In climates with short growing seasons, season extenders such as cloches, cold frames, tunnels and greenhouses are a great way to extend the growing season.
The Advantages of Season Extenders
Season extending structures help gardeners get an early start on spring planting. Tender vegetables such as tomatoes and peppers can be planted earlier and covered if a late spring frost is predicted. Late season vegetables such as winter squash or still‐ripening tomatoes can be covered in the autumn in case of an early frost. Season extenders can also enable gardeners to plant later in the season and prolong harvests through late autumn or even into winter.
Garden coverings such as cold frames are also used to “harden off” seedlings started indoors. Putting them in a cold frame before planting them in the garden acclimatises them to outdoor weather.
Season extenders can also be used to warm up the soil in the spring. While black plastic is commonly used for this purpose, cold frames or a row of cloches will work as well. Season extenders also retain moisture and may reduce the need for watering. They can also protect from harsh winds and cold rain.
Coverings can also be used to prevent insect infestations and to protect plants from birds, rabbits and other crop‐eating pests.
The simplest way to protect just‐planted crops from a spring frost is to cover them by hilling up soil around them. The young plants will still emerge through the soil once warm weather returns. This method is useful with early potatoes, for example. Other easy coverings include loose straw, dry leaves or a layer of compost.
Another simple solution is to cover a newly‐planted row with tarpaulin, newspaper or other thin layer to protect against an overnight frost. Use rocks, bricks or soil to weigh down these coverings to prevent them from blowing away.
Cloches and Tunnels
Cloches have been used for centuries – one well‐known example is the Victorian bell jar. The purpose of a cloche and other season extenders is to trap the heat of the sun’s rays, creating a warmer environment for the plants underneath.
A cloche works using the “greenhouse effect”. The sun’s short ultraviolet rays pass easily through the glass, warming the air and soil underneath. Within the cloche, the ultraviolet rays are converted to heat in the form of longer infrared rays, which pass back out through the glass more slowly. Soil, in particular, is a good heat collector, maintaining the heat within the cloche and releasing it slowly throughout the day and night. Note that if temperatures are too low or there is little sunshine, cloches will not produce this effect.
Cloches may be made from glass or plastic. Glass traps the heat better than plastic, but requires careful handling to avoid breakage. Plastic is inexpensive and less breakable, but does not trap heat as well.
Cloches may be dome‐shaped, conical, or bell‐shaped enclosures for individual plants. They may also be tunnels, or continuous cloches, which cover an entire row of plants. Continuous cloches can be tent‐ or barn‐shaped glass structures; rigid plastic tunnels; or polythene (polyethylene) over flexible hoops. Cloches and other season extenders must be ventilated to prevent too much heat buildup.
Cloches can be purchased or homemade. A recycled plastic drink or milk container with the bottom cut out is one option. Taking off the cap provides ventilation. A glass cloche can even be made from a large wine bottle by cutting off the bottom using a glass cutter and sanding the cut edge with emery paper. A tomato cage or hardware cloth covered with plastic is another idea.
Cold frames are like mini greenhouses. They can be purchased or homemade. Cold frames are often made using old wooden windows, but clear plastic can also be used. Cold frames may be permanent or portable. A permanent frame can be made of bricks or rocks, both of which hold heat well. A portable frame can be made from lighter material such as plywood or bales of straw. The glass cover is best positioned at least a 45 degree angle, facing south. Double cold frames use two lids with a hinge, like a row of glass‐roofed houses.
While cold frames are relatively inexpensive, greenhouses can be a major purchase. Greenhouses may be made of panes of glass or rigid plastic framed in timber, aluminum, or plastic. Another variation is the hoop‐style greenhouse. Greenhouses are often heated to further extend the growing season.
Try a few of these season extenders to stretch your gardening season and boost your harvests!