Himalayan Knotweed

Himalayan knotweed is a persistent and invasive weed that can grow quickly in many environments. and becoming as problematic as its well-known relative, Japanese knotweed.

The plant sucks up minerals from deep underground through its roots while reproducing quickly and competing with native plants that we rely on for food or shelter, so it needs to be removed before it becomes the dominant plant on your property.

This blog post will provide an in-depth guide on what Himalayan Knotweed are, how they grow, and how to eradicate them from your garden.

Quick Facts

Common names: cultivated knotweed
Scientific name: Persicaria Wallichii
Family: Polygonacea
Origin: non-native
Flowering season: June to November
Habitat: woodlands, stream embankments, roadsides, railway banks and waste ground

Himalayan Knotweed Identification

This plant looks similar to Japanese knotweed, but its leaves are longer and more pointed. It’s a huge, thicket-forming plant that’s gotten established on stream banks, hedgebanks, woodland borders, roadsides, railway banks, and waste ground, reaching up to 2m tall.

Himalayan Knotweed is a lesser-known relative of the notorious Japanese Knotweed (Fallopia japonica) yet is forecast to cause many of the same economic, environmental and social problems once it becomes better established.

Displaces native species by forming dense stands up to 2m tall and reducing availability of nutrients in the soil.

What does Himalayan Knotweed look like?

Characteristics that make up the Himalayan Knotweed leaf, stem, root and flower are detailed below.

Once established this plant grows into extremely dense stands that out-compete all native vegetation.

Himalayan knotweed leaves - long and tapered
Himalayan knotweed leaves – long and tapered

Leaves: The long, tapering leaves are 4 to 8 inches long, with brown, persistent sheaths at the bases of the leaf stalks.

Himalayan knotweed leaves are very narrow, often half as wide as they are long. This is the main feature that distinguishes it from Giant knotweed and Japanese knotweed.

Despite the difference in leaf shape, Himalayan knotweed will grow in a similar fashion to Japanese knotweed throughout the year.

Stems: Normally these are green although they can have alternating red-green colourisation.

Bamboo-like stem with red nodes. The stems are thick, hollow, and easily exceed one meter (40 inches) in length.

Himalayan knotweed roots
Himalayan knotweed roots

Roots: They are called “knotweeds” because they grow in tangled clumps that resemble knots, which makes them difficult to remove from a lawn or garden.

Spreads by rhizomes and vegetative propagation as the roots spread far and deep.

Himalayan knotweed flowers in full bloom
Himalayan knotweed flowers in full bloom

Flowers: These can range in colour from white to pink and are loosely clustered.

Flowers on Himalayan knotweed have a pink hue to their colour, which distinguishes them from the pure white of Japanese knotweed flowers.

Seeds: The seeds can be found in dense clusters at opposite ends on each stem. The seed of a Himalayan Knotweed plant is able to germinate whether it’s planted on top or below ground level.

Himalayan knotweed growing by a roadside
Himalayan knotweed growing by a roadside

Himalayan Knotweed Seasonal Changes

Similar to Japanese Knotweed, P. wallichii overwinters as rhizomes and is unidentifiable until the spring. During the growth period, identification is based on the distinctive red petiole, the lanceolate leaves and the clusters of small creamy-white/pinkish flowers.

Himalayan Knotweed in Spring

Himalayan knotweed will start with reddish shoots in the Spring that will quickly develop into thick foliage, reaching heights of around 2m by the Summer.

Himalayan Knotweed in Summer

They thrive in moist soil and full or partial sun. Knotweeds can spread by seed, root fragments, and stem fragments, making them very difficult to control.

Himalayan Knotweed in Autumn

In the Autumn, the foliage begins to wilt as the leaves turn yellow before the plant turns into pale hollow stalks by the Winter.

During autumn, leaves will begin to wilt as displayed by a pronounced yellow colouration until winter, where the plant will appear dead, existing as pale hollow stalks.

Himalayan Knotweed in Winter

This weed is a perennial plant that dies back during the winter months.

Himalayan knotweed flower stalks
Himalayan knotweed flower stalks

How to get rid of Himalayan Knotweed

The best way to get rid of Himalayan knotweed is by using herbicides and pulling it out by hand. If you don’t want to use herbicides, then try digging up the roots with your hands or using a shovel.

It’s important not to let this weed take over your garden because it can grow quickly and spread easily. Make sure to keep an eye on any new growth so that it doesn’t become too big before taking action against it again.

Method One – Herbicide Treatment

To effectively manage this weed with a herbicide, use one containing glyphosate, triclopyr, 2,4-D, picloram, and imazapyr as active ingredients as these have been demonstrated to be efficient in reducing knotweeds, either individually or in combination.

Because of its large rhizome and sprouting potential, we recommend managing Himalayan knotweed at a landscape level. Cutting, mowing, digging, and pesticide applications cause it to resprout strongly, especially early in the growing season, until at least August.

It’s likely that eradicating just one patch will take more than a year, and in most cases, numerous treatments. Herbicide use will almost probably be required in the control strategy for large-scale projects and sites.

It resprouts vigorously the following cutting, mowing, digging and herbicide treatments, especially early in the growing season, until at least August.

Successful eradication of just one patch is likely to take more than one year and multiple treatments in most cases. Landscape-level projects and large sites will almost certainly require integrating herbicide use into the control strategy.

Method Two – Excavation

Put on some gloves and grab the plant by its base. Trim it down and then cut off all the roots in the ground. Dig deep if you have to, but make sure there are no roots left behind! Repeat this step every few months for the best results.

Mowing can also be done with a brushcutter or mower, cutting as low as feasible and as often as possible during August, but at least every 2-3 weeks. If the knotweed is in soft soil, it can be pulled out by the root crown, with as much of the root system as feasible removed.

The plant will not die instantly, but its root mass will be reduced. New sprouts can emerge at a distance of up to 6 metres from the original plant and should be plucked as soon as possible.

Cut the stems to the ground level and cover the area with thick black plastic or many layers of cardboard. At least 2 metres beyond the plant’s base should be covered (preferably 7 m). This should be done at the start of the year or after the plant has been clipped a couple of times in the spring.

Method Three – Root Barrier

Using a root barrier membrane will provide a necessary barrier to prevent this weed from growing back. Throughout the growing season and well into the following, the covering material should be left in place.

Himalayan knotweed leaves and stems
Himalayan knotweed leaves and stems

Himalayan Knotweed Management

Himalayan knotweed is a weed that is difficult to get rid of. It spreads rapidly and can take over large areas of land. There are, however, many methods for controlling this invasive species and here we’re going to go over the most effective ways for removing it from your property.

First, you need to cut off all of the roots in order to kill the plant completely. You should also make sure that you use a herbicide on any remaining vine pieces as well as on any new growths that pop up after cutting back everything else because these parts will grow back without any problem at all if they receive water or sunlight.

In Conclusion

One of the most important things to remember when removing Himalayan knotweed is that it can take a long time and require many treatments.

Have a treatment plan in place and both the time and knowledge to tackle this persistent weed will result in the outcome you desire – a clean weed-free garden.