by Nick Gromicko, CMI® and Kate Tarasenko
A greenhouse is a transparent structure used to grow plants within. The primary purpose of a greenhouse is to trap light and heat, which allow plants to survive and thrive in climates where they would otherwise die. Greenhouses encourage the germination of new seedlings, extend the growing season for mature plants, and allow tropical plant species to grow in temperate climate zones. Commercial agricultural operations and hobbyist gardeners alike take advantage of greenhouse technology. The use of greenhouses for growing vegetables dates back to the Romans. Italians used the first modern greenhouses to propagate tropical species of plants that were brought back by early explorers.
Greenhouses vary in strength and durability, and are usually constructed with glass or polycarbonate walls and roofs. Here are some basic types:
- Cold-frame greenhouses are those that do not supply supplementary heating or cooling systems.
- Lean-to designs are "half-greenhouses" that stand up against and are attached to the side of an existing building. They require less space than a free-standing unit, and may be assembled and disassembled seasonally.
- Free-standing greenhouses may come as a kit or may be assembled in a do-it-yourself manner. Most units have a base, a wooden or metal frame, and glass or polycarbonate panels. A hoop-shaped design is preferred for many beginning users.
How do greenhouses work?
The walls and roofs of greenhouses are transparent and allow light from the sun to penetrate into the structure. Plants and soil on the inside absorb the solar radiation and re-radiate it, trapping it inside. Some heat is lost but, overall, more is gained and, consequently, the indoor temperature is higher than the outdoor temperature in colder climates. Greenhouse structures heat up slowly and tend to maintain their warm temperatures for longer periods than the outside air. Some greenhouses are very basic and offer little in terms of environmental controls. Others utilize high-tech, automated gauges, timers and computers to maintain proper conditions.
Maintaining the proper temperature is important in encouraging seed germination and plant survival. Dark-colored barrels of water may sit on the greenhouse floor. These drums absorb daytime solar heat and release it slowly at night. Electrical space heaters may also be used if sufficient heating is not attained through this method. Different temperature zones may be created so as to encourage a range of plant varieties. Soil-warming trays are another electrical heating option, especially when heating the greenhouse air is not necessary. A shade cloth may be draped over the structure in times of excessive heat.
A venting system may circulate new air into the greenhouse and will also regulate temperature. This may simply be comprised of two fans: one for bringing in new air, and the other for circulation. People may water plants by hand or they may install mechanical timers and dispensers, such as overhead spouts or booms.
Greenhouses also trap moisture. A hygrometer monitors humidity levels, and misters spray water as needed. Bees may enter the greenhouse to pollinate plant species that require it. In other instances, people may fertilize plants by hand. Plants may be transferred outdoors once colder weather warms sufficiently to suit them. Species that are not acclimatized to the particular locale may remain growing in the greenhouse indefinitely.
Advantages of Greenhouses
- Greenhouses maintain a relatively stable interior climate, safe from harsh fluctuations in outdoor temperatures.
- Careful regulation of all environmental factors may be accomplished within a greenhouse, as opposed to the unpredictable nature of the outdoors.
- Greenhouses make it possible to grow vegetation in climates where it would otherwise be impossible, sometimes throughout the entire year.
- Animals that might pose a hazard to vegetation are kept at bay.
- Greenhouses shield plants from excess precipitation and wind.
- Soil-warming trays minimize the cost of supplemental heating, since they deliver the necessary warmth to the plants without requiring that the entire greenhouse be heated.
- Building a greenhouse can be relatively simple and inexpensive, especially in the case of homemade hoop designs.
Disadvantages of Greenhouses
- Supplemental heating and lighting systems in colder climates cause increased use of energy and associated costs.
- Structures must be maintained and materials replaced as needed.
- Greenhouses that require glass panels are often prohibitively expensive for small-scale gardeners. Replacing them if they crack or break can also be expensive.
- Polycarbonate panels are less durable than glass panels and may blow down in strong wind. They are usually not designed to withstand heavy weight, which may pose a major problem in areas that receive large amounts of snow.
- Greenhouses may attract unwanted insects and rodents.
- Irrigation must be provided, since plants do not receive any rainfall.
- Soil may need to be rejuvenated more regularly than in outdoor plants.
Safety and Inspection
- Greenhouses should be built in an area of land that receives the most possible sunlight. They shouldn’t be constructed near trees, for instance, so as to avoid shading and falling branches.
- Ventilation systems should always be installed and function properly. Stagnant, humid air may lead to the growth of hazardous fungi. Excessively high temperatures can kill plants.
- Leaks should be promptly sealed. Holes that are unattended to may attract insects, excess moisture, fungi, pathogens, and excessively cold air.
- Operators should regularly inspect plants for insects. In cases where pests are present, action should be taken immediately to eradicate them.
- Extra caution should be taken when handling greywater for irrigation.
- Make sure that electric heaters are designed specifically for the greenhouse setting and do not pose a fire hazard.
- Structures should be as large as possible so as to capture a maximum of the sun's rays.
- Soil should maintain the proper balance of nutrients, depending on the plants' needs.
- Plants should be grouped together according to their temperature needs in greenhouses that utilize different temperature zones.
- Areas should be sterilized and free from visitors (including pets) who might cause inadvertent contamination.
- Pools of water should be swiftly dried up so as to avoid excess evaporation and interior humidity.
- Containers should be placed at the bottom of walls to collect dripping water for later use.
- Mold should be immediately cleaned off of all surfaces.
- Safe handling practices should always be followed when using harsh chemical pesticides.
- Greenhouse walls should be sufficiently thick, especially in colder climates.
- If not using concrete, crushed stone or grass, make sure that the floor of the greenhouse has a dry, anti-fungal, non-skid surface.
- Greenhouses that are lean-to styles or which use the home's structure as one of its walls should be regularly checked for moisture buildup, errant plant growth, and other potential biological concerns that can encroach on the living area or threaten the home's structure.
- Greenhouses attached to homes should also be inspected for pests that may migrate from plants and soil to the home's interior.
- Make sure that any electrical devices in the greenhouse use the proper type of outdoor electrical receptacles, and that these have covers, which will prevent an electrical hazard caused by water.
- Unplug and safely stow any electrical devices in the greenhouse that are not in use.
- If children are present in the home, use a childproof latch or lock on the greenhouse door to prevent them from entering the greenhouse unattended, where electrical, chemical and water dangers lurk.
- Check with local building ordinances or your HOA to find out how large and what type of structures are permitted in your area.
In summary, greenhouses are an excellent technology for gardeners looking to regulate the climatic growing conditions of their plants and seedlings. Safe practices should be followed to get the most out of them.