Weather Risks: Strategies to Mitigate the Risk of Winter Injury
Table of Contents
This in the 3rd in a series to help apple and tender fruit growers in Ontario assess the weather risks that can damage their trees and crops. It is important to recognize the weather risks at each location, and develop strategies to reduce or eliminate the impact on your business operation.
What it is
Winter injury is freezing damage to wood and bud tissues, caused when cold temperatures reach a critical level. Fruit trees have complex mechanisms within their cells that cause them to "harden off" or acclimatize for winter. Exposure to short days and colder temperature through the fall and winter encourages trees to export water from their cells into the spaces between cells. This protects the cell structure, because the crystals that form on freezing can damage tissue structures. It is better to have the crystals form between cells where they do not damage the cells. More and continual exposure to cold temperatures moves trees into deeper dormancy and more tolerance to extremely cold temperatures.
When it occurs
Winter injury occurs when temperatures drop below the critical level that each species can tolerate. Usually wood is more cold tolerant than flower buds. Tree trunks (especially at ground level) and branch crotches are the slowest to harden off, and the most vulnerable to cold temperatures. If cold temperatures occur in late fall or early winter, the injury is often seen as trunk splitting or damage in crotches. If temperatures warm us, or swing erratically in short periods of time, winter injury is more likely. Unhealthy or stressed trees are more vulnerable to winter injury. Winter injury can reduce yields, kill the tree immediately, or cause a shorter tree life expectancy by making the tree more susceptible to pests (ie. cankers and borers)
Where it occurs
Winter injury is a common weather risk for tree fruit growers, especially where trees are grown on the northern limits of their adaptation. Some species like peaches and nectarines are vulnerable to damage most winters, while for more cold tolerant species like apples, winter injury is more common at colder locations, or away from the influence of the Great Lakes. Some cultivars are more winter tender eg. Loring peach, Mutsu/Crispin apple.
What can you do
Some of these potential mitigation strategies may help reduce or eliminate damage due to winter injury:
- Production insurance (PI): Production insurance is purchased before winter, and can give you peace of mind that at least some of your input costs will be covered if winter cold kills your fruit buds. Apple trees are also covered if winter kills them, although tree death must reach 7% of the trees before a claim is triggered. Your premiums will depend on the coverage you choose, your claim history, and the yield potentials of your orchards. Over time, your premiums can be reduced if you are lucky enough not to have claims. However, some growers struggle with the premium costs (especially in their start-up years or if they have claims) and the fact that PI is not intended to fully cover your loss, either in yield or price. There is also the problem of reduced coverage levels in the years after your crop is reduced, due to the effect of the loss on your long-term average yields. Also, spot loss insurance is not available, so growers with multiple orchard sites may be penalized when good yields occur on the non-damaged sites.
- Selecting sites less susceptible to cold: Avoiding low-lying areas, analyzing the effect of buildings and windbreaks, and seeking sites with good air drainage or located near large bodies of water can help avoid winter injury. These are not available to all growers, but should be considered when choosing your orchard site. Remember that other factors like suitable soil may be even more important when choosing a site.
- Thinning hedgerows or clearing forested areas: This may reduce the area of a frost pocket, or promote better air drainage. However, the benefit from wind protection through the season may be lost, making spraying more challenging and allowing soil movement and sand blasting. Generally this needs to be done in advance as well. Remember that bin piles may affect air movement.
- Selecting hardier cultivars: Some cultivars can tolerate more cold, although this will change throughout the winter depending on when the cold occurs. McIntosh, Honeycrisp, and Northern Spy apples tolerate more cold than Golden Delicious, Ambrosia and Empire. This is not always feasible due to your climatic location, market demand, or length of growing season, but needs to be considered.
- Restrict soil-applied nitrogen to early season: This encourages early growth in trees, while allowing strong pollination and fruit set, so the hardening off process can start in late summer. Summer foliar nitrogen or post-harvest urea for bud strength or scab sanitation in apples should be matched to precisely what the tree needs, so excess N is not available to promote late growth. Be sure to apply the proper rate of N in the spring to avoid deficiencies that are hard to correct.
- Establish sod alleys or grow cover crop between rows after July: The sod or cover crop absorbs excess N from the soil to encourage trees to start to mature and harden off. These row covers also promote better fruit colour, and provide a strong surface for equipment, especially in rainy seasons. There is a cost of seed and mowing to establish and maintain this cover, and if not mowed, it may impede harvest and attract rodents.
- Avoid pruning in the fall, or early winter: Fall pruned trees are more vulnerable to winter injury, especially in the tissue around the pruning wound. Even in the winter, damage may occur if severe temperature drops occur after pruning. Boring insects and canker disease often take advantage of damaged tissue around pruning wounds to enter the tree. Delaying pruning to late winter or early spring may require more labour to complete the job, and there may be less time to manage the brush before spraying starts, but it's better to protect your valuable trees.
- Paint tree trunks with white latex paint: The white colour reflects sunshine and its warmth, avoiding trunk warming than might encourage sap flow in mid-winter. This prevents trunk freezing and splitting known as Southwest injury. Good quality latex paint should be used for good coverage and retention. Painting trees is labour-intensive and hard to mechanize for good coverage, and re-applications are needed every couple of years.
- Use of wind machines: Research from other crops (grapes) has shown that wind machines can be used to reduce winter injury when temperatures drop if they are not below critical killing temperatures. These machines may have potential for reducing winter injury in tree fruit crops as well.
Avoiding or reducing winter injury will keep trees healthier and ensure healthy blossoms to set a full crop of fruit. Erratic temperature patterns in the late fall, early winter and through to bud break may make trees and buds more vulnerable. As we push our trees for higher production, we need to be aware of affecting their hardiness and ability to withstand drops in temperature. Site selection, choice of cultivars and management for reasonable growth with help reduce winter injury.
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