Richard Rosecrance, Professor of Plant Science, CSU Chico
Luke Milliron, UCCE Farm Advisor, Butte, Glenn, and Tehama Counties
Franz Niederholzer, UCCE Farm Advisor, Colusa and Sutter/Yuba Counties

The California prune industry relies on hand pruning to thin fruitwood, improve fruit size, reduce alternate bearing, and control tree size and shape. A number of prune growers, however, mechanically prune their orchards instead of the standard hand pruning of mature, bearing trees. Mechanical pruning typically entails hedging all 4 sides of the tree and topping every year, resulting in a box-shaped tree. Other growers have experimented with less severe mechanical pruning, hedging one or both sides of the tree. Even for growers who have gone mechanical, some hand cuts are still needed to remove suckers, as well as diseased and dead wood.  Hand pruning involves selective annual removal of branches using loppers and ladders as needed.  In this article we will provide the latest information on the “why” of mechanical pruning, yields of hand vs. mechanically pruned trees, as well as the when, and how to mechanically prune.

Why Mechanical prune?

The two main incentives for mechanically pruning your orchard are:

1. Cost of labor
2. Availability of labor

Hand pruning is one of the most expensive field operations, accounting for over 25% of the total cultural costs to produce prunes (UC Cost Studies, 2018). The UC Cost Study estimates about $3 per tree to prune, however, growers have mentioned that this figure can be $5/tree or more. Hand pruning costs and mechanical pruning costs are shown in Table 1. Savings ranged from $330 to $400 per acre using mechanical hedging vs. hand pruning. Mechanical hedging can be an important tool in decreasing pruning costs, reducing alternate bearing, and helping to maintain trees in their allotted space. A range of practices including mechanical topping and/or side hedging (one or two directions) are beginning to be practiced by prune growers. Growers are currently experimenting with mechanical pruning to limit tree size and encourage renewal growth at the canopy perimeter.

Mechanical pruning is a labor-saving technology, important because farm labor is in short supply in California. Farmers are finding it difficult to obtain employees, despite raising wages and benefits. A recent survey conducted by the California Farm Bureau Federation in collaboration with the University of California, Davis found that 56 percent of farmers had experienced employee shortages. The survey indicates that farm labor shortages are getting worse thereby forcing some growers to adopt labor-saving technologies such a mechanical pruning.

Yields of hand vs mechanically pruned trees

Current findings comparing treatment yields in hand, boxed, and hedged both sides of the tree, indicated no significant prune yield differences (Figure 1). Preliminary finding in 2019 and 2020 also indicated no differences in fruit size between mechanical vs. hand pruned trees. This trial, however, will continue for another two years, so stay tuned for more results.

Pruning weights and canopy volume

Hand pruned trees had larger canopy volumes than mechanically hedged trees (Figure 2).  Hand pruning typically consists of thinning cuts which removed large branches to allow light penetration into the center of the canopy, but had minimal effects on total canopy volume.  In contrast, hedging makes heading cuts where many small branches, leaves, and fruit were removed, significantly reducing the size of the canopy.  Hedging can produce a fruiting wall where most of the fruit are located on the tree periphery.

When to mechanically prune?

Mechanical pruning operations should be timed to minimize strong vegetative regrowth. Strong regrowth following hedging can decrease fruit size in the current year and return bloom during the following year. Late spring following the growth flush and early fall following harvest have been identified as good times to mechanically prune. Previous studies have shown that return shoot growth is minimized during these time periods. Results to date do not favor one timing over the other. However, if trees require fruit thinning, the spring treatment may be favored because substantial amounts of fruit are removed during spring mechanical hedging.

Concerns with mechanical pruning

Disease risk: Hedging results in thousands of indiscriminate cuts that are potential entry points for rain-splashed fungal spore infection from diseases such as Cytospora and Botryosphaeria (figure 3). Spraying with a fungicide protectant like Topsin-M® soon after any pruning can reduce the risk of infection. Learn more here.

Figure 3. A prune orchard mechanically ‘boxed’ for several years showing severe branch dieback in September 2019. Botryosphaeria canker was identified by the lab of Dr. Florent Trouillas (UC Davis Pathology Specialist at Kearney Ag Center). The orchard block was subsequently removed because of the severe dieback. Topsin-M® had not been regularly applied after mechanical pruning.

Reduced fruitwood thinning: Mechanical hedging doesn’t selectively thin fruitwood to improve fruit size. Instead, shaker thinning, as needed, is relied on to achieve good fruit size. Reduced fruitwood thinning may also result in fruit on positions that are more difficult to shake, such as long “hanger” spurs. If a grower is going to stop detail pruning in an orchard, estimating fruit per tree and shaker thinning – when needed – will be essential to avoid growing a big crop of small, low value fruit.

Unknown long-term results: Although previous studies and the first two years of the ongoing study point to mechanical pruning saving costs without compromising productivity, long-term results remain unknown. The rise in labor costs and reduced availability come at a time when Cytospora and Botryosphaeria cankers have dramatically increased. With adoption of best practices like not pruning when rain is forecast and using a protectant like Topsin-M® these cankers might be minimized.  Long term results of mechanical pruning on yield, dieback and overall orchard longevity are unknown.