Pruning is an essential task utilized annually in tree fruit production, affecting various aspects within the orchard such as: crop load, fruit size, light penetration, water absorption, fruit quality, fruit bud initiation, along with nutrient uptake and mineral content. Pruning is also used to reduce the spread of infection throughout an orchard and is usually performed in late winter or early spring. Although there are many benefits to growers by pruning their trees, negative effects and an increase in cost can occur if pruning is performed too early or too late in the season. Pruning is also about balance, as excessive pruning can result in excess vigour, and lack of pruning can result in shading leading to less fruit bud formation.
Best Time to Trim
There is not one correct answer when it comes to the right time to prune your orchard. Many factors influence the best time to prune but the most significant factor is weather. Weather initiates the acclimation and de-acclimation of dormancy in apples, which is pertinent in determining the ideal time to prune. Apples should be pruned in the late winter or early spring – after chances of severe temperature changes have passed. Overall, when determining the best time to prune, weather, dormancy, cold hardiness and winter injury need to be considered before clipping.
Depending on the part of the tree, different levels of cold hardiness are observed and the physiology changes on how plant tissues acclimate to cold. Hardening off of trees begins at the branches and works inwards. The slowest parts of trees to harden off are tree trunks and graft unions, especially at ground level, along with tree crotches. This means that these parts of the tree are more susceptible to cold temperatures and could result in trunk splitting or damage in crotches if cold temperatures occur in late fall or early winter.
Deep supercooling occurs in the xylem and pith tissues in the wood and are able to retain cellular water in liquid form at low, subfreezing temperatures due to cryoprotectants. Apple buds and bark become dehydrated as water is drawn between the cells to decrease the possibility of damage and to retain cell structure when ice forms. This occurs through the creation of a water potential gradient preventing freezing inside the cell or penetration of ice crystals through the cell wall. Apple buds have a high tolerance to dehydration. Dehydration is crucial in the beginning and end of dormancy for apples to start acclimation and de-acclimation based on the season.
Dormancy of trees is separated into three different phases:
- Paradormancy (correlation inhibition or dormancy establishment)
- Begins when external physiological factors restrict growth of the bud
- Takes place in the late summer and early fall
- Endo-dormancy (true dormancy)
- Growth stops due to internal factors, even in favourable conditions
- Prevents buds from bursting in mild conditions
- Maximum hardiness is usually assumed to occur mid to late January
- Growth is prevented by unsuitable conditions
- The tree is freezing tolerant at this stage, but not deeply dormant
Eco-dormancy is the most important when it comes to late winter and early spring tree pruning as this phase occurs during this time, yet no visible external changes are seen on the tree. The tree is not fully dormant at this stage and temperature fluctuations can impact trees phasing out of dormancy or de-acclimation. The paradormancy phase is also important in the late summer and early fall in regard to pruning as the trees may not be fully dormant and new growth may even be initiated.
Late Fall/Early Winter Pruning
During the late fall and early winter apple trees are not fully acclimatized to tolerate cold temperatures, therefore pruning is not recommended. During this time, pruning trees would increase sensitivity to cold temperature injury. Another potential risk when fall pruning is the introduction of boring insects and canker disease that are still active at that time into pruning wounds.
Late Winter/Early Spring Pruning
Although this is the ideal time to begin pruning, severe changes in weather can be detrimental to the bottom line of your production. It can take up to two weeks for trees to regain their cold hardiness after pruning has occurred, as pruning stimulates meristematic cellular activity of latent buds near cuts. If pruning is performed within an orchard and a cold snap follows, severe damage, such as winter injury, and/or potentially tree death could occur. This is especially important in the late winter season, as trees could be in the eco-dormancy phase and with mild temperatures trees may de-acclimate prematurely due to warm spells. Another factor to consider when pruning in the late winter/early spring is that apples could reach their chilling requirement, meaning that the trees lose the ability to reacclimate to their previous cold hardiness levels attained earlier in the winter.
Apple trees are cold hardy, withstanding temperatures between -25 to -30°C without a killing frost, although cold hardiness is not guaranteed as variation depends on multiple factors. Variety, rootstock, location, production systems, tree stress (e.g. water, fertility, pests), weather occurrences and pruning practices all impact cold hardiness within the orchard.
