Weed management strategies in greenhouses – Part 1: Non-chemical weed control strategies
Non-chemical weed control methods inside greenhouses and enclosed structures for ornamental crop production.
Weeds are a persistent problem in greenhouses, hoop houses, propagation houses and other enclosed structures. Hairy bittercress (Cardamine hirsuta), yellow woodsorrel (Oxalis stricta), spotted spurge (Euphorbia maculata) and liverwort (Marchantia polymorpha) are some of the most common weeds that are found in these structures. Weeds can be found growing within the containers of stock plants, liners and finished plants (Photo 1), in container drain holes, under benches and near walkways within the structures. Weeds growing within the containers compete with crops for space, nutrients, water, light and oxygen, thereby reducing the overall growth and market value.
Any weeds growing within the structure can also harbor pests and diseases. Pest examples include insects such as whitefly and thrips, and other pests such as mites, slugs and snails, according to Neal, 2015. Bittercress and woodsorrel are known to be hosts for impatiens necrotic spot virus and tobacco spotted wilt virus, which may be vectored to susceptible host crops by thrips according to Neal, 2015.
For these reasons, it is important to regularly scout and manage weeds. In the first part of this article series, we will be discussing some of the effective non-chemical methods for weed control that can be applied inside greenhouses and other enclosed structures.
Prevention and sanitation practices
Proper sanitation and adopting preventive measures are the first and foremost step in an effective weed control program. Using clean and sterile substrate or media and containers are recommended for growing ornamentals. Avoid storing media under the benches and if reusing containers, wash them thoroughly with pressurized water and chemical disinfectants to remove weed seeds, pathogens and dirt. Make sure the storage areas for bulk goods and racks are not infested with weeds or debris. Concrete floors and weed barrier fabrics can help to reduce weed seed germination. Chemical disinfectant products such as quaternary ammonium chlorides are available in the market, which can be applied to hard and concrete surfaces to avoid algae, liverwort (Marchantia polymorpha) and moss growth.
Weed seeds and propagules (tubers, rhizomes, etc.) can be dispersed via human activity, therefore, tools and equipment need to be cleaned thoroughly each time they are used. Gloves and footwear should also be cleaned and disinfected thoroughly.
Weeds growing immediately outside the greenhouse need to be controlled to reduce the introduction of weed seeds through vents or sidewalls (Note: extreme caution is needed if using herbicides in these areas to prevent crop injury.). According to Smith, 2019, screen exclusions on vents may also help reduce the amount of wind-borne weed seeds from entering the greenhouse.
Finally, regular scouting inside the greenhouses and inspecting new shipments, including stock plants, for small weeds are critical, proactive steps towards weed prevention, according to Marble and Pickens, 2015.
Cultural control practices
Water use can contribute to weed germination and growth within enclosed structures. Algae, moss and liverwort all thrive in moist conditions and therefore overwatering can lead rapidly to their growth and spread of these weeds on container media, particularly on crops with long production times. Nostoc (bluegreen algae, similar to algae) is another problematic weed that causes surfaces to become very slippery, creating safety issues for greenhouse workers. Ensure irrigation systems are uniformly applying water and take weather patterns that decrease water demand by the crop (cool, cloudy weather) into consideration to avoid overwatering.
Properly maintaining greenhouse drains will also prevent water collection on the floor and surfaces. A less obvious consideration is reducing humidity levels, when possible, through ventilation and plant spacing as high humidity may increase weed germination rates, according to Marble and Pickens, 2015.
Strategic placement of controlled released fertilizers in containers can further help to reduce weed germination, emergence and growth. Instead of topdressing with fertilizers, incorporation or subdressing is recommended as it reduces weed access to fertilizers in the top 0.5-1 inch of media where they germinate. Topdressing larger containers with 1 to 2 inches of organic mulch materials, such as pine bark or rice hulls, is another strategy that can create a physical barrier and help reduce weed emergence and growth.
Manual removal or hand weeding
Frequent scouting and hand pulling of weeds inside the greenhouse can minimize the chances of the weeds to become established and generate more seeds. Some common weed species such as yellow woodsorrel and bittercress can produce thousands of seeds per plant and are known to have nearly a 100% germination rate. Therefore, it is highly recommended to hand pull these species when young, prior to flowering. After hand pulling weeds, remove weed materials from the greenhouse floors as these weeds are resilient and can re-root in the humid conditions, according to Marble and Pickens, 2015.