Unwanted plants or weeds can become problematic beneath benches and around walkways of greenhouses. These plants can harbor aphids, whiteflies, thrips, and mites that will damage desirable plants. Weeds can also be a source of viruses, such as tomato spotted wilt virus or impatiens necrotic spot virus, which are transmitted to crop plants by thrips. Weeds and seeds are easily transported into greenhouses by animals, people, and tools, or in infested growing media. Weed seed can also be dispersed inside greenhouses by water dripping from hoses or plant containers.
Several simple measures can be used to prevent weed dispersion in greenhouses. Maintaining a weed-free zone around the outside perimeter of the greenhouse will minimize seed brought inside by foot traffic. Also, fine-meshed screens on greenhouse vents or other structural openings will reduce entry of wind-blown weed seed and insects. Also, the use of sterile media will help limit weed infestations. When bringing container-grown plant material inside, especially for overwintering, make sure pots are weed-free before placing them in the greenhouse. Also, clean tools after each use to minimize weed seed dispersion or disease transmission.
Always try to control weeds at a young stage before they flower and produce seed. Just one common chickweed plant can produce and release as many as 800 seeds (Figure 1). The three common types of pigweed usually produce 10,000 to 30,000 seeds per plant. Thus, just one or two uncontrolled seed-bearing weeds can result in rapid multiplication of these unwanted plants in a greenhouse.
Weed block fabric can be used to cover areas beneath benches and walkways and prevent weed emergence. If the fabric is covered by gravel, spilled potting media, or plant debris, this provides an environment conducive for weed seed to collect and germinate. Thus, periodic sweeping of the fabric to remove spilled media and plant debris will aid in weed control.
When weeds become problematic in a greenhouse, they can be removed manually or eliminated using a post-emergent herbicide. Few herbicides are labeled for use in enclosed structures, such as greenhouses due to the potential for crop injury to desirable plants or harm to human health. Some formulations of herbicides, labeled for outdoor use only can volatilize (change from a liquid to a gas) and then drift away from the target area, causing harm to other crops. However, herbicides labeled for greenhouses are usually low-drift products and are applied when air-circulating fans are turned off.
A good time to apply a herbicide is when a crop production cycle is completed and the greenhouse is empty. Roundup® Pro (glyphosate) may be used for weed control at this time. When the greenhouse is weed-free, a preventative pre-emergent herbicide such as Marengo® (indaziflam) can be applied to gravel or the ground beneath benches before ornamental (non-edible) crops are placed in a greenhouse.
Most post-emergent herbicides labeled for greenhouse use are non-selective (i.e., will kill any plant contacted with spray), except for Fusilade® II (fluazifop-butyl) and Envoy ®Plus (clethodim) which control grasses only. Also, some herbicides, such as Finale® (glufosinate) and Reward® (diquat) may only be applied in greenhouses where ornamental (non-edible) crops are grown. In contrast, Axxe® (ammonium nonanoate), Scythe® (pelargonic acid), Reward® (diquat), and WeedPharm® (acetic acid) can be applied in greenhouses where edible crops are grown. Also, Sporatec® fungicide, which contains rosemary, clove, and thyme oil as active ingredients, can be used to control mosses, liverworts, and hornworts in greenhouses where edible crops are grown.
REVISED: February 21, 2017