Not only are weeds in the greenhouse unsightly they contribute to the increased incidence of disease, in particular virus diseases. Most, if not all of the weeds that are common in greenhouses are secondary or alternate hosts for insects and the viruses they vector. One of the most abundant weeds in the greenhouse is Chickweed. Chickweed loves moist, low light conditions. Exactly the type of conditions under the greenhouse bench. Chickweed is also an alternate host for Impatiens necrotic spot virus (INSV), and 40, yes 40 other plant viruses. Chickweed is also an alternate host for insects such as whiteflies and thrips thus becoming a source of virus for the entire crop. This weed should never be allowed to be established.
Another common weed in greenhouses is Oxalis. Unlike Chickweed, Oxalis likes the sun and likes the heat (Figure 1). Oxalis forms a seed pod that when touched or disturbed the seeds are ejected. Oxalis is a host for Shamrock Chlorotic Ringspot Virus. This virus can infect the commercially-grow Oxalis that is sold for the St. Patrick's Day holiday.
Weeds are not the only unwanted plants that can pop-up in the greenhouse. Volunteer plants can also be a problem. Figure 2 shows a volunteer tobacco plant that appeared in a greenhouse in southern Ohio. If you are growing tobacco, this is not a problem, however, in a bedding plant production situation this is not a plant you want in your greenhouse. Tobacco is a solanaceous plant related to petunia and commercially-grown Nicotiana. It is an alternate host for more than 200 plant viruses, whiteflies, thrips as well as many other bacteria and fungi. Volunteer hosts should be eliminated immediately.
Besides being alternate hosts for viruses, insects and other diseases, weeds can also have an influence on the greenhouse environment. Like all plants weeds transpire water. This in turn helps raise the humidity in the greenhouse and increased humidity equals more disease. Figure 3 shows a greenhouse with lots of weeds. The inside of this house was like a steam bath. There was algae and moisture everywhere. This greenhouse had so much disease you could smell the plants rotting. I have often said that a weed-free greenhouse is a disease-free greenhouse. Figure 4 shows what a weed-free greenhouse should look like. Even though the floor of this house is dirt there is not a weed to be found.
That said, the grower must be very careful when using herbicides to control weeds in the greenhouse. Never apply a herbicide in a greenhouse when there are plants in the house. No matter how careful you are, no matter how much ventilation the possibility of plant damage is very high. The chemistry of many herbicides is based on plant growth hormones and most growers are well aware of the effects of an overdose of plant growth regulators. The safest way to control greenhouse weeds is to keep the area under the benches as dry as possible. If you need to use a herbicide, use it weeks prior to putting plants in the treated house. One of the safest products to use is hydrated lime. You can spread this product liberally under the benches without fear of plant damage. This will kill weeds and help control insects.
