9 Management of Greenhouse Weeds

Contents

9 Management of Greenhouse Weeds. 1

9.1 Weeds in the Greenhouse. 1

9.2 Chemical Control of Greenhouse Weeds. 1

9.3 Outside the Greenhouse. 1

 

9.1 Weeds in the Greenhouse

Weeds such as creeping wood sorrel (Oxalis corniculata), hairy bitter cress (Cardamine hirsuta), prostrate spurge (Euphorbia humistrata), and others are persistent problems in greenhouses. Wood sorrel, in particular, can rapidly spread throughout a greenhouse crop. Dehiscent seed pods which disperse seeds by propulsion allow seed to be spread throughout the greenhouse. Not only do these weeds detract from the perceived quality of plants produced, but some are also known to harbor insects such as whiteflies, mites, and thrips. Therefore, the removal of weeds from greenhouse pots, benches, and floors is important for aesthetic and pest management reasons. Several options are available to the greenhouse manager for controlling these pests. The first and most important control measure is sanitation. Keep weed propagules out of the greenhouse by using sterile media, introduce only “clean” plant materials, and use management strategies to control weeds outside of the greenhouse. Where possible, screening vents and windows will limit the introduction of wind-blown seed as well as insect movement. Concrete, gravel or mulched floors will also limit weed establishment. Despite these measures, some weeds will get into the greenhouse. These should be removed manually or by herbicide treatment before seed set. If the weeds are already established in the greenhouse they can be killed by (1) manual removal, (2) emptying the range and allowing the weeds to dessicate, (3) emptying the range and fumigating (see Table 3.3.2), or (4) using a postemergence herbicide (see Table 9.2.1). Each method (except fumigation) will remove only the vegetation that is present but does nothing to prevent reestablishment from seed that will be present. Continuous removal can be expensive and time consuming. Currently no residual herbicides are labeled for greenhouse use. Where weeds are a continual problem, clean up the area, remove residual soil, or cover soil with gravel or mulch. Geotextile fabrics covered by gravel (or other mulches) have been successfully used in many greenhouses. Only under extremely rare circumstances would fumigation be recommended for weed control.

 

Although slimes, algae and molds are not weeds, these organisms can establish in greenhouse beds or walkways in damp condition, in the same areas weeds establish indoors.  New products, such as Terracyte, which destroy membrane integrity of slimes and molds are now available for enhanced control of these infestations both inside and outside the greenhouse.

 

9.2 Chemical Control of Greenhouse Weeds

Only three herbicides are labeled for use inside greenhouses (see Table 9.2.1). There are very specific restrictions on the use of herbicides in greenhouses. Read the label and carefully observe any precautions. When applying any herbicide, the greenhouse should be well ventilated (but not so strongly that air currents will cause drift) or empty at the time of treatment. Although organic-type products, such as acetic acid herbicides, are now available for use outside the greenhouse, they are not labeled for use in the house. These products should be applied with the same precautions, so that greenhouse windows and vents remain closed during external application to minimize drift and volatility issues.

 

9.3 Outside the Greenhouse

The primary objective of weed control outside the greenhouse is to eliminate a major source of airborne weed seed. Perennial weeds such as quackgrass or bindweed may also grow under the foundation and enter the greenhouse through openings or cracks. Many options are available for controlling these weeds outside the greenhouse. Mowing carefully around the greenhouse and perimeter will prevent the majority of weeds from setting seed. However, a vegetation-free strip is recommended immediately adjacent to the foundation. After application of a systemic herbicide such as Round Up Pro, use a geotextile fabric covered with gravel or other inorganic mulch to suppress annual and perennial weed growth. As an alternative to the geotextile or as a supplement when weeds grow in the mulch, postemergent and soil residual herbicides may be used. Treflan, Surflan (oryzalin), and others are often used successfully for short-term annual grass control. Apply Surflan with a calibrated sprayer to achieve a dosage of 2 to 4 lb. AI/A. Surflan may also be mixed with either *Reward or Roundup to obtain both pre and postemergent weed control. It is generally inadvisable to use auxin-type herbicides, such as those labeled for broadleaf weed control in turf, near greenhouses because of their volatility and the exceptional sensitivity of greenhouse crops to phenoxy herbicides. When spraying weeds around the greenhouse it is advisable to close windows and vents to prevent spray drift from entering the greenhouse. Vents and windows may be opened almost immediately after spraying.

 

Because no herbicide will provide complete control, some escapes will occur. Supplement the herbicide treatments with manual removal to keep the greenhouse clean. When sanitation, mulching, postemergence herbicide application, and manual weed removal are combined in a comprehensive weed management program, weed pressure will be reduced, thus resulting in less time spent removing weeds and lower costs for production. In addition, control of weeds under the benches will likely prevent weed infestation in plants growing on the benches and reduce other associated problems such as whitefly, mite, and thrips infestation. Try to prevent weeds in outdoor locations near the greenhouse from setting seed; frequent moving will aid in prevention of seed formation and dissemination.

 

 

 

Table 9.2.1. Herbicides labeled for greenhouse weed control
Use	Herbicide Name
Under benches, in walkways, or around foundation	*Reward (diquat) EPA No. 100-1091
Comments: Postemergent contact-type, nontranslocated herbicide. Good for control of small annual weeds; large weeds will be injured but not killed. Highly toxic: always use recommended safety equipment when spraying Reward. Chief advantages: rapid kill of seedling weeds, application can occur even when a crop is present in the house, low cost, limited injury (cosmetic damage) caused by spray drift injury. Chief disadvantage: relatively high mammalian toxicity.
	Roundup (glyphosate) EPA No. 524-475
Comments: Systemic herbicide that controls both annual and perennial weeds by translocation. Much safer than Reward. Chief advantages: systemic control of annual and perennial weeds and low mammalian toxicity. Chief disadvantage: small amounts of spray drift can severely injure greenhouse crops. Therefore, it is recommended that Roundup be used only in empty greenhouses (between crops) with the fans off. If drift occurs, wash sides of house six hours after treatment; otherwise, condensation containing Roundup may drip on plants.
Under benches, in walkways, and in woody, plant product benches	Scythe (pelargonic acid) EPA No. 53219-7
Comments: Contact-type, nontranslocated herbicide that controls small seedling weeds. Works best when air temperatures are relatively high (>80˚ F). Large weeds often be injured but will not be completely controlled. Chief advantages: lower toxicity than Reward and use while crops remain in the house. Also is the only herbicide that can be used to control weeds growing in woody plant production benches such as rose benches. In all applications, avoid contact with desirable vegetation. Chief disadvantages: higher cost, slower acting with reduced efficacy in comparison to Reward’s activity on larger weed seedlings. Additionally, the odor caused by Scythe can be persistent and offensive to some people.