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Cold weather gardeningPlant selectionStart plants indoors Harden off Polypropylene and cold frames The benefits of cold weather gardeningLight levels affect growth and most plants grow better when there is lots of light. In the Northern Hemisphere the longest day of the year is the 21st or 22nd of June and the darkest around 21st of December. In Stockholm at 60o north there is only to little light to grow vegetables between November and mid February, but the hardiest vegetables can only be grown outdoors from April to October because the temperature is to low in the spring. In the autumn temperature and light become limiting at the same time, but in the spring only temperature is limiting. There is as much light at the 21st of Mars as at the same date in September, but the daily average temperature is 10oC lower (in Stockholm). This means that it's in the spring that cold weather gardening techniques are most beneficial.There are at least three good reasons to use cold weather gardening techniques:
Plant selectionThere is actually two ways of making your garden microclimate more suitable to your plants. The first is plant selection. There are large differences in development times and tolerance to cold temperatures between different varieties of the same vegetable. Many varieties of French bean grow very poorly in Stockholm whereas others produce a decent harvest even in a summer that is colder than average. For the varieties of carrot available in Sweden development times vary between 80 and more than 110 days. Information can often be found on seed bags or in catalogues. The information that a variety gives an early harvest is often an indication that it also tolerates colder temperatures. It obviously grows well in the spring.Top Start plants indoorsWhen you have exploited the possibilities of seed selection you can only get further by increasing the temperature of the soil and air around the plants. To extend the growing season plants can be started indoors or in greenhouses and cold frames. The plants can also be covered with plastic and other materials to increase the temperature of air and soil.When plants are started indoors there are two main problems. The first is that the indoor environment tends to produce weak and spindly plants. The second is that plants grown indoors are not well adapted to the harsher conditions outside and may suffer when planted out. To develop well in the spring most vegetables need lots of light, but not to high temperatures. Indoors it's usually to warm and dark. I place my seedlings next to a window and turn the radiator below the window off. I shade the pots with white reflective paper as much as a can without shading the emerging plants to keep pots and soil cool. To grow a lot of seedlings in a small space is a sure road to failure. Allow some space. It's usually much better to get 20 good plants than 200 that are week and leggy. Use a special sowing compost. A good sowing compost is rather low in nutrients, which will encourage a good root system and reduce the risk of leggy seedlings. The growth will be slower which make the seedlings tougher when planted outside. In general, slowly growing plants are more resistant to environmental extremes, including low temperatures. Seed germination will also be better in soils low in nutrients because lots of dissolved nutrients interfere with seeds absorption of water.
Small biodegradable pots can be made from newspaper. Make a roll of desired diameter and height and fix it with a plastic paperclip or fold in two cm at one end to stabilise the pot. Fill with sowing compost and put them on a tray. Water on the tray. Sow two or a few seeds in every pot. When seedlings emerge, thin them out to one per pot. The plants can be planted out with or without the pot. Be careful not to disturb the roots if you remove it.
Harden offPlants sown indoors may be very sensitive to cold temperatures. Broccoli sown directly outdoors will usually not be affected by light frosts while broccoli plants sown and cultivated indoors may be badly damaged or even killed by temperatures above the freezing point. Plants will be hardened if they are exposed to colder temperatures, but the change in temperature regime must not be too abrupt. The plants must be gradually accustomed the harsher outside environment. First you move them outside during the day, but back inside again over night. After a few days you can put them in a cold frame (still in their pots) and leave them there day and night. After a few days more they are ready for transplantation into their final place. If this is too much trouble, try to transplant them strait outside. The least you can do is to cover them with thermally bonded polypropylene. Pick a day when the first nights can be expected to be relatively mild. The coldest nights are those with clear skies and no or little wind. These nights can be fierce on all your plants, but especially on those recently moved outside.
Heat lowing veggies like peppers, tomatoes, and cucumber generally need a longer period of hardening than e.g. Brussels sprouts, lettuce and celeriac. They must also be transplanted much later in the season when all risk of frost is past.
Polypropylene and cold framesThermally bonded polypropylene is very useful in a vegetable garden. It raises the temperature and offers some frost protection. It retains moisture and is an effective pest barrier, but it is still porous enough for rain and air to pass through it. Unfortunately it's not very durable. It needs careful handling and is still not likely to last for more than two seasons. Cover the soil two weeks before sowing or transplanting to warm it up. The material is very light and can simply be laid on top of plants. The plants will lift it as they grow. I prefer to suspend it on wire hoops. I make the hoops from 5mm thick zinc-coated steel wire. The hoops can be widely spaced. Burying the edges in the soil or using stones or boards holds down the sheets. Beware of rough edges that are likely to tear the material. If hoops are not used, sufficient slack must be left to allow the material to rise with the plants as they grow.
A simple cold frame can be made in ten minutes from 4 boards nailed together at the corners and an old window or a clear plastic sheet. I use several of these portable cold frames. Frost hardy plants can be sown up to a month earlier and tender plants at least 2 weeks earlier than would be possible without the frames. A cold frame offers better frost protection than polypropylene, but also needs more attention. During the day the cold frame must be ventilated or the carbon dioxide that plants need for photosynthesis will be depleted. On a warm and sunny day the temperature within the cold frame may also be terribly high if air is prevented from circulating. On a cold night the frame should be closed and preferably covered with e.g. an old carpet for better insulation. If nights are mild the lazy gardener can leave the frames open. I remove the frames and polypropylene as soon as the plants can do without them, because too much pampering often produces vegetables with softer tissue and less flavour. If this is the kind of vegetables you want, you don't have to grow your own. You can buy them.
vegetables@frideen.com Last major update 2004-12-10 Copyright© Anders Frideen |