Start at the door

Prevention is the best way to manage greenhouse weeds

Weeds can be trouble in greenhouses. They starve plants of nutrients, water and light. They also protect insects and other pests.

“Not only are weeds a problem in and of themselves ... ( but) many greenhouse weeds harbour other pests, including white fly, and may be an alternate host for important viral diseases,” said Joe Neal, a professor of weed science at North Carolina State University.

So what’s a greenhouse grower to do?

“When it comes to weed prevention in greenhouses, a good place to start is at the greenhouse door,” said Lee Stivers, an extension educator with Penn State University.

Controlling weeds outside around the perimeter of the greenhouse means fewer weeds sneaking in through the door, she said.

Weed management in greenhouses, hoop houses and other enclosed settings can involve prevention, sanitation, landscape cloth, herbicides and pulling by hand. Some ways to go about it:

Weeding: Spreading landscape cloth over greenhouse floors can be an effective deterrent, but it must be kept free of potting soil, Stivers said. “Weeds growing on the greenhouse floor ... can easily be dispatched by hand-weeding or using a trowel or hoe,” she said.

Sanitation: “Adopt a zero tolerance for weeds in your greenhouse,” Stivers said. “One weed plant can produce hundreds of seeds,” she adds. “Remove weeds from the greenhouse; don’t just throw them in the trash can in the corner.”

Herbicides: Use chemicals carefully or not at all. “Most of the herbicides labelled for greenhouses require that the house be empty at the time of treatment,” Neal said. “Tender, greenhouse-grown plants can be injured by small amounts of herbicide drift.” Many common herbicides can volatilize or evaporate and spread as a vapour, Neal said. “Within a closed structure, these vapours can be trapped. The trapped vapours can injure crop plants that would otherwise not be injured ... applied outdoors.” Grow organically, but use care:

Products labelled “natural” or “naturally derived” aren’t necessarily safe, Neal said. “Common natural products containing acetic acid (vinegar), and natural oils can cause severe eye irritation. So read the label carefully.”