How to get rid of crabgrass in vegetable garden

CORVALLIS, Ore. – Vegetable gardeners declare war on weeds every summer.

Knowing more about weeds can give gardeners a leg up in the fight, said Ed Peachey, a weed specialist for the Oregon State University Extension Service.

"Different kinds of weeds require different kinds of strategies. Weed identification makes all the difference in how you approach the job," Peachey said.

First, gardeners should know how to distinguish between perennial and annual weeds.

Vegetable gardeners most commonly deal with annual weeds, which spread only by seed and die when the weather turns cold. Yet seeds from annual weeds can survive in the soil for several years.

Perennial weeds thrive year after year with root systems that are tough to eradicate. They spread both by seed and creeping root systems.

Here are three summer annual weeds common to Oregon vegetable gardens.

• Common lambsquarters — The seed's leaves and early true leaves are a dull bluish green on top and often a purplish red below. Seed leaves are narrow and oblong- to lace- shaped. This weed can grow up to 5 feet tall. Leaf surfaces, especially on new growth, are covered with a fine white powdery coating. Tiny green stalkless flowers cluster at the tips of the main stem and branches.
• Crab grass — Seedling leaves are light green and smooth. True leaves are dark green and smooth. The leaf blade is around ¼ inch across and up to 5 inches long and pointed. It often grows together in clumps or patches. Sometimes a reddish tint is visible at the base of the leaf. The seed head is unique and resembles an antenna. Leaf stems are flattened.
• Common purslane — Prolific, taprooted annual succulent in the Portulacaceae family. Its reddish stems start from a central root, radiating out like spokes of a wheel. The stems can grow up to 12 inches. Leaves are stalkless, oval, and smooth, varying from ½ to 2 inches in length. It produces small five-petaled yellow flowers. Difficult to kill with cultivation or hoeing because it can survive even when uprooted. Best to hoe and carry plants out of the garden.

It's easier to identify weeds when they flower, according to Peachey, but warns gardeners not to wait that long to do something about them.

"It's best to control weeds when they're small. The smaller the better," Peachey said. "Even if you can't tell which species, know that when the weed is young, it's time to kill it with a hoe or cultivator."

Herbicides are available but better for bigger gardens, Peachey said. The herbicide trifluralin will kill all three species of weeds. But read the label carefully, because it can also kill crops such as corn and cucumbers, Peachey advised.

To manage weeds, dig up with a hoe or pull them out by hand. Get rid of the roots completely and do not compost weeds that are producing seeds. Before you plant in early spring, you can use a propane flamer to kill early weeds. Other strategies include adding mulch and covering the soil with a clear plastic sheet for 4-6 weeks. Or gardeners can apply herbicides. Preventive methods include rotating crops and mixing up weed-control strategies every year. And most importantly, do not let weeds produce seeds.

The PNW Weed Management Handbook offers methods of managing and identifying weeds, including suggestions for herbicides. You can also bring samples of weeds or photographs for Master Gardeners trained by OSU Extension to identify. Find your local Extension office at Local OSU Master Gardener Programs.

Want to learn more about this topic? Explore more resources from OSU Extension: Weeds