Protecting plants against cold damage :
With winter approaching it is time to look at ways to lessen the damage that very cold weather can cause.
What happens within the plant?
It is when the temperature falls below freezing that plants are most likely to be damaged. Some of the water contained within the plant cells flows out the cells to the spaces between the cells and freezes. When the temperature warms it melts and moves back into the cells without causing too much damage (provided it caused no bursting of any of the plant’s parts). Although the dissolved salts within the plant cells act as the plant’s equivalent of anti-freeze and lower the point of freezing, if it gets cold enough the water within the cells also freezes. When this happens the ice crystals damage the cell walls and as the crystals melt the cells dry out and die off.
What climatic conditions lead to cold damage?
Air may be invisible but it flows in the same manner as water. Cold air, being dense and heavy, flows and settles closest to the ground. If the flow of cold air is slowed down or stopped by walls, fences and hedges it will begin to flow upwards. Because the coldest air is closest to the ground it initially causes damage to the lowest parts of the plant but over time the damage systematically moves up the plant.
Wind exacerbates cold damage; this is because it reduces the temperature on the surface of plants through evaporation. Windbreaks can prevent this additional damage but they must also be designed in such a way that they don’t trap cold air.
What can a gardener do?
Gardeners in cold climates are not totally helpless – there are ways to fight back!
• Pick the right plants – choose plants suitable to the climate and micro-climate of your garden. Endemic plants, for example, will stand a much better chance against the cold than exotics from more temperate climes.
§ Keep them healthy – plants that have had timely treatment against diseases and pests are better able to withstand cold weather.
• Good feeding – apply the correct fertilizers in spring and summer and stay away from nitrogen-rich fertilizers in late autumn. Nitrogen stimulates new growth, especially during sunny days, so should rather not be applied after March.
• Leave the leaves – dense foliage prevents heat loss so don’t prune tender evergreen plants in late autumn or winter and don’t cut away dead plant material, unless it is very unsightly. It is also best to delay winter pruning as long as possible because pruning stimulates new growth (most especially in unexpected warm weather) and new growth is most susceptible to frost damage.
• Cover up with mulch and fleece – organic mulches are the ideal protection against water and heat loss from the root area of plants. By preventing the loss of these two elements microbial activity can continue and this, in turn, generates heat. Protect vulnerable plants with horticultural fleece or frost cover, or by making enclosures around them using plastic (keep the tops open). The roots of plants in containers are especially susceptible to cold damage so pack the pots and planters closely together and cover them with hessian or grass.
• Moist soil helps – because it cools down at a slower rate than dry soil. Ensure that your plants never dry out and make a point of watering well when a cold front is expected. Watering at daybreak before the sun’s rays reach frozen plants will help them to defrost slowly thus lessening the damage – provided the air temperature rises above freezing point. Change your summer irrigation programme from April – you can still irrigate for the same length of time per session, but gradually increase the time between sessions.
• Don’t plant now – avoid planting and transplanting perennial plants in late autumn as they are unlikely to have established themselves properly by the time the worst winter cold strikes. Ideally in the summer rainfall areas with cold climates it is best if planting only starts again in August.
An Article from The Gardener Magazine – June 2008