Pruning is an important horticultural practice for plum trees, serving to shape the tree, maintain its health, and maximize fruit production. The timing of these cuts is not uniform, as different seasons offer distinct benefits and risks depending on the tree’s age and the grower’s specific goal. Pruning involves two main windows, each serving a different purpose for the tree’s long-term well-being and annual crop yield. The choice of when to cut is influenced by the desired outcome, such as establishing a strong framework or controlling excessive growth.
Structural Pruning During Dormancy
The primary window for significant shaping and large-scale cuts occurs in late winter or very early spring, generally between January and March. This timing is just before the buds swell and break dormancy. Pruning while the tree is dormant directs stored energy into the remaining branches, promoting strong new growth where desired once the growing season begins.
During structural pruning, the focus is on removing dead, damaged, or diseased limbs to improve the tree’s sanitation. This is also the time to select and maintain the main structural branches, often aiming for an open-center or “goblet” shape. This shape allows light penetration and ensures a strong, balanced structure capable of supporting a heavy fruit load. Removing inward-growing or crossing branches helps prevent future damage, especially benefiting younger trees establishing their framework.
Summer Pruning for Vigor Control
A secondary, lighter pruning period takes place in mid-summer, typically after the main flush of vegetative growth slows down (June or July). This timing is used less for major shaping and more for managing the tree’s size and energy. Summer pruning involves lighter cuts, mainly to shorten new growth and remove thin, unwanted shoots called water sprouts.
Pruning during the summer growing season reduces the tree’s vigor because removing active foliage limits its ability to photosynthesize and produce energy. This practice is useful for established trees that tend to grow too large or produce excessive woody growth at the expense of fruit production. Thinning the canopy in mid-summer allows more sunlight to penetrate the interior. This aids in ripening developing fruit and improves air circulation, which reduces disease pressure.
Critical Timing: Preventing Disease
The most important consideration is knowing when to avoid pruning, which is from late autumn through early winter (generally September to December). Pruning during this cool, damp period creates open wounds highly susceptible to infection from airborne fungal spores. Low temperatures and moisture slow the tree’s natural healing process, leaving the tissue exposed for an extended time.
This timing carries a substantial risk of infection by Silver Leaf disease, caused by the fungus Chondrostereum purpureum. This pathogen gains entry through fresh pruning wounds and is particularly active when the weather is wet and cool. Once inside, the fungus spreads throughout the wood, releasing a mycotoxin that causes the leaves to develop a silvery sheen. This eventually leads to branch dieback and often the death of the entire tree. Pruning established plum trees in mid-summer or late spring ensures wounds heal quickly during warmer, drier weather, significantly reducing the opportunity for fungal spores to infect the wood.