Some plants, like cherries and blackcurrants, need
plenty of chilling over winter for good growth in spring...
but frost and frozen ground is not the gardener's friend
First, the good...
The dormant state of plants in winter is vitally important for growth and the production of fruit in the spring.
A certain amount of exposure to chilling temperatures is required before certain plant can start its active growth in the spring.
From a technical viewpoint, the chilling required for a given plant is often measured in the accumulation of ‘chill units’ below a certain threshold (which is often very low, but rarely freezing).
Sweet cherries, for instance, require the accumulation of 1,000 chill units, at around 3.5°C to complete (or ‘break’) its winter dormancy. If these figures are not achieved by the spring, the plant’s flowers and growth buds will be somewhat impaired.
Blackcurrants also require chilling, to ensure uniform growth and ripening of wood, and as a result fruit yields will be increased. Most blackcurrants prefer temperatures of below 7°C, however each variety will require a different number chilling units to reach peak productivity.
Not just cherries and blackcurrants, but many other fruits will exhibit poor cropping productivity if they do not receive their quota of chilling units, at very specific temperatures according to species, and so not complete a full period of dormancy.
Ornamental plants are also affected by insufficient dormancy. Beech, for example, requires large quantities of chilling units – this is why it is one of the last trees to come into leaf. Hawthorn, on the other hand, requires much less chilling, and is happy to leaf early in mild winters.
If we can generalize for a moment, plant growth is active at temperatures of around 5°C. There are no visible signs of this activity during dormancy, however internally the plant is actually quite busy producing both leaf and flower initials, plus some extra root formation in readiness for the increase in temperature and day length.
And all of that winter work goes towards the sprightly burst of energy in spring that we all know and love.
And the not-so-good...
If the temperature falls too far, a plant can be damaged when ice crystals are able to form in the tiny spaces between plant cells.
The length of time a plant is frozen has little effect on the long-term damage caused – but it’s the speed at which ice crystal formation occurs (and the speed of thaw) that determines how badly affected the plant will be. The faster the freeze and thaw, the more damage will be caused.
If freezing temperatures are accompanied by snow, it could be the plant’s saving grace as it insulates the plant from the worst of the freezing temperatures, and also shields it from icy winds, which can desiccate evergreen plants as they struggle to keep liquid water in their leaves. There is a negative, however, and that’s the weight of the snow, which can damage branches and stems.
For evergreen plants, it also follows that frozen soil can be a problem; these plants have leaves that will still be transpiring, albeit at a reduced rate during the winter, but transpire they do. When air temperatures rise, an evergreen plant will try to draw more water from the ground, but if that groundwater is still frozen, it will lead to the plant’s leaves being scorched.
If you have evergreen hedges near a road, pay attention to when that road is gritted. Road salt can be splashed up by passing traffic, onto your evergreen leaves, eventually scorching them. Give evergreen hedges a rinse or two to prevent those unsightly brown marks on your leaves.
Signs of damage
Spring is the time when any damage from hard winter temperatures will become apparent. Evergreen plants in particular will show late and retarded growth, and any damaged roots will also be susceptible to diseases such as phytophthora and pythium.
And if any stems have split thanks to that freeze / thaw cycle, the plant will be both structurally weaker, and much more susceptible to the entry of diseases.
So, there’s the good and the not-so-good of freezing weather – pay it due respect, and care for your plants best you can.
You’ll be richly rewarded in spring if you do.