Least-Toxic Control of Knotweed Choose a different pests


Pest type: Plants

According to UC IPM:
Common knotweed, Polygonum arenastrum, is also known as wiregrass, wireweed, matweed or doorweed. It is an annual species that is native to Europe that has established itself throughout most of the United States and Canada. Common knotweed germinates in late winter or early spring, when sufficient moisture is available. It often germinates in soil cracks in compacted soil. Though it germinates in early spring, it grows slowly and upright before becoming prostrate. If mowed, it remains prostrate and spreads. It can form mats 3 to 4 feet in diameter. Seeds develop on the plant low to the ground and seedlings readily survive mowing. Like other species in the genus Polygonum, seed in the soil are probably long-lived. Flowering may occur from March through October.

Polygonum argyrocoleon or silver sheathed knotweed grows more erect to a height of one foot or more. It has long rose colored flowered spikes.

Is it a problem?

Weeds may cause allergies or lead to skin rash on contact. Most weeds are simply a nuisance because they are considered unappealing in a lawn. A few weeds should be tolerated, but weed infestations that overtake turfgrass are signs of unhealthy soil.

Pest prevention practices

Remove potential habitat
Foster natural resilience

In-Depth Information:

Common knotweed readily invades areas where other weeds may have difficulty surviving such as trampled and compacted areas. It is tolerant of soils compacted by trampling with foot traffic and is therefore frequently found along paths and walkways. Examples include turf, roadsides, sports fields, vacant lots, gravel parking areas, gardens, agricultural crops, foot paths, and dirt roadways. It is particularly troublesome in alfalfa fields, where soil is compacted from wheel traffic. In turf it invades open areas caused by heavy wear.

Having healthy soil, using at least two native turfgrasses, proper watering, and a sensible landscape design can all help your lawn ecosystem develop a natural resistance to weeds. Indeed, knotweed and other weeds in your lawns are often a sign that the balance of your soils are off. Low pH and soil compaction tend to promote knotweed growth, so aerate your lawns and apply lime to raise the pH levels. A lack or excess of nutrients can also add to the problem. Knotweed is commonly found in soils with an excess of potassium and magnesium, and a lack of calcium. Soil testing can help to identify these problems so that you can properly remedy them.

Monitoring and record-keeping

Keep an eye out in your lawn for areas where knotweed begins to take root. The first ones may be a sign that your soils are compacted and too low in pH. A few knotweed plants, however, are not harmful. Predatory (beneficial) insects are attracted to the flowers which they use as a food source.

Non-chemical and mechanical controls

Create a barrier
Aerate soils
Proper watering
Remove debris and habitat
Handpick and destroy

In-Depth Information:

According to UC IPM:
One of the most important management methods is to prevent soil compaction, which provides the conditions under which this weed grows best. Arrange landscapes so that soil is less likely to become compacted. Spread out foot and vehicle traffic over a broader area. Use fences or hedges to reduce traffic and install rock or pavement pathways where traffic cannot be avoided. Do not trample areas soon after irrigation or rainfall. Arrange soccer fields and athletic areas so that heavily used areas such as goals, midfields, and sidelines can be rotated. Loosening the soil in lawns to provide better drainage and a better environment for more desirable species can be beneficial. If areas are compacted, loosen the soil and overseed with a locally adapted grass seed.

A variety of mulches can be applied to planting beds and other landscaped areas to prevent establishment of common knotweed. Rock or organic mulches such as bark or compost can be used over the top of landscape fabrics. If used alone, organic mulches should be 3 to 4 inches thick. Finer mulch material is not desirable since weeds seed may easily grow in it. Coarser material will drain readily and reduce seedling establishment of common knotweed. Mulch needs to be replenished each year to maintain cover thickness and eliminate light penetration to the soil.

Prevent knotweed from producing seed by controlling young plants. This will reduce the amount of seed present in the soil in succeeding years. These are easy to remove with common weeding tools, such as a swivel hoe. For the home gardener, frequent manual removal along with mulching should be sufficient to manage this weed in most situations.

Many herbaceous plants and some woody species can be pulled out or dug up. It’s important to remove as much of the root system as possible; even a small portion can restart the infestation. Pull plants by hand or use a digging fork, as shovels can shear off portions of the root system, allowing for regrowth. To remove larger woody stems (up to about three inches in diameter), use tools that are designed to remove the above ground portion of the plant as well as the entire root system. It’s easiest to undertake this type of control in the spring or early summer when soils are moist and plants come out more easily.

Another option is cutting or mowing the knotweed. This technique is best suited for locations that you can visit and keep up with on a continual basis. To be effective, you will need to mow or cut infested areas three or four times a year for up to five years. The goal is to interrupt the plants ability to photosynthesize by removing as much leafy material as possible. Cut the plants at ground level and remove all resulting debris from the site. With this treatment, the infestation may actually appear to get worse at first, so you will need to be as persistent as the plants themselves. Each time you cut the plants back, the root system gets slightly larger, but must also rely on its energy reserves to push up new growth. Eventually, you will exhaust these reserves and the plants will die. This may take many years, so you have to remain committed to this process once you start, otherwise the treatment can backfire and make the problem worse.  

Biological controls

Goats are herbivorous foragers that are very effective at controlling weeds since grass is their least desirable food choice. They can be especially effective for roadside management, along railroad tracks, parks, forests, etc. Many people now make a living by contracting themselves and their herd out for weed control around the nation.

Least-toxic chemical options as a last resort

  • Horticultural vinegar, or acetic acid, is also effective at killing certain weeds. Avoid spraying other green vegetation, such as turfgrass, since this is a nonselective plant killer.
  •  Herbicidal soaps are highly refined soaps that can penetrate the waxy coating on plant leaves, causing them to dry out.

Acetic Acid (Vinegar)

Corn Gluten Meal


Chemicals to Avoid

Look at your product labels and try to avoid products containing those chemicals listed below:

(A = acute health effects, C = chronic health effects, SW = surface water contaminant, GW = ground water contaminant, W = wildlife poison, B = bee poison, LT = long-range transport)

dithiopyr (SW-URBAN, GW, W, B)

Oryzalin (C, W)

Pendimethalin (C, GW, W)


Trifluralin (C, SW, GW, W, LT)