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.

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
The most important means to manage weeds is sanitation. This involves using weed block fabric to cover the floor and removing any weeds that grow in along the edges. Weed seedlings can be removed either manually or by using an herbicide before weeds go to seed. It is best to leave the fabric covered floor bare so it can be easily swept. Some growers have covered the fabric mulch with stone or other material. This creates a nice environment for weed seedlings to germinate after media has fallen onto the floor and settled in the gravel.

Using Herbicides in the Greenhouse
Few herbicides are labeled for use in a greenhouse due to the potential for crop injury or death. 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 (herbicides changing from a liquid to a gas). Herbicide vapors can easily buildup within an enclosed greenhouse and injure susceptible plants.

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 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 and Touchdown Pro are best applied to actively growing weeds when temperatures are above 50°F. Note that Roundup and Touchdown Pro 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.

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 2005-2006 New England Greenhouse Floriculture Recommendations.

Table 1. Herbicides labeled for use inside greenhouses.

Trade Name	Common name	Use w. crop in GH	Target Weeds	Comments
Envoy Valent USA Corp.	Clethodim	Yes	Selective, contact, post-emergence weed control. Annual and Perennial grasses.	Does not control sedges or broadleaf weeds. Apply to actively growing grasses from 2”-6” tall. 24 hour REI.
Finale Bayer Environmental	Glufosinate- ammonium	Yes	Nonselective, systemic, postemergence weed control. Annual and perennial grasses, broadleaf weeds	Apply to actively growing weeds under benches. Avoid drift and direct contact with desirable vegetation. Do not use in greenhouses containing edible crops. Lacks residual and pre-emergence activity. 12 hour REI.
Scythe Dow AgroSciences LLC	Pelargonic Acid	Yes	Nonselective, contact, postemergence weed control. Annual and perennial broadleaf and grass weeds as well as most mosses and cryptogams	Apply to young, succulent weeds. Cool or cloudy weather may slow down activity. Thorough coverage needed. Do not use over desirable plants. Lacks residual and pre-emergence activity. 24 hour REI.
Reward Syngenta	Diquat dibromide	Yes	Nonselective, contact, cell membrane disruptor.	For general weed control beneath benches. Avoid contact with desirable foliage as injury may occur. Do not use on food crops. High mammalian toxicity.
Roundup ProDry Monsanto Co.	Glyphosate	No	Nonselective, systemic, postemergence weed control for non-crop areas. Annual grasses and broadleaf weeds	Do not mix, store, or apply in galvanized steel containers. Desirable vegetation must not be present. Air circulation fans must be turned off. Lacks residual and pre-emergence activity. 4 hour REI.
Roundup Pro Monsanto Co.	Glyphosate	No	Nonselective, systemic, postemergence weed control for non-crop areas. Annual and perennial weeds	Do not mix, store, or apply in galvanized steel containers. Desirable vegetation must not be present. Air circulation fans must be turned off. Lacks residual and pre-emergence activity. 4 hour REI.
Touchdown Pro Syngenta	Glyphosate	No	Nonselective, systemic, postemergence weed control for non-crop areas. Annual and perennial weeds	When applying Touchdown Pro to control weeds in and around greenhouses, desirable vegetation must not be present and air circulation fans must be off. No residual soil activity.

Outside the Greenhouse
Managing weeds outside of the greenhouse is important to eliminate a major source of air borne weed seed and to prevent perennial weeds such as bindweed from growing under the foundation and into the greenhouse. Weed control around the greenhouse will also reduce populations of flying insect pests. There are several options for controlling these weeds. One option is mowing. Mowing, when done regularly, can prevent the majority of weed seed formation. A better solution if possible, is to maintain a weed free barrier around the greenhouse. Some sources suggest that a 10 to 20 foot weed free barrier around the greenhouse is adequate. Weed block fabric mulch or postemergent and soil residual herbicides may be used. Surflan (orzyzalin) has been used successfully for residual weed control or Surflan combined with Finale or Roundup can be used for post and preemergent weed control. Do not use auxin-type herbicides, such as those labeled for broadleaf weed control in turf or brushkillers, near greenhouses. While spraying weeds around the greenhouse with any herbicide, close windows and vents to prevent spray drift from entering 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.

References

Neal, J. 1997. Greenhouse Weed Control. Horticulture Information Leaflet 570. North Carolina Cooperative Extension Service. 3 pages.

New England Greenhouse Floricultural Recommendations 2005-2006. New England Floriculture Inc. Section E. Weeds and Algae.

* * *

Updated 2/05
Prepared by Tina M. Smith
Extension Educator, Floriculture Program
Dept. of Plant & Soil Sciences
University of Massachusetts

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.

• 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.)

• 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.