Gardening Tips: How to cope with frost

Frost is one of the hazards which gardeners in cooler climates have to learn to live with. In Britain, only a relatively short period of the year (between early and late summer) can normally be relied upon to be frost-free, and this period is even shorter in the north. Gardeners in other countries apparently better favoured may be almost as much at risk, for it is not the prolonged frosts which persist day after day in winter that do the most damage, but those surprisingly severe frosts which occur unexpectedly in spring or autumn, for which the gardener is usually unprepared.

Frosts in winter often do little or no harm to fruit or vegetables unless they are sudden and severe; on the contrary, they may even do good by killing overwintering pests.

Weather and frost

In general frost forms in calm air. Anti-cyclonic weather—which brings settled, dry weather in summer—is therefore likely to bring frost in autumn, winter or spring. On the other hand, cyclonic or oceanic weather sweeps in cool air from the sea, laden with moisture. There is little danger of frost when it is windy and the sky is covered with rain clouds. This is because the clouds act as a reflecting layer lessening the loss of heat from the earth by radiation.

When the wind drops and the clouds disappear, however, there is a danger of frost. On clear nights the earth loses heat rapidly by radiation. This radiated heat raises the temperature of the air lying above the ground; as the air becomes warmer, it becomes lighter and moves upwards, to be replaced by the heavier cold air from above. This cold air, in turn, is warmed by radiated heat and rises, to be replaced by more cold air. These convection currents of air are formed more readily on cloudless nights, as there is no reflecting layer, and more and more heat is lost from the earth until the temperature falls below freezing, and a frost results.

Unfortunately, a change in the weather often occurs at night, presenting to the unwary gardener a grim spectacle of white, sparkling ruin in the morning.

Types of frost

The type of frost described above is known as a radiation frost; this is the most common sort. A wind frost may occur when the temperature is below 0°C (32°F), the sky is overcast and there is a cold wind blowing at more than about three miles per hour. Wind frosts often occur over high ground exposed to strong cold winds, but although less likely to be of concern to the gardener, they are more widespread than radiation frosts. In contrast to the situation with radiation frosts, there is often very little difference between daytime and night-time temperatures. Winds also cause damage to plants by their drying action. The degree of trouble experienced with radiation frosts depends mainly on the local climate and the physical features of the garden. A slight wind may lessen the danger of radiation frost by mixing the warmer air above with the cold air at ground level. If, however, the garden is protected from the wind, for example by a thick hedge, then this cannot occur and the risk of frost will be increased. On the other hand, a dense shelter hedge, wall or other barrier, may lessen the likelihood of radiation frost by reflecting the heat back towards the ground: remember that radiation frosts are much more likely beneath an open sky. In addition, it will protect crops against the drying effects of strong winds. The drawback with this is that few crops are able to grow in the shade cast by such a barrier.

Frost pockets

Cold air flows downhill; like water, it always finds the lowest level, and will collect there to form a pool, known to gardeners as a frost pocket. The warm air is forced upwards to higher ground.

Frost pockets can be large or small. A garden lying at the foot of a steep hillside, especially if sheltered from the wind by trees or walls, or even a whole valley, can be entirely submerged in a frost pocket. If you have a garden in such a situation, there is obviously little you can do to change the climate, but you can choose your crops wisely and make sure that you protect them properly.

Frost pockets are easily detected, especially in autumn, by observing where the mists lie on a calm morning, soon after daybreak.

There are also ‘mini’ frost pockets within gardens, formed by slight depressions in the terrain or by the shelter of walls or hedges. The north side of a dense hedge or bush is obviously a cold spot. On the other hand, there is equal danger in a frost pocket on which the morning sun strikes early, for direct sunlight on frozen plants can intensify the damage.

It is most important to avoid creating a frost pocket in a sloping garden by planting hedges or bushes or building walls across the slope which will hold up the downhill movement of the cold air and allow it to build up. It is not necessary to abandon the idea of a shelter belt, but you must make sure that there is a wide gap in it to allow cold air to escape and continue its downhill flow.