Cold hardiness of apple trees can decrease when:
- Pruning occurs too early in the winter
- Too much pruning occurs the previous fall
- Infected buds or wood is present
- Crop load isn’t managed
- Fall applied nitrogen
- Deficient in nutrients (Nitrogen, Potassium, Zinc, Magnesium and Boron)
Compromising the cold hardiness of apple trees potentially impacts the growth and production of fruit and increases the susceptibility of winter injury to trees.
Winter injury occurs when freezing damage to leaves, bud tissues or wood is found, caused by cold temperatures. Winter injury can cause so much damage that trees are no longer recoverable or could lead to the death of the tree.
Winter injury on the trunk tends to have a dark external bruise on the bark that can be soft to the touch. To confirm observations, peeling back the bark would reveal a dark brown inner stem tissue. This can be further clarified during the growing season if patchy, irregular and poor growth are observed.
Winter injury to bud tissues is more difficult to determine. The best way to confirm winter injury of buds is through observation. First, collecting buds throughout your orchard and then cutting the buds horizontally and vertically to analyze the ovules and styles. If brown or black tissue is revealed, winter injury or frost damage has occurred (Figure 1). To better understand when winter injury damage could occur on apple buds Table 1 demonstrates the critical temperatures during the spring in relation to growth stage.
Table 1. Critical Spring Temperatures in°C for Apples
1/2 Inch Green
Winter injury can be found in many different forms on various parts of the tree, including:
- Blackheart: Occurs when the pith is killed, and the heartwood is darkened. Gumming also occurs with cell death.
- Cambium injury: Gumming could be a symptom. Cambium is one of the last parts to harden off, where cambium injury results in a weakened tree.
- Crotch injury: Could result in injury up and down limbs. Crotches are one of the last parts of the tree to harden off, where narrow crotch angles are more at risk.
- Crown injury: Results in the killing of the bark close to the ground. Certain varieties could be more susceptible.
- Southwest Injury: Occurs when the tree trunk is heated during a sunny day followed by a dramatic drop in temperature. Use of white latex paint is recommended.
- Trunk splitting: Commonly occurs most often in the late fall or early winter due to a rapid temperature drop. Cracking and splitting can occur all the way to the pith and may not heal over or close.
- Shoot death and dieback: Normally occurs with very cold weather when trees are not fully acclimated. Prevalence could be increased with nitrogen fertilization and late pruning.
- Leaf and flower bud injury: Temperatures required to damage buds varies between 5˚ and 6˚C due to acclimation and de-acclimation of trees. Below -35˚C could result in tree damage.
- Root death: This is exacerbated by bare ground and can begin at -4˚ to -12˚C.
Winter injury not only impacts the growth of the apple trees but increases the susceptibility to other pests and diseases. This leads to more expense through mitigation strategies and loss of income through death of trees.
The possibility of winter injury increases if pruning is performed too early in the winter, followed by severely cold weather. Fall pruning also enhances vulnerability to winter injury, specifically around the pruning wound.
Mitigation of Winter Injury
Improving the cold hardiness of trees can increase the resilience against winter injury.
Cold hardiness of apples trees can increase when:
- Timely pruning occurs
- Trees are healthy
- Crop load is managed
- Growth Regulators are used
- Late season Nitrogen application are avoided
To reduce or eliminate damage from winter injury utilize the following mitigation strategies:
- Avoid pruning in the fall or early winter
- Prune cold hardy varieties/species first
- Prune older trees first (late winter) younger trees (early spring)
- Use soil-applied nitrogen in the early season
- Select hardier cultivars
- Select sites with minimal low spots or location with low air movement
- Use weather risk reduction technologies
- Paint tree trunks with white latex paint
- Choose cold hardy rootstocks
- B.9 (Budgovsky 9) has excellent winter hardiness
- O.3 (Ottawa 3) has good winter hardiness
- V.2 (Vineland 2) has moderate to good winter hardiness
- M.9 (Malling 9) has moderate to good winter hardiness
- M.27 (Malling 27) has moderate to good winter hardiness
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