Himalayan knotweed found

Jul 12, 2016
3 min read

Himalayan knotweed has been found in the walled garden and also growing near the first pond in The Dell. This was tackled in the past but has obviously made a come back.

It usually takes at least three to four seasons to eradicate Himalayan knotweed using glyphosate. Professional contractors, however, will have access to more powerful weedkillers that may reduce this period by half.

You could be fined up to £5,000 or be sent to prison for up to 2 years if you allow contaminated soil or plant material from any waste you transfer to spread into the wild.

Himalayan and Japanese Knotweed are very similar and although rather attractive, Japanese knotweed (Fallopia japonica)(syn. Polygonum cuspidatum) is a real thug as it spreads rapidly. In winter the plant dies back beneath ground but it is a perennial weed and by early summer the bamboo-like stems shoot to over 2.1m (7ft), suppressing all other growth. Eradication requires steely determination as it is very hard to remove by hand or with chemicals. New legislation now covers its control.

The canes have characteristic purple flecks, and produce branches from nodes along its length. These branches support shovel-shaped leaves.

Japanese Knotweed above. Photo by Ancatdubh43 at English Wikipedia [Public domain], via Wikimedia Commons

Himalayan Knotweed.

The problem

Japanese knotweed was introduced from Japan in 1825 as an ornamental plant. The plant is not unattractive but its rapid annual growth and relentless spread, allows it to easily overwhelm other garden plants. Where established as a wayside weed, native plants are also aggressively over-run.

Although it does not produce seeds, it can sprout from very small sections of rhizomes and, under the provisions made within the Wildlife and Countryside Act 1981, it is an offence to cause Japanese knotweed to grow in the wild. Much of its spread is probably via topsoil movement or construction traffic.

Glyphosate Perhaps the most effective and simplest method for the home gardener to tackle Japanese knotweed is with the glyphosate-based weedkiller Scotts Roundup Tree Stump & Rootkiller. This has label recommendation for controlling Japanese knotweed, instructing it to be applied to the cut canes. Bayer Garden Super Strength Weedkiller also has label control for this weed

Alternatively, we can try other tough formulations of glyphosate (e.g. Scotts Roundup Ultra, Scotts Tumbleweed, Bayer Tough Rootkill, Doff Maxi Strength Glyphosate Weedkiller or Westland Resolva Xtra Tough Concentrate)

Glyphosate is usually applied to the foliage and is passed within the plant to the underground parts

It is useful to cut away old stems during the previous winter to allow good access. As with other weeds, the most effective time for spraying Japanese knotweed with glyphosate is at the flowering stage in late summer. However, it is difficult to spray at this stage, when the weed is 2.1m (7ft) or more high

A more practical approach is to allow Japanese knotweed to grow to about 90cm (3ft), which will usually be reached in May, and spray then. There will be regrowth and consequently a second application in mid-summer is useful. Check during September and if it has grown once more, spray again before growth begins to die down in the autumn. Check again the following spring

Avoid spray coming into contact with garden plants. Glyphosate-treated knotweed will often produce small-leaved, bushy regrowth 50-90cm (20in-3ft) in height the following spring. This is very different in appearance to the normal plant and it is essential that this regrowth is treated.

We will notify the park authorities that this brute is making a come back.

SEPTEMBER 1st update

Barriers have been around the 2 affected areas, but vandals have since ripped down the barrier tapes in the area around the top pond.