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Weeds can be especially troublesome in greenhouses. They starve plants of nutrients, water and light. They also protect insects, mites, rodents and other pests.
“Not only are weeds a problem in and of themselves ... (but) many greenhouse weeds harbor other pests, including white fly, and may be an alternate host for important viral diseases,” said Joe Neal, a professor of weed science and department Extension leader at North Carolina State University.
And if your greenhouse crops are to be potted outdoors later, “then the weeds tag along,” he said.
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.
“Anything or any being going through the door is a potential carrier of weeds and weed seeds — incoming potted plants, potting soil, muddy work boots, tools and equipment, dogs and even wind,” Stivers said. “Vigilance at the doorway will go a long way toward preventing weeds from becoming established in the greenhouse.”
Controlling weeds outside around the perimeter of the greenhouse means fewer weeds sneaking in through the door, she said.
“Similarly, you could put screening up on any ventilation fans to keep seeds from blowing in,” she added.
Weed management in greenhouses, hoop houses and other enclosed settings can involve prevention, sanitation, landscape cloth, herbicides and pulling by hand.
“Prevention and sanitation are key, but once weeds are in the greenhouse, you want to have an integrated approach to preventing their spread and reproduction,” Neal said.
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. “If there are just a few weeds growing on the greenhouse floor, they 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,” and some, like hairy bittercress, “are quite crafty in how far they can make their seeds travel. 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 labeled 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 vapor, Neal said. “Within a closed structure, these vapors can be trapped. The trapped vapors can injure crop plants that would otherwise not be injured by the same herbicide applied outdoors,” he said.
— Grow organically, but use care. Products labeled “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.”