It is difficult to control weeds in greenhouses because the types of plants grown are generally sensitive to herbicides and weeds are often hard to reach with an herbicide application. Only one preemergence herbicide is registered for use in greenhouses (indaziflam) and that is for beneath benches and greenhouse floors only. Only a few weed species are common problems in greenhouses. They are all closely associated with high moisture and nutrients and spread rapidly if they are allowed to become established.
The most common weeds in and around greenhouses include annual bluegrass, common chickweed, creeping woodsorrel, lesser-seeded bittercress, liverwort, moss, and pearlwort. Others that may be present include cheeseweed, cudweed, fireweed, prostrate and spotted spurge, sowthistle, and willowherb. Controlling these weeds will also help reduce the reservoir of insects and plant pathogens that are often associated with weeds.
Reducing standing water or high moisture around the interior perimeter of the greenhouse will help control these weeds as they often become established there and can spread from those habitats.
Sanitation is the best method for weed control. Weeds may be brought into the greenhouse in potting mix or with cuttings, bulbs, or other plant material or on dirty pots and tools. If weeds do get in, they should never be allowed to flower and seed. This is especially true of bittercress and creeping woodsorrel (oxalis). Maintain trash cans in the greenhouse for weeds that are pulled during maintenance, so they can be readily removed before flowering. Hand-weed frequently (daily or weekly) so no weeds go to seed. If the floors are concrete, regularly wash or sweep away soil that drops to the floor so that weeds will not establish or seed. When crops are rotated, clean weeds out of the greenhouse and clean benches, tubing, and walls to remove seeds that may be sticking to them. Irrigate with water that is free of fungal spores and weed seeds.
If using raised or self-contained beds, pasteurize the soil before planting by either steaming or solarizing. Solarization is described in GENERAL METHODS OF WEED MANAGEMENT and steam fumigation in CONTAINER NURSERIES.
Only Marengo (indaziflam) is currently available for preemergence use in greenhouses and only under benches or on the floor, not in the container. Many of the other preemergence herbicides are quite volatile at greenhouse temperatures and can move or accumulate or both in greenhouses to levels toxic to crop plants. Even though some herbicides may be labeled for use in a crop, the label must specifically state that it can be used in greenhouses to be legal and safe.
On the greenhouse floor and under the benches a postemergence herbicide can be used to reduce weed populations and to keep the weeds from flowering and seeding. Provide good drainage and level the gravel or soil under the benches to reduce water collecting in low areas. Wet areas increase the chance of liverwort and moss infestations. Air movement at the floor level will help dry off the floor and will also reduce the chance of infestations of weeds favored by wet areas. After a crop has been harvested, remove any weeds to keep them from seeding so new seeds will not be added to the seedbank in soil.