Weed of the Month: Prostrate Knotweed
 | Author:
Alison Kosto, Broadwater County Extension Agent
MSU Broadwater County Extension Agent |
MSU Extension Corner
Weed of the Month: Prostrate Knotweed
Allison Kosto, MSU Broadwater County Extension Agent
Having an uncanny ability to grow where other plants cannot on compacted soils, between sidewalk cracks and on driveways makes prostrate knotweed a frustrating weed!
Description & Habitat
Native to Europe, prostrate knotweed is now found throughout North America and is widespread in Montana. Prostrate knotweed prefers full sun and drier soils. It has the ability to grow in poor soils where other plants cannot and is often found in highly compacted soil including road and driveway edges, sidewalk and pavement cracks, pathways, waste areas and lawns. It can withstand some trampling. An annual plant (life cycle of one year) in the Buckwheat family, it is a low-growing plant that can form mats along the ground. Stems are wiry with many branches and can grow up to two feet long. Mats can be four feet in diameter. Leaves are very small and lance-shaped. The flowers are inconspicuous. It has a small taproot and only reproduces by seed. Seeds mature fairly early in the summer. Prostrate knotweed is often confused with spotted spurge which has a similar look and growing pattern. An easy way to distinguish them is that spotted spurge has a milky white sap when the stems are broken. The plant can quickly cover bare soil and can prevent native or desirable species from establishing. Prostrate knotweed is also a host for several fungi, viruses and nematodes.
Management
Thankfully, prostrate knotweed weed is fairly easy to control. The most successful defense is a healthy lawn. Prostrate knotweed thrives in compacted soil, so aeration may improve weed control. As with most annual weeds, the key is to prevent the plant from going to seed. If you find it in your lawn, hand pulling can be highly effective, and the root is usually easily pulled. If the plants are along a driveway or sidewalk, water the area before pulling and the plant should pull right out. Mulch, at least three inches deep, is also an option where practical. Since the plant prefers compacted soil, tilling will discourage development but is often not practical where this weed is typically found. Mowing is not effective because the plant grows along the ground, below mower height.
Herbicides can also be used to control prostrate knotweed. Commonly used chemicals include 2,4-D, Banvel, Chaser, MCPP, SpeedZone, Trimec, TZONE and Roundup. For best control, spray herbicides at the seedling stage. As the plant becomes more mature, it will become more resistant to herbicides. The plant is moderately resistant to 2,4-D even at the seedling stage, and it should be combined with another herbicide. Pre-emergent herbicides can also be effective in lawns and perennial landscapes. Always read and follow the entire label when applying herbicide. Common chemical names are used for clarity but does not imply endorsement of a product or brand.
For assistance on weed identification and management, contact the MSU Extension Office in Broadwater County at 406-266-9242.
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