Greenhouse Gardening 101
A greenhouse (sometimes called a hothouse) is a useful structure for growing plants, especially in a cool climate. It is usually constructed with a wooden or metal frame and covered with a material that allows the maximum amount of light to shine through. Unlike growing outdoors, a greenhouse is a controlled environment, where you and the sun are in control. Therefore, daily care throughout the growing season is required. When considering if a greenhouse is right for you, ask yourself whether or not it is a commitment you are able to make. If you will be away for a few days or longer, youâll need to make arrangements for someone with some experience to look after the greenhouse. Construction MaterialsThere is quite a choice of materials available for covering a greenhouse. Your decision will depend on the amount of money you wish to spend, whether the greenhouse is a permanent or temporary structure and the type of plants you plan to grow.The best (and most expensive) material for covering a greenhouse is glass, but you must have a special structure that is strong enough to support the weight of the glass and prevent damage from wind and snow. If you plan on investing in a glass greenhouse it is usually best to buy a greenhouse kit, which includes the metal framing as well as the glass. A permanent glass greenhouse, either attached to the house or freestanding, can be considered an investment that will add value to your property.Covering with double-layer polycarbonate sheets is more economical than glass, and can create a permanent greenhouse that is energy-saving because of the double layer. It must be certified for greenhouse use, otherwise it may become discoloured and deteriorate over time. Polycarbonate is easy to install over a wood or metal frame, and greenhouse kits using this material are also available.A fibreglass covering is another option, but be careful to select fibreglass that is designed for this purpose. Fibreglass tends to discolour over time, reducing the amount of light transmitted.Polyethylene that is specifically made for greenhouses is the most economical type of covering. Greenhouse poly is manufactured to prevent ultraviolet light from breaking down the plastic and causing it to become brittle and crack. It will last three years or more when securely installed, and all the stress points are cushioned with a material such as carpet underlay. (To contrast, construction-grade plastic likely wonât last through a single summer.) Greenhouse poly is available in rolls of four- or six-mil thickness, and it is important to use at least four-mil to resist tearing. A double layer of polyethylene can be used to conserve energy, reducing heating costs by up to 40 per cent.The floor of the greenhouse must be covered with a material that allows water to drain away. Paving stones set in sand will work, as will landscape fabric covered with pea gravel. This will prevent weed growth, which is critical to maintaining a healthy greenhouse environment.Size & LocationThe size of the greenhouse depends on what you want to grow and how much you wish to spend. A small 8' x 10' greenhouse can grow more than enough cucumbers and tomatoes during the summer and fall to supply the average family. It can also be used to grow greens and start off plants to be transplanted outside in the garden. Also, consider if youâd like to use your greenhouse to double as a sunroom for relaxation, with room for a fountain or other features.Choose a site that will offer the most sunlight to your greenhouse and, if at all possible, ensure that the long side of the greenhouse faces south.Temperature ControlSuccessful greenhouse growing largely depends on ventilation to control the amount of heat. If the temperature rises above 30°C (which can easily happen without ventilation), tomatoes and cucumbers will not be properly pollinated, plants will dry out quickly and diseases will develop. Natural ventilation can be achieved by having manually controlled vents both in the top and bottom of the greenhouse. A small greenhouse can be ventilated simply by opening doors at both ends. A thermostatically controlled fan at one end and a vent at the other eliminates the time-consuming work of opening and closing vents every day. At the very least, greenhouse growers should use thermometers to measure the temperatures of both the air and soil.To extend the growing season, invest in a source of heat for the spring and fall. Small electric or oil-fired heaters are not very expensive, and you may only require just enough heat to keep the plants from freezing. A great way to conserve energy is to build a plastic tent inside the greenhouse and install a thermostatically controlled heating cable to provide the correct temperature for germinating seeds, growing early greens and rooting cuttings. Get GrowingVegetable plants like tomatoes and cucumbers can be grown in beds on the ground, but it is more convenient and easier to clean up if the plants are grown in containers. Plants must be given space to grow so that the air can circulate around them, reducing the likelihood of developing diseases and attracting insects. For example, a tomato plant should have at least two or three square feet per plant. Greenhouse plants must be watered every day when it is sunny. Installing a water and electrical supply is very convenient and reduces the amount of time required to grow plants. Removable benches make it easy to clean up the greenhouse after the growing season and are essential for starting off seedlings. Other accessories, like a potting bench, can make work much easier. - By Ross TraverseRoss Traverse has been a horticultural consultant to gardeners and farmers for more than 40 years. His "Down to Earth" column appears in each issue of Downhome.