For the same reason, when planting fruit (or ornamental) trees and vegetables in a sloping garden, you should plant them up and down the hill rather than across it. Also, it is obviously better to grow crops at the top of a sloping garden than at the bottom.

How frost damages plants

Frost damages plants by freezing the water within their cells, causing the cell walls to rupture, and also by forcing water out of the cells.

The damage caused by frost usually becomes apparent only during the thaw following the period of frost. A quick thaw is more harmful to plants than a slow one. During a quick thaw, these changes become reversed suddenly. There is a sudden rise in temperature and sudden strong sunshine, which causes water to be lost from the leaves rapidly. Combined with the fact that the soil and roots may well still be frozen, this can cause serious damage as the plant cannot make up the water loss and dries out. During a slow thaw, on the other hand, the water can re-enter the plant cells gradually and uniformly and the water loss problem is not so acute.

Frost and vegetables

Because it usually occurs early and late in the growing season, frost affects vegetable crops chiefly in their first and last stages. They are most at risk when they are seedlings and then again at harvest time.

If you waited until all danger of frost was past before sowing your seed, you would not be sowing until early summer, which, for many crops, would not allow for a long enough growing period. It is therefore essential to make plans for protecting seedlings against frost.

Greenhouse and other protection In early spring a heated greenhouse is a tremendous asset. It need not be one heated to 16°C (60°F) for the growing of tomatoes and other tender plants; what is needed is one with a night heater installed in it to ensure that the temperature does not fall below 4.5°C (40°F), or, ideally 7°C (45°F). In such a greenhouse it is easy to achieve a start of a month or two over outdoor sowings.

If your greenhouse is unheated, you can still use it for bringing on early seedlings. The glass will protect them from the chilling effects of cold winds, though not from a sudden drop in temperature on a still, clear night. When a hard frost threatens, it is possible to give some protection by covering the seedlings with sacking or even newspaper. This method, however, will not give sufficient protection against a really severe frost, however, and it is worthwhile investing in an inexpensive night heater.

An alternative to heating the entire greenhouse is to install plant propagators, which are, in a way, greenhouses within greenhouses.

In effect, cloches, polythene tunnels and garden frames are miniature cold greenhouses. They too are effective in protecting plants against cold winds, but not nearly so effective against low night-time temperatures. Nevertheless, glass cloches will stave off several degrees of frost.

You should always remember that all frost protection depends on retaining warmth rather than on keeping the cold out. Glass performs this function best because it lets through ultra-violet light in the daytime, which warms the soil, and does not allow the infra-red rays to pass out, which would cool the soil at night. PVC and cellulose acetate also possess good properties in this respect.

The small size of cloches and frames makes it possible to protect plants against moderate frost simply by covering the cloches or frames with matting, sacks or hessian — this usually proves adequate to protect even tender plants against the degree of frost likely in late spring. For earlier crops, sown while frosts are more frequent or more severe, it may be worthwhile installing an electric soil-warming cable, which is not an unduly expensive operation.

Degrees of frost resistance

Plants are classified, according to their resistance to frost, in three categories: tender, half-hardy and hardy.

Tender plants cannot withstand any frost at all, and need to be given protection unless the climate or season is warm enough.

Hardy vegetables, by contrast, are fairly frost resistant, especially when fully mature, though the seedlings of some of them need protection.

Half-hardy plants will grow outdoors in summer but will not survive temperatures below freezing. Some half-hardy vegetables, such as beetroot, cauliflowers, kohlrabi, calabrese and celery, need protection as seedlings, but beetroot, calabrese and celery (except for the summer varieties) will stand a certain amount of frost when mature.

Cauliflowers and lettuces have been split into a huge number of specialized varieties, some of which are hardy enough to stand quite severe winter conditions, while others are exclusively for growing in summer, and yet others have been developed for forcing under glass. With peas, too, there are certain varieties which may be sown in autumn to stand a moderate winter.

