Least-Toxic Control of Japanese Knotweed Choose a different pests


Pest type: Plants

Japanese knotweed, also known commonly as “bamboo,” is a native of Japan, brought to the United States in the late 1800s as an ornamental. Japanese knotweed is a shrub-like herbaceous perennial (which dies back to ground each fall) that can grow to ten feet in height and form dense thickets of stems that exclude native vegetation and alter natural ecosystems.

Japanese knotweed can tolerate a variety of adverse conditions including full shade, high temperatures, high salinity and drought. It is often seen along stream banks, in wetlands, and along roadways. It has distinctive stout, hollow, bamboo-like stems and large, broad, ovate alternate leaves. Tiny white or greenish-white flowers develop in late summer and grow in clusters that form a mass of white over the plant in full flower.

Is it a problem?

Japanese knotweed can crowd out native vegetation and alter natural ecosystems. It poses a significant threat to river bank areas, where it can survive floods and is able to rapidly colonize shores and islands. Once established, populations are persistent and difficult to control.

Pest prevention practices

In-depth information:
Japanese knotweed thrives where there is available water such as streams or damp areas with plenty of sunlight, so using proper water techniques, staying away from excessive overwatering, and making sure there is adequate drainage can help keep japanese knotweed out of your lawn.

It is important that an effective japanese knotweed management program is established. All cut or pulled stems of japanese knotweed should be treated with extreme care as they can potentially re-sprout and cause spread. They should be kept on site, or disposed of in a licensed landfill site that can carry out deep burial to prevent spread onto other sites. Treatment of colonies on riverbanks should be treated as soon as possible because bank erosion can lead to plant material breaking off and dispersing downstream.

As stems, crowns and rhizomes readily regenerate, they must be allowed to dry out thoroughly after they have been pulled or cut. This can be helped by putting the material onto a plastic sheet, or raised wooden platform, rather than on the ground. Regular checks should be made to ensure that this material is not contaminating waterways or other sites, or developing roots. Thoroughly burning plant material on site after cutting and drying, if current laws allow, can be an effective means of disposal provided that the waste is burnt on site and not removed to other land.

Ensure that machinery, tools and work clothes are free of fragments of Knotweed before leaving the site. Tracked vehicles, off-road tires, tools and even work boots can harbor fragments of Knotweed and could potentially cause spread to another site. These items should be thoroughly cleaned before leaving a Knotweed contaminated site.

Monitoring and record-keeping

As with all invasive species, Japanese knotweed is most effectively controlled by recognizing their appearance early and removing isolated plants before they begin to spread.

Non-chemical and mechanical controls

Remove debris and habitat
Handpick and destroy

In-depth information:
Regular pulling will, after a number of years, eventually exhaust the rhizome and kill the plant. This is only an effective method of control if it is carried out continually over a number of years. Cutting and mowing can be used to prevent spread, to reduce vigor, and to reduce underground biomass. With all these methods, the main problem is the safe disposal of the cut or pulled stems to prevent spread.

Cutting can be carried out with loppers, cutters, hooks, scythes, bladed cutters such as those with a metal circular blade or strimmers with a metal blade. Do not use bladed cutting equipment such as brush cutters or strimmers near a watercourse (even ditches) as fragments of Knotweed could enter the water and spread downstream. Cut material should be collected up, then dried out and burnt on site or removed and taken to a licensed landfill site. Ensure workers use protective clothing and face visor and they clean equipment before leaving the site and to avoid leaving fragments of Knotweed to spread around the site.

In the UK, studies have shown that with four cuts a year the plant loses vigor and underground biomass. The first cut should be carried out when the first shoots appear and the last cut should be done when the plant is at its most luxuriant in late summer but before it dies back in the autumn (September/October). Annual cutting will be required. You should monitor the extent of the Knotweed stand to make sure it is not spreading sideways as there is evidence that cutting can cause the rhizome to spread laterally.

Regular mowing in parks, gardens and on roadways is a good way of controlling the growth of Japanese Knotweed. However, if mowing is stopped, the plant can take over the site again. Avoid using a flail mower as this may result in small, viable fragments of stem regenerating in neighboring, non-infested areas. It is preferable to use a mower with a collecting bag for Knotweed areas. Mowing should be collected up and left on site. Compost should be checked for re-growth.

Pulling is another method of eradication. It is an excellent method to use on sites with native or sensitive species growing (e.g. on wildlife sites or riverbanks) as it specifically targets Knotweed plants. Care should, however, be taken to avoid trampling valuable flora in the vicinity.

Stems should be pulled regularly when they reach full height. They should be pulled near the base to include some rhizome. Control of a small infestation could be achieved in 3 years but this method requires regular, sustained treatment to work.

Covering with heavy plastics or geotextile fabrics has had some success. Cut stems to ground surface and then cover the stand, being sure to extent coverage at least 10 feet beyond the farthest stems. Weigh down the edges and monitor for sprouts. Leave cover in place for at least two growing seasons.

As knotweed is removed from a site, you will want to encourage the growth and establishment of native plants. If you are working in your yard or garden, choose either native species or non-natives that do not have invasive properties. In some situations native plants will readily re-establish themselves without any help. However, you may want to replant the area to jumpstart the process of re-vegetation.

Biological controls

Japanese Knotweed is used as animal fodder, and cattle, sheep, horses, donkeys and goats graze the plant. Animals prefer the young shoots as they emerge in the spring -- after about June, the stems become rather woody. Grazing may reduce shoot densities and height but will not eradicate it. Although grazing can help reduce the spread into non-infested areas, it is not a method of control. Dead stems should be cut back in winter as these can deter grazing in the spring. Continued grazing will ensure the supply of new shoots throughout the growing season. Grazing is therefore not an eradication tool but is helpful in suppressing the plant and reducing spread.

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.

Acetic Acid (Vinegar)

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)

2,4-D (C, SW, GW, W, B)

Glyphosate (C, SW-URBAN, GW, W, B)

Triclopyr (GW, W)

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