Managing weeds in the greenhouse

For greenhouse growers, weeds may not be the first — or second or third — problem considered. But, if not taken care of, they can be a source of frustration for any grower and lead to more problems, no matter what type of plants they encroach on.

“Weeds can be a persistent problem in both production and retail greenhouses, and they’re a problem for a couple of different reasons,” says Lee Stivers, an extension educator at Penn State University. “They can be a source of pests, such as aphids, thrips and mites. Growers can be pulling their hair out trying to control these pests in the crop on the bench, because new waves of infestations are coming from weeds in the greenhouse. They can also be a primary inoculant source for a couple of important greenhouse plant diseases, such as impatiens necrotic spot virus and tomato spotted wilt virus.”

How weeds develop

According to Stivers, weeds often come into the greenhouse when seeds are brought into the production space. This can happen in a variety of ways, ranging from mud on workers’ boots to equipment brought inside to irrigation systems.

“It’s pretty easy for seeds to get in there,” Stivers says. “And once they get in and start growing on the floor under the benches, they are setting seeds and contributing to a weed seed bank tremendously quickly.”

Growers should look for weeds on the floor or in protected spots, Stivers says. Most commonly, weeds will grow on the floor, harboring pests and being unsightly. Less frequently, weeds will infest potted plant material, lowering product value and even impeding growth.

This is where weeds can start to be a hassle. Stivers says weeds growing under benches can be perfect hiding places for pests and viruses that can ruin plants.

Preventing damage

According to Stivers, the best way to deal with weeds and the potential problems they can cause is work ahead to prevent them from spreading in the first place.

“They say that an ounce of prevention is worth a pound of cure, and nowhere is that more true than in a greenhouse,” she says.

Stivers says growers can take preventative measures such as practicing basic sanitation and using sterile growing media, and not mixing in field soil, to prevent weeds from taking root. If weeds do pop up and have to be removed, she says they need to be removed and disposed of outside the greenhouse to keep the weed seed bank low.

In her work, Stivers has talked to growers about how they deal with weeds. One operation had no issues with weeds and it was because they had a no-weeds policy in place. “It’s almost like you have to have an attitude to keep them out,” she says.