PROSTRATE KNOTWEED & ITS CONTROL Deborah L. Brown Extension Horticulturist

Prostrate Knotweed (Polygonum aviculare) is a low-growing, broad-leaved (non-grass) annual that germinates very early in spring. The mass of young seedlings are sometimes mistaken for grass or crabgrass, but as they grow they branch and spread widely across the ground.

Prostrate knotweed's branches do not root to the ground as they grow. They may extend a foot or more in length, so one sprawling plant could extend over two feet across. This low-growing plant is anchored by a thin, white taproot. Tough, wiry branch stems are covered by small, oval, blue-green leaves. When stems or leaves are broken, any sap that exudes from the wounds is clear, not milky. At each point along the stem where a leaf is attached, there is a small, papery sheath.

Prostrate Knotweed (Polygonum aviculare) Illus. Common Weeds of the United States

Prostrate knotweed strongly prefers soil that is hard, compacted, and poorly aerated. It also seems to be rather salt tolerant, but not shade tolerant. Thus it typically is found along streets where plows have piled road salt-laden snow and slush, along paths and sidewalks, and hard trampled, sunny areas in lawns.

Controlling prostrate knotweed will not be successful unless conditions which favor its growth are changed. Relieving soil compaction is most important. For small areas, this might be done by using a spading fork or a pitchfork. The ground should be in a good workable condition when you work on it, neither dry and hard nor mushy and soggy. Push the fork in about every 6" to 12", pull back slightly, and pull out of the ground.

For larger areas, rent a machine called a core aerator or "plugger" to take plugs of soil out of the ground. Some lawn services also can do the work for you. The best time to aerate is between August 20 and September 15. Follow aeration with overseeding to help replace thin, weed- infested turf with good grass.

Other cultural practices that promote lawn growth but lessen knotweed include high mowing (2½" to 3½") in hot weather, thorough, deep watering during summer and other dry periods, and fertilizing, primarily in the fall.

Broad-leafed herbicides can also be used to eliminate young knotweed. A mix of 2-4,D and MCPP (mecoprop), can work when used early in the season. It may have to be used more than once to achieve good control. A mix of 2,4-D, MCPP, and dicamba may be more effective, but be careful not to use it beneath young trees as it can be absorbed by their roots.

The best time to spray knotweed is in spring, when wind is calm and temperatures range between the high 50s and the low 80s and no rain is expected for 24-48 hours. The weeds must be growing actively. Be sure to read and follow the product's label instructions and precautions.	Illus. Weeds of the North Central States
Lawn chemical application companies may be able to prevent knotweed with an early application of the pre-emergent herbicide isoxaben, which is not available to homeowners directly.

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Reviewed 9/98