University of Illinois Extension


Barbara Larson
Unit Educator, Horticulture
Boone and Winnebago County Units

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Season Extenders

An interesting and fun aspect of gardening is pushing the limits of the environment. Experienced gardeners plant borderline hardy shrubs and flowers to attempt to grow something unusual. Tomato growers often have friendly competitions to produce the first tomato of the season. In northern Illinois, extending the gardening season is one of the most common ways to cheat Mother Nature.

Several tools and techniques will lengthen the growing season, making it possible to begin earlier in the spring and end later in the fall. These tools are: cold frames and hot beds, hoop houses, cloches, and floating row covers. Although most of these methods apply primarily to vegetable growing, several of them may be used for flower culture.

The most useful method with the most applications is a cold frame, a simple structure that provides warmth from the sun and blocks the wind. The sun’s rays enter through a transparent cover. This creates a greenhouse effect that heats the interior of the cold frame.

The most common use of cold frames is to expand the growing season one to three months. Many gardeners use cold frames to harden off transplants, but another good use is raising a few salad vegetables. Lettuce, radishes, and scallions will grow to full size in a cold frame before their regular outdoor planting season. Or in fall these same crops may be grown in the cold frame through November.Cold frames may be used in winter to force bulbs, store root vegetables, or propagate trees and shrubs by hardwood cuttings.

Permanent cold frames should be sturdy enough to withstand years of sun and weather. Most cold frames are made of wood and have a hinged covering. Wood and glass windows make a great covering, but they are heavy and breakable. Alternative covers may be made of plexiglass or double layer of clear plastic. Doubling the plastic creates dead air space for additional insulation in the cover.

Cold frame lids should be hinged for easy opening. On a sunny day, air in cold frames can get too hot for plants, therefore the lid should be propped open so cool air may enter the frame. Some mail order garden catalogs offer temperature controlled cold frame hinges that automatically open and close to vent the frame.

In general, cold frames should be located against a south or east wall near the building foundation to take advantage of its heat. Portable cold frames built of lightweight material allow the gardener to move the frame to different sun exposures as seasons and plants change. Portable frames can also be set up on concrete blocks or bricks to add height for tall plants.
A cold frame may be made into a heat bed by adding heating cables. The bottom heat of a hot bed encourages root growth in plants. A waterproof thermostatically controlled heating cable should be buried in a layer of sand two inches beneath the plants. An alternative method is to place fresh manure in the bottom of the cold frame. As the manure decomposes, it releases heat, creating a natural hot bed.

With a hot bed, vegetable and flower seeds can sprout and grow in sunlight instead of artificial light. Many seedlings require constant warm soil temperatures to germinate, so a hot bed gets them off to a better start.

A hoop house is similar to a cold frame, only larger. Metal or plastic pipes are bent into a series of hoops that are stuck into the ground or attached to a raised bed. The hoops are covered with 4-6 millimeter polyethylene which is tucked into the soil along the sides. Gardeners can expect an additional six to eight weeks of growing time inside a hoop house in the spring and fall. Like cold frames, hoop houses must be ventilated on warm days. This may be accomplished by lifting the ends or making openings in the top of the plastic.

Frequently used for tomatoes or peppers, cloches and hot caps add three to four weeks to the spring growing season. There are many variations on cloches, but generally they are any transparent “house” that covers a single plant. The most common cloches are empty gallon milk jugs with the bottom cut out. Another inexpensive cloche is made by covering a tomato cage with clear polyethylene. Like cold frames and hoop houses, cloches should have some type of opening to allow hot air to escape on sunny days.

Floating row covers are made of spun polyester or polypropylene and look like fabric. They are permeable to light, water, and air. Floating row covers have multiple uses in the garden. Row covers keep covered plants 5-10 degrees warmer than the surrounding air and provide frost protection to a low of 28 degrees. They protect tender plants from wind and rain damage. In addition, row covers are an excellent barrier to insects.

Part of the art and science of horticulture involves fooling Mother Nature. It is fun and educational to try techniques and tools that help plants grow when the environmental conditions are not ideal.


February - March 2004: Seed Starting | It’s Never Too Early to Prepare to Compost | Season Extenders | Winter Damage to Home Lawns

Past Issues

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