Transplant Weed Management

Weeds are rarely an issue during transplant production. However, weed seeds can infest growing media leading to weed issues later in production. In greenhouses, weeds are hosts of aphids, whiteflies, thrips, mites, slugs. and diseases. 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), though they show few symptoms. Thrips can then vector the virus to susceptible vegetable crops. Weeds can also carry other plant-damaging viruses that are vectored by aphids. Integrated weed management includes sanitation and physical barriers, along with direct controls, such as hand weeding and 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, which provides an ideal environment for weed growth. Regularly pull any escaped weeds before they go to seed. Repair tears in the weed block fabric.

Herbicide use in a greenhouse when plants are present is not recommended. 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 do not 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. Herbicide 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 can injure plant foliage. Use a dedicated sprayer that is clearly labeled for herbicide use only.

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. 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 Labeled for Greenhouse Use

The symbol ^OG indicates a pesticide is listed by the Organic Materials Review Institute (OMRI) as approved for use in certified organic production.

ammonium nonanoate (Axxe^OG): REI 4h. Non-selective, contact, postemergence herbicide. Avoid contact with desirable vegetation.

caprylic acid and capric acid (Fireworxx^OG): REI 12h. Non-selective, contact, postemergence herbicide. Herbicidal soap. 

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.

indaziflam (Marengo) REI 12h. Preemergence herbicide. Labeled for greenhouse use on greenhouse floors in an empty greenhouse. For control of grasses and broadleafs. 

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.

Weed Control Outside of Greenhouses

In addition to mowing, herbicides may also be used outside of greenhouses. Before spraying weeds around the greenhouse with any herbicide, close 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. Greenhouse vents and fans can also draw in herbicide vapors from herbicides that are applied outside the greenhouse causing severe damage.