In greenhouses, weeds are a primary source of insects such as aphids, whiteflies, thrips, and other pests such as mites, slugs and diseases. Low growing weeds help maintain moist conditions, a favorable environment for fungus gnats and shore flies. Many common greenhouse weeds such as chickweed, oxalis, bittercress, jewelweed, dandelion and ground ivy can become infected with Tospoviruses including impatiens necrotic spot virus (INSV) and tomato spotted wilt virus (TSWV) while showing few, if any visible symptoms. Thrips can then vector the virus to susceptible vegetable crops. Weeds can also carry other plant damaging viruses that are vectored by aphids. An integrated weed management program will help to effectively manage weed populations. This approach includes preventive measures such as sanitation and physical barriers, and control measures such as hand weeding and the selective use of postemergence herbicides.
The use of a physical barrier such as a weed block fabric is an effective method to limit weed establishment on greenhouse floors. The weed fabric should be left bare so it can be easily swept. Covering the weed fabric with gravel makes it difficult to remove any spilled potting media providing an ideal environment for weed growth. Regularly pull any escaped weeds before they go to seed. Repair tears in the weed block fabric.
Overall, it is best to avoid herbicide use in a greenhouse when plants are present. However, if herbicides are used read and follow all the information below and consult labels. If the label does not say that it can be used in the greenhouse, then don't use it. Herbicides that are not considered volatile in field situations can cause significant injury through vapor movement in warm and enclosed structures.
Few herbicides are labeled for use in a greenhouse due to the potential for severe crop injury or death to desirable plants. This injury may occur in a number of ways including: 1) spray drift if fans are operating at the time of application; and 2) volatilization (changing from a liquid to a gas). Herbicide vapors are then easily trapped within an enclosed greenhouse and 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. Use a dedicated sprayer that is clearly labeled for herbicide use only.
Avoid use of preemergence herbicides in the greenhouse! Preemergence herbicides are applied to soil to prevent the emergence of seedlings. They can persist for many months and in some cases over a year. Preemergence herbicides can continue to vaporize in the greenhouse, causing significant damage to young transplants. Only one preemergence herbicide, indaziflam (Marengo) is labeled for greenhouse use on greenhouse floors in an EMPTY greenhouse.
Postemergence herbicides are applied after the weeds have emerged. Several postemergence herbicides can be used under greenhouse benches and on the floors. Contact herbicides are best applied to small seedlings. Large weeds will be burned but not killed.
Herbicides for Use in Greenhouses
Ammonium nonandate (AxxeOG): REI 4h. Non-selective, contact, postemergence herbicide. Avoid contact with desirable vegetation.
Glyphosate (Roundup Pro, Roundup Pro Concentrate): REI 4h. Non-selective postemergence herbicides. Translocated/systemic. For use in an empty greenhouse between crops and outside greenhouses.
Pelargonic acid & related fatty acids (Scythe): REI 12h. Non-selective, postemergence, contact herbicide. Cool or cloudy weather may slow down activity. Provides no residual weed control but leaves a strong odor. For use when crops are in the greenhouse.
The symbol OG indicates a pesticide is listed by the Organic Materials Review Institute (OMRI) as approved for use in certified organic production.
Weed Control Outside Greenhouses
In addition to mowing, herbicides may also be used outside of greenhouses. Before spraying weeds around the greenhouse with any herbicide, close windows and vents to prevent spray drift from entering the greenhouse. Avoid using auxin-type herbicides, such as those labeled for broadleaf weed control in turf or brush killers, or herbicides with high volatility near greenhouses. Select herbicides with low volatility.