While greenhouses provide a way for individuals to grow exotic plants or vegetables throughout the year, there are a number of pests that can get into a greenhouse and cause problems for the plants. Animals like rodents and insects, as well as fungi, are common greenhouse pests.
Mildew is one of the most common greenhouse problems, because a greenhouse is a moist environment that is closed off from dry air and sometimes air movement, especially in greenhouses without fans. Mildew appears as a white or grey powdery substance; it often begins growing in the corners, along windows and can even begin to grow on the plants themselves if left untreated. Clean the greenhouse each spring and fall to reduce the likelihood of mildew problems.
Mice are a problem in many greenhouses, as they come into the building for warmth and to eat greenhouse plants. Mice droppings are unhealthy for humans, and exposure to them can cause problems. The presence of mice droppings, especially around the storage area of planters, indicates a mouse problem. Additionally, mice will sometimes eat greenhouse plants and vegetables. Cleaning out the storage area to prevent hiding places for mice and laying traps can solve this issue.
As in the outdoors, aphids cause major plant damage when they appear. An aphid infestation can damage or kill many plants, especially delicate exotic plants that don't handle insects well. The clearest sign of aphids is the presence of small holes appearing in the leaves and stems of plants. Aphids can be taken care of in two ways. Chemical pesticides are effective in killing aphids but can also damage plants. Releasing ladybirds or lacewings to kill aphids is also effective and less harmful to some plants.
Whiteflies are white insects that are about half an inch long; they commonly infest greenhouse vegetable gardens. Whiteflies prefer poinsettias, tomatoes, fuchsias, cucumbers and lettuce and breed and live on the undersides of leaves. Heavy applications of fertiliser are necessary to control a whitefly infestation, according to the University of Kentucky Agricultural Extension. Additionally, releasing a small, parasitic wasp called Encarsia formosa will deplete the number of larvae, since the wasps parasitise them.
Although insects and fungal pests can cause a great deal of plant damage if left untreated, there are other factors that need to be taken into account, as well. Even sunlight can become damaging to greenhouse plants. If the greenhouse is too warm, young plants and exotic plants are in danger of being scorched, which can kill them. Shade delicate plants with hardier ones or invest in greenhouse blinds and use ventilation properly during the hottest parts of the day.