour available in 2020, there are several reasons to consider supplementing hand thinning with blossom thinning.
Peaches only need 5-10% of flowers to set for a full crop. Blossom thinning helps to remove unwanted fruit early, increasing fruit size and decreasing the amount of labour needed for follow up hand thinning. Risks of blossom thinning include concerns with frost and removing too many blossoms. Researchers suggest that even if frost occurs after mechanical thinning of blossoms, there is a reduced natural drop of fruit. Many growers might understandably be cautious about using blossom thinners this years as a result of recent frosts. Concern about crop loss due to frost is valid, however there may still be some merits to using mechanical blossom thinners in some varieties. Blossom thinning in frost years, may be beneficial on varieties that produce an excessive amount of blooms and early varieties that have issues with fruit sizing. Researchers suggest that even if frost occurs after blossom thinning, there is a reduced natural drop of fruit (Schupp personal communication). There are two methods of thinning blossoms including mechanical thinning and chemical thinning.
Extensive research trials have been conducted with the Darwin mechanical blossom thinner (Figure 1.0) over the past 10 years. While mechanical thinning hasn’t been adopted in apple and pear crops due concerns with fire blight, peach and cherry growers in Pennsylvania and other areas have started to use mechanical thinning in commercial orchards.
Research conducted in Ontario (J. Cline et al., 2009) and Pennsylvania and other areas have shown that the Darwin string thinner can remove approximately 40 to 70% of the peach tree blossoms resulting in a significant reduction of labour and associated costs for hand thinning. The use of mechanical thinning can reduce hand thinning time from 12 to 51% (depending upon site, rotation speed of the strings and the number of strings used) resulting in a reduction of thinning costs of $119 per acre to $200 per acre. Mechanical thinning can increase fruit size by 5 to 18%, depending upon the variables. The Darwin thinner is a non-selective thinner, so it is important to evaluate the settings as you progress to ensure that it is working effectively. The Darwin thinner is not recommended for young trees (ie. 3 years old) that aren’t stiff enough to string thin, and the trees may be completely defoliated. Although the Darwin thinner can be used in the open vase training system, it performs better in narrow canopies (Quad V, perpendicular V) that have been pre-pruned. (Schupp, personal communication).
Figure 1. Mechanical thinning with a Darwin thinner (Picture courtesy of Matt Peters, N.M. Bartlett Inc.)
Hand blossom trimmer
Hand blossom trimmers/thinners are commercially available and are starting to be used in some orchards in the US. Hand blossom trimmer is a hand-held battery-powered rotating rod with strings (ie. Washington State University’s string thinner, the “Electro’flor” made by the French company Infaco, and the Cinch string thinner developed in Michigan by Phil Miller (Figure 2).The hand held tools may be beneficial to use as they are more maneuverable to use on larger trees. The hand held blossom thinners still require labour to use them, and take min/tree to thin as compared to the Darwin thinner that can thin a tree in seconds.
Figure 2 Cinch blossom thinner. (Picture courtesy of Bill Shane)
Chemical thinning for peaches
Chemical thinning has long been used in apples, but is more challenging to use in peaches. Chemical blossom thinning is highly variable partly due to different levels of bud viability after winter injury, severity of pruning and environmental conditions. Researchers have shown that while the chemical thinners generally increases fruit size, they are inconsistent in reducing the need for follow-up hand thinning. Research (Osborne and Robinson, 2008) suggest that chemical thinner use on peaches can be inconsistent, resulting in over thinning in some years resulting in excessive reduction in fruit set and yield.
Defruiting young peach trees
Defruiting young peach trees that are just coming into production, may be another alternative to reduce the amount of labour needed during harvest. The benefit of defruiting young peach trees is that it can reduce the need for labour at harvest, and the trees will go into the winter being more resilient to cold temperatures. The challenges with defruiting the trees is that the lack of fruit will increase tree vigour, potentially causing issues with shading (and blind wood). In addition there will be an increase in flower buds or next years crop.
Options to help minimize the vigour in defruited trees include:
- Reducing the fertilizer rate by about half
- Delaying thinning 60 Days after bloom to reduce vigour.
- Summer pruning 50 Days after bloom to increase flower bud initiation.
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