The first and most important control measure is sanitation. Where possible, screening vents and windows will limit the introduction of wind blown seed as well as insect movement. Concrete or mulched floors will also limit weed establishment. Some weeds will get into the greenhouse. These should be removed manually or by herbicide treatments before going to seed.
Continuous removal is expensive and time consuming.
Currently there are no residual herbicides labeled for greenhouse use. Where weeds are a continual problem, clean up the area, remove the soil or cover it with a mulch. Only under extremely rare circumstances would fumigation be recommended for weed control. Only three herbicides are labeled for use inside greenhouses. There are very specific restrictions on the use of herbicides in greenhouses.