| | Many horticulture crops are perennial, with a significant amount of the total production returns achieved beyond the first season. Some crops are maintained for 2 to 3 seasons (e.g. strawberries), whereas other crops may grow for 5 to 10 years before being harvested (e.g. nursery crops). Ensuring that crops survive the harsh Alberta winters will protect against lost investments.
Generally over-wintering of all plants involves the same basic guidelines:
Use hardy plant material
Plant species and cultivars vary in their relative hardiness; their ability to withstand winter temperatures and in some parts of Alberta, winter Chinooks. Careful consideration must be made prior to selection of varieties or cultivars.
Current hardiness zone maps
Ensure excellent plant health throughout the growing season
Plants that are healthy and have minimal stress are generally less susceptibility to winter injury. Proper cultural practices includes:
- Ensuring that plants are well established if transplanted (up to 6 weeks before soil freeze up).
- Ensuring adequate moisture throughout the growing season.
- Avoiding excess applications of fertilizers, especially late in the summer.
- Ensuring plants are healthy and free from disease and pests.
Ensure that plants have acclimated prior to harsh winter conditions
Acclimation involves the gradual shutdown of plant parts and an overall increase in the ability to withstand decreasing temperatures. Winter hardy plants respond to environmental signals, such as changing day length and decreasing temperatures. The following factors can affect the ability of a plant to acclimate.
- Plants that are native or have been bred in a different climatic region, may be adapted to longer frost free periods or greater heat units and may not react to environmental signals and acclimate prior to winter.
- An excess or un-timely application of nutrients (particularly nitrogen) may result in a resurgence of growth late in the season or a failure of plants to acclimate.
- Excess water (irrigation or precipitation) late in the summer may results in late acclimation of plants.
Prevent desiccation and moisture stress
Many types of winter injury are a result of a deficiency of moisture in plant tissues. Desiccation leading to winter injury can be prevented by:
- Ensuring that there is adequate moisture in the root zone. Roots of many plants are active to soil temperatures of 5°C. A slow and heavy application of water should be applied after deciduous trees have lost their leaves, prior to covering strawberries and prior to freeze up of the soil. Roots are not active once the soil is frozen so watering is not necessary.
- Protecting plants from winds using shelterbelts. This can also increase snow accumulation, which may insulate plants.
Protect sensitive plant parts from extreme temperatures
Plant parts are not equal in their ability to withstand extreme winter temperatures. Despite the ability of many plants to acclimate, some damage can occur if additional protective measures are not taken. These can include:
- The use of snow fencing and shelterbelts to reduce winds can help build up an insulating snow cover, protecting plants.
- Using straw or bark mulches to cover whole plants or root systems.
- Protecting roots system in pots i.e. container-grown nursery crops require winter protection that insulates against the cold temperatures, such as covering with snow, straw, insulating foam etc.
Protection of sensitive plant parts from temperature fluctuations
Winter damage often occurs when temperatures fluctuate or when there are rapid and extreme changes in temperature (e.g. Chinooks in Southern Alberta). When conditions improve (i.e. increase in temperature), plants begin to lose their acclimation, which leads to a resumption in growth and the emergence of sensitive plant parts. If cold winter conditions resume after a period of warm conditions, plant injury can occur. This is particularly a problem in flowering tree or shrub species (e.g. Saskatoon berries).
Bright winter sunlight can cause localized warming of plant tissues, triggering water flow, which can later freeze and the ice crystals cause tissue damage. Desiccation of plant tissues in this circumstance is important also – damage would be hard to distinguish in either case but it mostly happens on the south and west sides of plants. Protection of susceptible plant parts can be accomplished using paints or trunk wraps, although this general practical in a commercial operation.
Mulches can protect plants by buffering temperature fluctuations. Mulches should be left in place until spring air temperatures have stabilized (i.e. frosts have passed).
The use of any type of over-wintering strategy should be adjusted based on suitability for the plants and cropping system. Some crops require special care in over-wintering.
Strawberries - Fifteen to twenty cm (6-8 inches) of clean (weed/disease free) straw is typically applied once day-neutral strawberry plants are dormant in late fall. This layer is maintained until new growth is observed in the spring. June-bearing strawberries should also be covered at the same time as above.
Container-grown nursery crops - Nursery crops (trees and shrubs) grown in containers have special winter management requirements in order to prevent winter injury. Management includes:
- Prevention of physical damage (breakage of limbs, etc) due to being blown over in high winds. This can be accomplished through the use of guy lines, consolidating containers (self-support), burying containers or laying containers on their sides under shelter or in groups.
- Protection of sensitive roots by burying &/or consolidating containers, using mulches &/or micro-foam blankets to insulate, or placing containers in a sheltered location (hoop house, barn, storage, etc.) with insulation or covering.
Prepared by Robert Spencer, Ag-Info Centre, Alberta Agriculture and Forestry |
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