Prostrate knotweed can form a dense mat on the ground, outcompeting other vegetation.

The enemy

Prostrate knotweed (Polygonum aviculare L.) This annual was introduced to our country as a contaminant in crops and landscape materials. This invader is a close relative of the Japanese knotweed that grows like bamboo, but it grows flat to the ground, produces very small white to pink flowers at the joint of its leaves and stems and normally blooms from spring to mid-summer.

I have seen this plant colonize itself into a great outdoor carpet (is this always bad? I think not, better than dust or mud). It has small 1⁄2-inch to 1-inch-long oblong leaves. This plant generally grows along the driveways, landscaped and other highly compacted sites. This plant is also not a succulent, which differs it from a lookalike weed, prostrate purslane.

As with many annuals this plant produces thousands of seeds per plant, thus it spreads throughout a landscape, driveway, or into fields. It is great at robbing the soil of nutrients as well as covering the surface of the soil so that sunlight cannot penetrate to assist smaller plants in growing. The plant winds itself around other plants thus can inhibit the desired vegetation from growing erect.

This plant mostly spreads in dirt and mud from equipment or footwear, so make sure that you are not spreading it from site to site by cleaning your shoes and equipment. In older barnyards it will become like a carpet (not necessarily bad as it is a good ground cover).

The defense

As this is an annual it spreads rapidly and produces many seeds. Preventing the plant from going to seed is very tough. Hand pulling it in moist soil works, but almost always just breaks off at the root when you are trying to pull it along a sidewalks, driveways, or other traffic sites. If you see it along a landscape in a crop field you can selectively kill it by using 2,4-D and Roundup (make sure that you don’t overspray or drift onto desirable vegetation). Using products that are true pre-emergence are the best herbicides as the weed dies just after germination. These products in a landscape may include Preen, Snapshot, Gallery, Dimension, and others.

The majority of agriculture herbicides will keep this weed from becoming a problem. Most people see the plant in late summer as it may be the only weed left being green, unfortunately it is too late for most herbicides as the plant has already flowered and produced seed.