Weather Risks: Strategies
to Mitigate the Risk of Winter Injury
Table of Contents
- What it is
- When it occurs
- Where it occurs
- What can you do
- For more information
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
- 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
For More Information
For more information:
Toll Free: 1-877-424-1300