Control weeds outside the greenhouse to help prevent movement of their seeds into the greenhouse. Weed control in the area around the greenhouse will also reduce the chances that insects and plant pathogens that may live and reproduce on weeds will move into the greenhouse. If insects such as aphids, leafhoppers, lygus bugs, thrips, or whiteflies are abundant on the weeds outside the greenhouse, it may be desirable to control them before the weeds are removed or these insects may move into the greenhouse when the weed host dies.
Weeds outside the greenhouse can be controlled by cultivation or mowing where feasible and with herbicides. Take care when applying herbicide so that it does not move into the greenhouse by drift or runoff. Preemergence herbicides can be used if the soil slopes away from the greenhouse so no soil or water that may contain an herbicide gets into the greenhouse. Vents in the greenhouse and fans should be closed or shut off while herbicide applications are being made so that the herbicide will not be drawn into the greenhouse. Also, volatile herbicides (e.g., oxyfluorfen) should not be used around the outside of a greenhouse.