Japanese knotweed is a fast-growing invasive plant that can cause damage to roads, buildings, and other structures. Eradicating it can be difficult and expensive, but there are several methods that can be effective. Here are some ways to kill Japanese knotweed permanently:
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Herbicides: Glyphosate-based products, like Roundup, are effective against Japanese knotweed. These herbicides can either be sprayed onto the leaves or injected into the stem of the plant. The plant should be sprayed with weed killer at several stages during its growth.
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Smothering: Smothering the plant can be an effective way to kill it without using herbicides. This involves cutting the knotweed at the base and close to the ground around the first week in June, piling the cut stems on an impervious surface such as a tarp, plastic, pavement, etc. so they can dry out, and allowing the knotweed to resprout. Do not do any management until after flowering, which usually occurs in early to mid-September. This method eliminates the need for chemicals, and there are also no soil disturbance/erosion issues.
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Digging: Digging up the plant and its roots can be effective, but it is a labor-intensive process and must be done carefully to avoid spreading the plant. All plant parts must be removed and disposed of properly.
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Stem injection: Stem injection is another technique for killing knotweeds. This involves cutting or breaking stems in early summer, then applying herbicide spray to the regrowing shoots in late summer. Cutting should be timed so that plants do not exceed three feet in height when sprayed.
It is important to note that knotweed eradication can be expensive and difficult. Small stands may be killed by repeated cutting, but this treatment must be repeated six times or more per season, for many years. Removing plant material from the site is not recommended unless all plant parts can be incinerated, buried deeper than five feet, or chopped into pieces smaller than ½ inch and composted. Any weed management activity should include a plan to reintroduce native or desirable landscape or crop plants. This allows the area to recover and resist future invasions. Monitor the site for at least two years following treatment to ensure that the knotweed has been completely killed.
