There are herbicidal chemicals on the market that are specifically engineered to keep weed seeds from germinating, stopping the threat of weeds before they emerge from the soil. Within the United States, it is illegal to use these chemicals within greenhouses. This is to protect the gardeners as well as the consumers.
Herbicides that are pre-emergent are effective against weeds. They work by creating a chemical barrier around the seeds. This makes it impossible for new seeds to sprout and take root within the garden. Otherwise, the chemicals can cause young seedlings to die shortly after germination, which is also very effective. These chemicals must be mixed into the soil to have any effectiveness against seeds and are noneffective against plants that have already broken the surface.
Greenhouses are specially designed inside environments that are created to absorb and trap heat. Plants are able to use this atmosphere to better thrive than they would typically do outdoors. Unfortunately, this intense heat makes pre-emergent herbicides extremely volatile and toxic. The fumes released can easily mix with condensation, and irrigate and damage crops and become hazardous to humans. There are no herbicides currently on the market that can be used for this purpose.
Unlike pre-emergent herbicides, post-emergent varieties can be used within greenhouse atmospheres. They are effective means of reducing propagation of plants that have already broken the surface. Selective varieties will kill weeds without harming other crops. Caring for plants, properly watering and fertilizing them, and removing weeds manually are the first steps in keeping weed threats to a minimum.
Granular and Liquid
Herbicides that can be safely used within a greenhouse come in two forms--liquid and granular. Granular herbicides control weeds through slow release, giving gardeners the ability to specifically target difficult areas. Liquid herbicides, on the other hand, often leave chemical runoff that can hurt plants, especially sensitive ornamental species.
Many gardeners have begun to use gluten meal as an effective alternative to chemically produced weed killers. Corn gluten meal applied to the soil surface will reduce the root and shoot development of weeds by up to 275 percent. It must be placed on the soil before weed germination to be effective.