“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.”