![]() Read the label and carefully observe any precautions. They are not for use in pots or ground beds where crop plants are growing. With one exception, the products described below are only for use under benches, in walkways and around the foundation of the greenhouse. There are very specific restrictions on the use of herbicides in greenhouses. Postemergence Control of Greenhouse Weeds Skip to Postemergence Control of Greenhouse Weedsįive postemergence herbicides are labeled for use inside greenhouses. Other chemical fumigants currently on the market, such as Basamid and Metham Sodium, are not well adapted to greenhouse fumigation but may be useful in some situations. The most effective fumigant, Methyl Bromide, is no longer available. Soil / substrate temperature must remain at or above 180☏ for at least 30 minutes to kill most weed seed. In plant beds or under benches, steam sterilization can be effective. The soil and media should be moist but not wet and between 50 and 80☏. Before fumigating, kill above-ground portions of existing weeds and wash the interior walls, benches, and glass or plastic to remove and moisten weed seed. However, if fumigation is considered for other reasons, be sure the conditions are right for weed seed control as well. Only under extremely rare circumstances would chemical fumigation be recommended for weed control. ![]() Prior recommendations suggesting that hours of heating were required were based on soil solarization research with lower temperatures. The latest literature states that you only need to hold this temperature for a few minutes to kill weed seeds. Your goal is to reach about 175 F (~80C). Regardless of the method, it is important to monitor the soil temperatures. There are surface steam systems that are manually operated and are convenient for treating smaller areas, or areas with obstructions that make it impractical to cover the ground with a tarp. The typical method is to place steam emiter hoses on the ground, cover with with a tarp, then inject steam. Steam sterilization can be used to kill soil-borne pathogens and weed seeds, and is gaining in popularity. Geotextiles or "weed block" fabrics covered by gravel (or other mulches) have been successfully used in many greenhouses. Where weeds are a continual problem, clean up the area, remove the soil or cover it with a mulch. Continuous removal of weeds is expensive and time consuming. Even solarization rarely produces sufficient heat to effectively kill weed seed.
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