Cold frames are another way to stay frost free

Central Alberta has on average 90 frost-free days per year. For many vegetables, this is long enough for the vegetable to mature if the seed is planted directly in the ground in early spring.

Central Alberta has on average 90 frost-free days per year. For many vegetables, this is long enough for the vegetable to mature if the seed is planted directly in the ground in early spring.

Other plants need to grow for a longer period of time or have warmer growing conditions. One way to accomplish this is to plant out starter plants when there isn’t a danger of frost.

Another method is to extend the growing season in spring and fall by providing the plants warmth and protection.

Cold frames are frames covered in plastic that are not heated. They come in all shapes and sizes.

During the day, sunlight heats the structure, increasing the soil and air temperature, which encourages the plants to grow faster.

The temperature within the structure will cool down at night but not to the same extent as air outside the structure. Openings or vents are needed to allow excess heat to escape during warm days.

Unheated greenhouses are what most people think of when a cold frame is mentioned but they are one of many available.

The initial investment can be high but good quality ones will last for years.

They are also big enough to contain the plants in spring and fall.

Tunnels are another type of cold frame. They are a plastic or polyspun cover that is held off the ground and plants by short wire hoops.

Covers can be kept on the plants throughout the season producing early crops, unless pollination is required for the plant to develop produce.

Lean-tos, on the south or west side of a building, can be very effective. These can consist of a simple wooden frame that rests on a building. Plastic, glass or clear fibreglass covers the frame, protecting the plants.

Traditional cold frames are low boxes that have a taller wall on the north side than the south.

The lid is made of an old window or covered with plastic.

Once the plants outgrow the space, the covers are removed.

CozyCoats are small individual cold frames that are often used to protect tomatoes, peppers or cucumbers when they are transplanted outside.

Commercial varieties are open-ended plastic tubes that are filled with water.

The top of the tube is pushed together when the plant is small and can be opened as the plant grows.

The water absorbs the heat during the day and radiates heat during cooler hours.

A ring of two-litre pop bottles filled with water and held together with duct tape has a similar effect.

Bell jars or cloches are made of glass or plastic.

They are placed over top of plants when it is cold and removed when temperatures become warm.

Most have vents that allow some circulation of air.

Raised gardens are warmer than flat gardens as the sun will warms the sides of the garden as well as the tops.

Raised gardens can be permanent fixtures or temporary mounds of soil that are built yearly.

Permanent raised gardens can be made from untreated wood, stone or large tractor tires.

Smaller tires will fit around individual plants, attracting heat during the day and providing protection at night.

At the beginning of the season, the tires can be stacked together to cover the plant as it grows, providing protection from the wind and absorbing warmth.

Seeds —with the exception of squash, corn and melons — can be planted when the soil is cold but will not germinate until the temperature increases.

Covering the garden area with a sheet of plastic before planting will increase the soil temperature and speed up germination.

Covering the ground with a biodegradable sheet, then making holes for the small plant seeds will continue to keep the soil warm and weeds in check all summer.

Plastic will protect the garden and keep the soil warm for a few weeks after planting but it has to be removed to allow air to circulate and moisture to penetrate the soil.

A floating row cover, of polyspun cloth, will warm up the ground and air underneath, speeding up plant growth.

The floating row cover can sit on top of the plants and stay in place until the plants begin to flower. At this time, it will need to be removed to allow insects in to pollinate.

Come fall, covering plants with plastic and or polyspun cloth when there is danger of frost can keep the garden growing for another few weeks.

Linda Tomlinson is a horticulturalist who lives near Rocky Mountain House. She can be reached at or

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