Keep compost weed-free: Time, temperature and turning critical factors

LAWRENCE, Kan. — Compost can be a gardener’s best friend. It can deliver dramatic improvements in soil quality and nutrients to nourish your plants throughout the growing season.

But unless you use the right techniques, you may also find yourself with a bumper crop of weeds.

“Composting is a biological process that decomposes leaves, lawn clippings and other organic materials until they look like rich soil,” said Gary Wade, Extension horticulturist with the University of Georgia. “But many of the rich ingredients in the mix can also be a source of weed seeds.”

When lawns are mowed, for example, seeds can be collected with the clippings. Seeds can also survive in leaf debris or on mature weeds that you pull from your garden.

Time and temperature

If you want to keep weed seeds from sprouting and flourishing in your flower beds or garden, you’ll need to ensure your compost gets really warm. With the proper temperature, the volume is reduced, odors dissipate and those pesky weed seeds die.

Within a week, temperatures in a properly constructed compost pile will reach 130 degrees. That quickly kills many seeds and stabilizes the composted material. But it takes 30 days of exposure to temperatures of 145 degrees or more to kill seeds from tougher weed species.

To achieve that temperature, you’ll need the right mix of materials and moisture.

Among the weed seeds that need high temperatures to decompose are common groundsel (Senecio vulgaris), bird’s-eye speedwell (Veronica persica), round-leaved mallow (Malva pusilla), common lambsquarters (Chenopodium album), spiny sowthistle (Sonchus asper), ladysthumb (Polygonum persicaria), wild buckwheat (Polygonum convolvulus), field bindweed (Convolvulus arvensis) and broadleaf dock (Rumex obtusifolius).

How do you know whether you’ve achieved the right temperature? Though compost thermometers are ideal, you can also simply reach into the pile. If it is uncomfortably hot to the touch, you’ve probably achieved the temperatures you need.

Turn your compost pile

For maximum seed destruction, it’s also important to turn your compost. That’s because the exposed surface and localized internal cool spots give weed seeds an opportunity to survive.

By turning and mixing, you can minimize the problem. Turning also can help you determine when your compost is ready to use.

If the pile fails to reheat after turning, it most likely has reached a biologically stable state. It should have a fine texture and little or no odor. Remember, though, that effective weed control doesn’t end with proper compost preparation.

“The same nutrients that benefit bedding plants and crops will also benefit any existing weeds and give them a real boost,” said Lee Van Wychen, science policy director for the Weed Science Society of America. “So it’s important to make ongoing weed management a priority on composted beds and gardens.”

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