Potatoes are highly vulnerable to frost in spring, when the green shoots are first appearing above ground. Many gardeners have had the unfortunate experience of finding their young potato shoots blackened by an unexpected late frost. When that happens, all is not lost, for the tubers will send up new shoots, provided the frost has not been too severe. A useful procedure is to cut off the blackened shoots as far down as the frost damage extends. The crop will then be a little later, but not as late as it would have been if planting had been delayed until all danger of frost was past. The deeper potatoes are planted, the longer they take to emerge and therefore the less risk they run of frost damage, but shallowly-planted potatoes usually bear heavier crops earlier.

Broad beans are frost-hardy, but if sowing in autumn be sure to choose a variety bred for sowing then. Do not sow them too early, or the beans will grow too tall, and the winter frosts, though perhaps not killing them, will cause the stems to fall over and become weakened.

In autumn, the following vegetables will be ruined by the first frost: cucumbers, French beans, marrows, pumpkins, runner beans, squashes and tomatoes. The next group will stand a fair amount of frost, but should be harvested and stored before hard and prolonged frosts settle in: beetroot, celery, potatoes and turnips. Carrots should also be harvested quite early, for frost causes them to lose their flavour.

Peas, broad beans, calabrese, kohlrabi and maincrop onions will normally be harvested before the first frosts. So will sweetcorn, though it is not damaged by a moderate frost. The resistance of cauliflowers and lettuces to frost depends on the variety. Winter broccoli (especially purple sprouting), Brussels sprouts, savoys, kale, leeks, parsnips, scorzonera, salsify, swedes and chicory are highly frost-resistant.

It is generally considered that a touch of frost improves the flavour of parsnips and swedes. However, some protection, in the form of a mulch of dry straw or bracken, is useful for even hardy root vegetables, largely to ease the problem of digging them out of the frozen ground.

Leeks will stand all frost, and will start to grow again in spring. Broccoli and winter cauliflowers often stand quite a hard winter, but become rather more susceptible to frost in early spring, when new growth begins. When the morning sun strikes a brassica or other vegetable crop that has been exposed to a hard night frost, it is beneficial to syringe the leaves with cold water.

It must not be forgotten that, as far as the soil is concerned, frost is the gardener’s ally. Dig as much land as possible in autumn, especially that which has not been dug before, and leave it rough during the winter. By pulverizing and crumbling the clods of soil, frost will do most of the hard work of cultivating your soil.

Frost and fruit

The fruits we normally grow in the open garden are all winter-hardy in temperate climates. The flowers, however, are highly vulnerable to frost. A night frost will cause the petals to turn brown, and in severe frosts complete trusses of apple blossom will wither and dry up. With strawberries and other fruits, the flowers with ‘black eyes’ are a well-known sign of frost damage. In these black-eyed blossoms, the ovary has been killed.

In instances of less severe frost, the blossom may still set fruit, but these will show signs of damage which will remain until maturity: an area of russet skin, sometimes cracked, starting from the blossom end. This is common in apples and pears. Fruit may become very misshapen because the seeds have been destroyed on one side only.

Peaches, nectarines and apricots may also be scarred by russet marks, but they are more likely to fall off the tree.

Blackcurrants may react to frost by shedding flowers and tiny berries over a period of several days. This may happen sometime after the frost has ended, and is often mistaken for poor pollination.

Cold weather may cause a different problem when temperatures are not freezing but are low enough during the daytime to discourage pollinating insects from doing their vital work.

Damage to roots

Although the roots are normally safe from frost, there is one frost danger to which thev may be at risk during the first winter after planting. The expansion of soil moisture as it freezes may cause a lifting of the soil so that the close contact between soil particles and the finer roots is lost, a situation in which re-establishment will be delayed. It is advisable to look over all newly-planted fruits immediately after a period of frost, and to press the soil firmly back in place with your foot wherever necessary. Pay particular attention to strawberries.

03. September 2013 by admin
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