1. Used cold frames (hoop houses)

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by Liesbeth Leatherbarrow
by Liesbeth Leatherbarrow

Liesbeth has written for several western gardening publications, including Gardens West and The Gardener for the Prairies. She has also co-authored three gardening books: The Calgary Gardener (with the Calgary Horticultural Society), The Calgary Gardener, Volume Two: Beyond the Basics (with Lesley Reynolds), and 101 Best Plants for the Prairies (with Lesley Reynolds).

April 8, 2007

There isn't a gardener among us who has not at one time or another lamented Calgary's sometimes precarious, always unpredictable, and relatively short growing season. A few more frost-free weeks in the spring and fall would make THE difference and some years we are lucky. Other years, occasional or repeated June and September frosts make gardening in our climate a challenge. Surprise August snowstorms aside (for which there is no known solution), there is a simple way for gardeners to extend the growing season - build a coldframe.
Coldframes come in a variety of shapes and sizes but most can be described as a wooden frame or box, often partially sunk in the earth, covered with an old glass window, sheet of clear flat fibreglass, plastic film or other transparent material. Ideally a coldframe is situated in a well-drained, open, sunny location sheltered from the wind. Its top should slope to the south, southeast or southwest to collect the maximum amount of early spring sunlight and to warm up quickly during sunny spells. As such, a coldframe provides a daytime microclimate considerably warmer than that of the surrounding area. The soil's capacity to retain heat acquired during the day also helps maintain "warmer" nighttime temperatures.
A coldframe can easily be converted to a heated frame or hotbed by the addition of a bottom heat source to warm up the air and soil even earlier still. Traditionally, heat generated by decomposing manure was used to warm hotbeds but these days electric soil heating cables paired with a thermostat do a better job. With a thermostat setting of 20-25°C, hotbeds can even be used when there is snow on the ground.
Both coldframes and hotbeds require frequent attention since their limited airspace can warm up or cool down quickly. A thermometer mounted inside the coldframe is useful to help monitor temperatures. Plants growing in a frame can be scorched during periods of strong sunshine. As the day warms up, the glass top of a frame must be opened gradually to prevent overheating and the reverse applies as the day cools down. Automatic pneumatic window openers can be purchased to simplify this task. Shading compounds can also be applied directly to the glass to help control temperatures or netting/muslin/white polythene can be secured to one edge of the frame top and rolled in or out as needed. It is important to use insulation or electric lights as a heat source during cold and frosty days and nights to prevent freezing of the plants. 
Good ventilation in coldframes is necessary to prevent the damp, stale atmosphere that encourages diseases such as botrytis. A dry atmosphere is especially important in an unheated frame in the winter.
Watering plants in a coldframe may be done by hand and frequently during warm days as the soil dries out quickly in the sun. Trickle hoses, drip lines and capillary matting are also suitable and in some ways preferable to hand watering since they eliminate the need to open the frame too often, thereby preventing unnecessary heat loss.
There are many ways in which coldframes can be used to full advantage:

  • Raising summer bedding-out plants and vegetables for later planting in the garden; this takes more work than starting them inside, but most gardeners agree that the exposure to outside air produces plants that are sturdier and healthier;

  • Permanent growing location for heat-loving veggies started inside such as tomatoes, cucumbers and melons;

  • Hardening off bedding-out plants and vegetables started inside before planting them in the garden; by controlling the temperature in the coldframe plants are gradually acclimatized to outdoor conditions;

  • Vegetative propagation - stem cuttings of hardy and tender plants can be rooted in heated or unheated frames;
    raising and growing potted plants - e.g. bulbs, calceolaria, primula, begonias, pelargoniums, alpines;

  • Overwintering - ideal winter storage place for resting and dormant plants; place plants directly in soil or "plunge" pots into a 12" depth of insulating medium such as peat moss, ashes, dry leaves or pulverized bark to protect plant roots from the frost (bulbs can be forced this way too);

  • Overwintering tender plants - a heated frame where frost-free conditions are maintained is suitable for overwintering tender plants, bulbs, corms and tubers.

A few tips for maintaining coldframes:

  • keep the glass very clean;

  • scrub thoroughly once a year;

  • treat wooden frames regularly with a horticultural wood preservative;

  • if algae become a problem, wash the frame with an algicide;

  • rake the soil often to prevent the growth of moss and liverwort;

  • sterilize the soil regularly or replace it entirely every few years.

Detailed plans for constructing coldframes can be found in many gardening books.
So . . . as you lament yet another short prairie growing season, consider putting a coldframe on your "to do" list. It will always keep you one step ahead of Jack Frost and that's good when it comes to gardening.

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