Pre-emergent herbicides are chemicals that are used to control weeds. As their name suggests, these chemicals work by killing weeds before they emerge. Unfortunately, chemical pre-emergent herbicides are not legally permitted to be used in greenhouses, due to the fact that doing so poses too many risks.
There are two possible modes through which pre-emergent herbicides work. They may either inhibit cell division in the young root system of weeds, causing the young seedling to die shortly after germination, or they may work by creating a barrier seal around the seed, making it impossible for the seed to sprout.
Pre-emergent herbicides must be applied before annual weeds emerge. Generally, this is early in the spring. Pre-emergent herbicides are applied directly on the soil.
There are no chemical pre-emergent herbicides currently available for use in greenhouses. The reason for this is that these pre-emergent herbicides are extremely volatile in the greenhouse environment due to the high temperatures. Volatility results in the release of gases that can easily injure the entire greenhouse crop. Many pre-emergent herbicides can move and/or accumulate in greenhouses to toxic levels for crop plants. The soluble active ingredients can persist and mix with greenhouse condensation and rain down, damaging water drops onto your crops. Even though an herbicide may be labeled for use in a crop, it must specifically indicate that it's allowed to be used in greenhouses, in order to be legal and safe.
Post-emergent herbicides can be used in the greenhouse to reduce weed populations. Post-emergent herbicides are also effective at preventing the weeds from flowering and seeding, therefore reducing propagation. If the weeds get really bad, consider the viability of removing the plants from the greenhouse and transferring them outdoors where there are more chemical control options. After a greenhouse crop has been harvested, remove all weeds. Many weeds prefer damp conditions; keep humidity under control in the greenhouse by not over-watering plants, and by leaving adequate space between plants.
Until recently, most pre-emergence weed killers have been chemically produced. New to the market is an organic alternative: corn gluten meal. According to a study by Bingham et Al, corn weed blocker, made from 100 percent corn gluten is an effective pre-emergence herbicide. Like other pre-emergence herbicides, it must be applied to the soil before weed germination. In the study "Greenhouse Screening of Corn Gluten Meal as a Natural Control Product for Broadleaf and Grass Weeds," corn gluten meal was evaluated under greenhouse conditions, for its efficacy in controlling weeds. The results of the study were that root and shoot development of weeds was reduced by 275 percent. Although chemical herbicides are more effective; due to the fact that they are not labeled for use in the greenhouse environment, corn gluten is a possible alternative.