Structures From Trellis to Greenhouse

by David S. Ross.

David S. Ross is Extension Agricultural Engineer, University of Maryland, College Park.

Structures can add to your gardening enjoyment in a number of ways. The beginning gardener may get along with a minimum, just a place to keep tools and supplies. Others will find additional structures advantageous and reasonable.

Coldframes, hotbeds, and greenhouses provide a place to start seedlings or to grow an early crop of lettuce. A storage shed or area is needed for rakes, hoes, power tillers, fertilizers, and numerous other items. Gardening space can be expanded vertically by using a fence or trellis to support a growing vine or even a fruit tree. And when the harvest is over, a place must be available to store the surplus produce until it can be consumed.

The environment in plant growing and food storage structures is quite important. Environmental control equipment such as heaters, coolers, ventilation fans, and thermostats are necessary to maintain the appropriate temperature conditions. Light and water are essential for plant growing and must be supplied in a suitable way.

Plans and information releases from the U.S. Department of Agriculture are available from your county Extension office. These offices have additional information and will be able to answer questions on specific gardening subjects.

A hotbed or coldframe is well suited for starting seedlings early for transplanting into the home garden. It can double as a place to grow cool-season vegetables in the spring for table use or to extend the fall growing season.

The coldframe is generally a wooden frame box about 3 by 6 feet in size with the back (north side) higher than the front so as to slope the top to capture the most sunlight, and for rain runoff. The sloped top, attached by hinges, is made of window sash, storm windows, or a frame covered with polyethylene film. The hotbed differs by having a source of heat to warm the soil and in colder climates to warm the air.

A sunny well-drained location with wind protection is ideal. A location near the south side of the house is good because it is close to water and electricity, and the young plants can be given the frequent attention they need.

Good construction conserves the soil heat at night. Make the joints as airtight as possible. Soil can be banked up around the sides to keep it warmer. The wood parts should be painted with a primer and one or two coats of white paint to reflect the light.

A wood preservative, such as 2 percent copper naphthenate, is safe near plants and can be used to give additional protection before painting. Creosote and pentachlorophenol are toxic to plants and should not be used.

Coldframe.

A convenient size may depend on the building materials available, but a 3- by 6-foot coldframe or hotbed gives plenty of room for young tomato, pepper, cabbage, onion, and other plants for a big home garden. Height of the frame can be 12 inches in front (south side) and 18 inches in the back (north side). However, these dimensions can be reduced; the top may be open most of the time after the last frost when the plants are tall. The length can be in multiples equal to the width of the window sash or other material used for the cover.

Heat for the coldframe comes from the sun which warms the soil and air. At night the heat is slowly lost through the cover. The temperature must be controlled during the day so it does not get too high for the plants (maximum of 100 F). The cover must be raised to permit ventilation, or you can install a small ventilation fan controlled by a thermostat.

Cool-season crops, such as cabbage, cauliflower, and lettuce, can stand a day temperature of 60 to 65 F. Warm-season crops, such as eggplant, peppers, tomatoes, and melons, do better at 65 to 75 day temperature. The night air temperature can be 5 to 10 lower with good results. Cold-frame temperature may not be easy to control, depending on weather conditions and the frequency with which someone can check the temperature. However, with experience a satisfactory operating procedure should be found.

The hotbed is heated by an electrical cable placed under the soil. In northern areas of the United States, 12 to 16 watts of heating cable per square foot of bed area is needed. In southern areas 10 watts per square foot should be adequate.

Heating cables vary in length and wattage rating. Select the number of cables needed for the total wattage.

Be sure adequate electrical service with ground fault interrupter (GFI) is available to meet requirements of the National Electrical Code and any local codes.

Lay the heating cable on the ground 6 inches below the intended planting surface. Carefully spread the cable so it is uniformly spaced on the bottom. Avoid kinks. Do not lay cable across another cable as overheating damage may result.

Cover the cable with 2 inches of soil or sand. Then place 1/2-inch mesh hardware cloth across the surface of the sand to protect the cable from gardening tools. Add a 4-inch layer of soil or soil mix on top of the wire mesh for growing plants. Sometimes 2 inches of sand or vermiculite is placed below the cable to conserve heat.

Early hotbeds had 12 to 24 inches of animal bedding and manure in them to provide heat as the manure composted. These beds generally grew better plants because of carbon dioxide released by the manure.

A thermostat, normally built into the heating cable, is used to control soil temperature. A separate thermostat with remote sensing bulb works well because the sensing bulb can be placed in the root zone. A temperature of 70 to 75 F is ideal for germinating most seeds. A good thermometer should be used to check operation of the thermostat at seed depth and to measure air temperature.

Cost of operation depends on the weather, location, and construction. A 3- by 6-foot bed will use 1 to 2 kilowatt-hours of electricity per day.

Operating costs can be reduced by adding insulation to the sides, covering the bed at night with straw or other insulating material, and keeping the frame in good repair and airtight.

Watering is important. Try to have the bed moist at all times but not soaked. Apply water in the morning so plant foliage can dry by evening.