Prevention and sanitation are the grower's first line of defense. Weed seeds are easily blown into the greenhouse though vents and other openings. The use of a physical barrier such as a weed block fabric will help to limit weed establishment on greenhouse floors. Leave the weed fabric bare so it can be easily swept. Covering the weed fabric with gravel will make it difficult to remove any spilled potting media providing an ideal environment for weed growth.
Hand pull any escaped weeds before they go to seed.
Neither of these measures will prevent reestablishment from seed. Herbicide vapors are easily trapped within an enclosed greenhouse and can then injure desirable plant foliage. Always be sure the herbicide selected is labeled for use in the greenhouse. Carefully follow all label instructions and precautions. It is the applicator's responsibility to read and follow all label directions.