Japanese Knotweed (Reynoutria japonica)

Hearing the name of Japanese knotweed can strike fear in the heart of homeowners. When this aggressive and invasive weed is identified on a property, mortgage lenders sometimes deny loans or demand extensive chemical treatment. This is due to the fact that the root system of Japanese knotweed is so extensive that once established, it can be nearly impossible to eradicate.

I. Appearance and Characteristics 

Reynoutria japonica, synonyms Fallopia japonica and Polygonum cuspidatum, is a species of herbaceous perennial plant in the knotweed and buckwheat family Polygonaceae. Common names include Japanese knotweed and Asian knotweed. It is native to East Asia in Japan, China and Korea. In North America and Europe, the species has successfully established itself in numerous habitats, and is classified as a pest and invasive species in several countries.

The flowers are small, cream or white, produced in erect racemes 6–15 cm (2+1⁄2–6 in) long in late summer and early autumn. Japanese knotweed has hollow stems with distinct raised nodes that give it the appearance of bamboo, though it is not related. While stems may reach a maximum height of 3–4 m (10–13 ft) each growing season, it is typical to see much smaller plants in places where they sprout through cracks in the pavement or are repeatedly cut down. The leaves are broad oval with a truncated base, 7–14 cm (3–5+1⁄2 in) long and 5–12 cm (2–4+1⁄2 in) broad, with an entire margin. New leaves of Reynoutria japonica are dark red and 1 to 4 cm (1⁄2 to 1+1⁄2 in) long; young leaves are green and rolled back with dark red veins; leaves are green and shaped like a heart flattened at the base, or a shield, and are usually around 12 cm (5 in) long.

Mature R.japonica forms 2-to-3-metre-tall (6+1⁄2 to 10 ft), dense thickets. Leaves shoot from the stem nodes alternately in a zigzag pattern. Plants that are immature or affected by mowing and other restrictions have much thinner and shorter stems than mature stands, and are not hollow.

II. How to Grow and Care

Watering

Thriving in its native humid environments, Japanese knotweed has adapted to maintain a balance between moisture retention and drought resilience. This species typically requires moderate hydration, aligning with its preference for consistent soil moisture without being waterlogged. Accordingly, it benefits from watering every week to sustain its health. Given that Japanese knotweed is more commonly grown outdoors, it’s important to consider how natural rainfall contributes to its watering schedule, particularly as this perennial herb enters its vigorous growing season.

Propagation

Japanese knotweed’s ideal propagation season is during spring and autumn through division. It can be moderately challenging to propagate and may require pre moistening of the soil. Success is indicated by new growth. Ensure divisions contain both roots and shoots for optimal results.

Transplanting

The best time to transplant Japanese knotweed is in late fall to early spring (S3-S5). This gives the plant ample time to adjust before peaked growth in summer. A semi-shaded location is ideal, ensuring the root ball or clump remains completely covered after replanting. Remember, Japanese knotweed thrives in well-drained, fertile soil!

III. Uses and Benefits 

Japanese knotweed flowers are valued by some beekeepers as an important source of nectar for honeybees, at a time of year when little else is flowering. Japanese knotweed yields a monofloral honey, usually called bamboo honey by northeastern U.S. beekeepers, like a mild-flavored version of buckwheat honey (a related plant also in the Polygonaceae).

The young stems are edible as a spring vegetable, with a flavour similar to rhubarb. In some locations, semi-cultivating Japanese knotweed for food has been used as a means of controlling knotweed populations that invade sensitive wetland areas and drive out the native vegetation. It is eaten in Japan as sansai or wild foraged vegetables. In Europe, young shoots have been used in restaurant cuisine.

Knotweed can grow up to 30 cm per day, hence it is a fast-growing vegetable tolerant of poor quality soils. Because knotweed is resistant to over-harvesting, it is more often foraged than cultivated as a vegetable. The plant can be eaten raw or cooked.

Ground-feeding songbirds and game birds also eat the seeds.

In Japan

It grows widely throughout Japan and is foraged as a wild edible vegetable (sansai), though not in sufficient quantities to be included in statistics. They are called by such regional names as tonkiba (Yamagata), itazuiko (Nagano, Mie), itazura (Gifu, Toyama, Nara, Wakayama, Kagawa), gonpachi (Shizuoka, Nara, Mie, Wakayama), sashi (Akita, Yamagata), jajappo (Shimane, Tottori, Okayama), sukanpo (many areas).

Young leaves and shoots, which look like asparagus, are used. They are extremely sour; the fibrous outer skin must be peeled, soaked in water for half a day raw or after parboiling, before being cooked.

IV. How to Control

Japanese knotweed has a large underground network of roots (rhizomes). To eradicate the plant the roots need to be killed. All above-ground portions of the plant need to be controlled repeatedly for several years in order to weaken and kill the entire patch. Picking the right herbicide is essential, as it must travel through the plant and into the root system below. But also more ecologically friendly means are being tested as an alternative to chemical treatments.

• Chemical

The abundance of the plant can be significantly reduced by applying glyphosate, imazapyr, a combination of both, or by cutting all visible stalks and filling the stems with glyphosate. However, these methods have not been proven to provide reliable long-term results in eliminating the treated population.

• Mechanical

Digging up the rhizomes is a common solution where the land is to be developed, as this is quicker than the use of herbicides, but safe disposal of the plant material without spreading it is difficult; knotweed is classed as controlled waste in the UK, and disposal is regulated by law. Digging up the roots is also very labor-intensive and not always effective. The roots can extend up to 3 meters (10 feet) deep, and leaving only a few centimeters (inches) of root behind will result in the plant quickly growing back.

Covering the affected patch of ground with a non-translucent material can be an effective follow-up strategy. However, the trimmed stems of the plant can be razor sharp and are able to pierce through most materials. Covering with non-flexible materials such as concrete slabs has to be done meticulously and without leaving even the smallest splits. The slightest opening can be enough for the plant to grow back.

Soil steam sterilization involves injecting steam into contaminated soil in order to kill subterranean plant parts.

Trials in Haida Gwaii, British Columbia, using sea water sprayed on the foliage, have not demonstrated promising results.

• Biological

Research has been carried out on a Mycosphaerella leaf spot fungus, which devastates knotweed in its native Japan. This research has been relatively slow due to the complex life cycle of the fungus.

Following earlier studies, imported Japanese knotweed psyllid insects Aphalara itadori, whose only food source is Japanese knotweed, were released at a number of sites in Britain in a study running from 1 April 2010 to 31 March 2014. In 2012, results suggested that establishment and population growth were likely, after the insects overwintered successfully. In 2020 Amsterdam imported and released 5,000 Japanese Aphalara itadori leaf fleas, exempting them from a strict ban on the introduction of alien species, as one of the measures to contain the knotweed. The psyllids suck up sap from the plant, potentially killing young shoots and slowing or even stopping growth. It was hoped that the psyllid would hibernate over winter and establish themselves in 2021.

Anecdotal reports of effective control describe the use of goats to eat the plant parts above ground followed by the use of pigs to root out and eat the underground parts of the plant.