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| Managing
Weeds in Your Greenhouse
Weeds are a persistent problem in both retail and wholesale greenhouses. Weeds are unsightly and harbor insects such as whitefly, aphids, thrips, and other pests such as mites, slugs and diseases. Studies conducted through our UMass Greenhouse Integrated Pest Management program have shown that chickweed, oxalis, bittercress, jewelweed, dandelion and ground ivy are hosts for impatiens necrotic spot virus (INSV) which may be vectored to susceptible host crops by thrips. Therefore, the removal of weeds from greenhouse floors is important for the purpose of pest management as well as aesthetics. An integrated weed management program will effectively manage weed populations. This includes the use of cultural controls (prevention and sanitation), mechanical controls (hand-weeding), physical barriers (fiber cloth), emptying the range and allowing weeds to dry up (solarization) and selective use of chemical control (pre and postemergence herbicides). These methods will only remove the vegetation which is present, but will not prevent reestablishment from seed which will also be present. Even solarization rarely produces sufficient heat to effectively kill weed seed. Prevention Using Herbicides in the Greenhouse Always be sure the chosen herbicide is labeled for use in the greenhouse and carefully follow label instructions and precautions. Herbicides are generally classified according to the stage of weed growth affected. Preemergence herbicides are applied before weeds emerge and provide residual control of weed seedlings. There are currently no preemergence herbicides labeled for greenhouse use. (Note: Surflan (oryzalin) is no longer registered for use in enclosed greenhouses). Postemergence herbicides are applied after the weeds have emerged. In the greenhouse, several postemergence herbicides can be used under greenhouse benches and on the floors. Contact herbicides such as Scythe and Reward are best applied to small succulent seedlings. Large weeds will be burned but not killed. Envoy, a selective herbicide is best applied to actively growing grasses beneath greenhouse benches. Irrigating crops too soon after applying an herbicide can wash it off and reduce its effectiveness. Also, since the listed herbicides are generally nonselective, they should not come in contact with crop foliage. Systemic herbicides such as Roundup are best applied to actively growing weeds when temperatures are above 50°F. Note that Roundup can only be used in an empty greenhouse between crops. Finale is similar to Roundup, in that it is a translocated, non-selective herbicide. However, in contrast to Roundup, Finale produces symptoms more rapidly (often within 48 hours compared to ~7 days with Roundup) but may not control selected perennial weeds as well as Roundup. The following table was adapted from the 1999-2000 New England Greenhouse Floriculture Recommendations. Table 1. Herbicides labeled for use inside greenhouses
Outside the Greenhouse If weeds are currently growing up close to the greenhouse and the plan is to eliminate those weeds, use a knockdown insecticide on the weeds first. This will kill flying insects and prevent them from leaving the weeds and entering the greenhouse through vents. Then use a postemergence, non-selective herbicide to kill existing vegetation. With so few herbicides available for controlling weeds, it is important to practice exclusion and sanitation as part of a routine integrated crop management program. The following table was adapted from the 1999-2000 New England Greenhouse Floriculture Recommendations. References Neal, J. 1997. Greenhouse Weed Control. Horticulture Information Leaflet 570. North Carolina Cooperative Extension Service. 3 pages. New England Greenhouse Floricultural Recommendations 1999-2000. New England Floriculture Inc. Section E. Weeds and Algae. Pages E-1 to E-3.
Prepared by Tina M. Smith Links to Further Resources on the Web North Carolina Cooperative Extension's Horticulture Information Leaflet Greenhouse Weed Control focuses on prevention and non-chemical elimination of weeds, using herbicides as a last resort. Includes a useful table comparing the qualities of herbicides labeled for use inside greenhouses. University of Florida's Cooperative Extension Service has a factsheet titled Weed Management in Enclosed Greenhouses . Consists primarily of a table of chemicals weed control in greenhouses. Although not specifically about Weeds, the University of Kentucky's factsheet Managing the Greenhouse Environment to Control Plant Diseases stresses the importance of sanitation in the greenhouse. Weed Management in Greenhouses is another brief factsheet on this topic, this time from Rutgers Cooperative Extension. (Warning: some recommendations here are Out of Date. Check your current -- and local -- pesticide regulations.) Cornell also has a short section on Weeds and Algae in their excellent booklet on Integrated Pest Management for Bedding Plants Although not specifically about Weeds, the University of Kentucky's factsheet Managing the Greenhouse Environment to Control Plant Diseases stresses the importance of sanitation in the greenhouse. The University of Connecticut's factsheet on Western Flower Thrips discusses the fact that weeds may serve as a host for both WFT and the INSV virus, so that a 15 ft weed barrier is an important part of thrips management. See also our factsheet Weed Management for Outdoor Cut Flowers and the Web Resources listed there. Selected IPM Articles |
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As greenhouses become empty, take the
opportunity to thoroughly eliminate weeds and algae, and sanitize benches
and work areas to prevent problems for the next crop cycle.
