Using a large growing house or some type of small greenhouse setup can greatly extend the growing season for amateur gardeners and is essential for commercial production of many ornamental flowers, plants and vegetable transplants. However, the warm, moist environment of the greenhouse can lead to as many problems as benefits, including rampant disease and attacks by pests. Careful management of several factors can lead to a successful indoor growing operation.
The main advantage of the greenhouse is the availability of higher temperatures during cooler months. Most plants grow at an optimum rate between 70 and 80 degrees F; at temperatures higher or lower than this range, plants typically slow their rate of growth, delay flowering or can become damaged and die. Especially in warm, overly humid environments, plants grow to become leggy and spindly, usually with a very weak stem and long intervals between leaf nodes. Using a combination of vents and a heat source to regulate greenhouse temperature and humidity is essential to maintaining the proper environment for healthy plant growth.
Humidity and Watering
The enclosed environment of the greenhouse traps the moisture released from leaves during the process of transpiration, or plant respiration. Plants that are clustered closely together typically have a higher ambient humidity than plants spaced apart, and a higher level of humidity is more desirable than low humidity in the greenhouse. Higher humidity reduces the amount of watering that is necessary in the greenhouse; however, overly humid environments encourage the growth of mold, fungus and several serious bacterial diseases of plants which disfigure or kill specimens. Regulating humidity goes hand in hand with regulating temperature, as humidity increases with temperature.
Plants exposed to proper levels of natural sunlight or specially designed plant lights in the greenhouse typically feature dark green or glossy foliage, robust growth indicated by a stockier stem and thick, lush vegetation. A combination of proper temperatures and length of daylight--or a nighttime period--is the usual requirement for spurring a given plant into flowering. Plants growing in low light environments will appear to be sickly pale, with yellow or brown foliage, long, thin stems and a floppy appearance; deprived of light, most plants will gradually wither and die.
In greenhouses with poor cultural practices, such as not sterilizing growing media or growing containers, or with high levels of weeds, disease can be a serious problem. Several prevalent diseases of greenhouse plants include botrytis, or gray mold; powdery mildew; root rot fungus; blight; and several types of virus. Plants crowded too closely together can easily transmit disease to their neighbors, so proper spacing is essential. Maintaining internal humidity of between 25 and 70 percent is essential to managing incidences of plant disease, which deform plants by causing damage to foliage, blossoms, stems, fruit and roots but frequently also kill host plants.
Along with disease, management of greenhouse pests is essential to maintaining a healthy population of plants. Common greenhouse pests include aphids, thrips, whiteflies, spider mites and slugs; the main defense against pests is to practice excellent cultural standards by inspecting everything that enters the greenhouse. This includes, but is not limited to, examining plants for evidence of pests or damage by pests, ensuring sterility of soil and other growing media, cleaning surfaces and growing containers on a regular basis and installing barriers around entrances to prevent native pests from entering the greenhouse. Most greenhouse plant insects damage the host plant by piercing leaves and stems with sucking mouthparts, and inhibit the healthy growth of the plant by draining it of sap. They can also be disease vectors: some types of thrips are known to carry necrotic spot virus, which damages foliage and causes stunted growth.