Fallopia, Japanese Knotweed: Planting, Growing, and Care

Description and Botany

Fallopia is a plant with a tumultuous history. Belonging to the Polygonaceae family (which includes, for example, Buckwheat, Sorrel, Rhubarb or Persicaria), and native to the southern and oceanic regions of East Asia, it is indeed a genus whose name has changed three times. Its stems with prominent nodes first earned it the name Polygonum (from the Greek ‘poly’, meaning ‘many’, and ‘gonu’ for ‘knees’). Its vernacular name, Knotweed, also means ‘many times knotted’. Later, it was classified by the Guinochet flora (1973) in the genus Reynoutria, before finally becoming Fallopia, in homage to Gabriele Falloppio, a 16th-century Italian naturalist.

This genus includes climbing annuals (Fallopia convolvulus, F. dentatoalata, F. dumetorum), more or less shrubby perennials (F. forbesii, F. japonica, F. sacahlinensis), climbing perennials (F. baldschuanica, F. cynanchoides, F. denticulata, F. multiflora, F. scandens), semi-climbing perennials (F. cilinodis) and climbing undershrubs (F. aubertii). Cross-breeding has also given rise to hybrids such as Fallopia x. bohemica (from F. japonica x F. sachalinensis).

Originally, Fallopia japonica grew naturally in the humid undergrowth of China, Korea, Japan and Siberia.

In the early 19th century, Philipp Von Siebold, a physician and botanist then stationed in Japan, brought this plant back to his garden in the Netherlands. Due to its very elegant habit, beautiful autumn flowering and nectar-rich flowers, it quickly became highly prized both in Europe and the United States. It was even awarded a gold medal by the Agricultural and Horticultural Society of Utrecht (Netherlands) in 1847, and was described as “the most interesting plant of the year”, both as a fodder plant, melliferous plant and soil stabiliser. This is how Fallopia japonica, like Fallopia sachalinensis, which gradually naturalised on both continents, began their colonisation process, spreading to the point where they are now considered by the Nature Conservancy, particularly concerning Fallopia japonica, to be among the 100 most problematic exotic species. The Species Survival Commission of the International Union for Conservation of Nature also considers it one of “the world’s worst invasive alien plants”.

Fallopia sachalinensis

Today, it is widespread throughout our territory, with higher concentrations noted in the North-West, South-West and East. Its expansion is so dynamic that in some cases it displaces native species. Its management and eradication in wild spaces have become problematic.

With very rapid growth, some species are evergreen, while the most commonly used ones are deciduous. Their vegetation dries up and disappears in winter, only to regrow vigorously from the stump the following spring. Depending on the species, the hollow stems are erect, voluble or creeping. They are sometimes tinged with red, especially in early growth. The leaves are alternate, single and entire, triangular, cordate or more or less ovate and sagittate, and borne on petioles whose insertion at the stem is surrounded by a thin, brown, cylindrical membranous sheath called an ochrea, a typical organ of the Polygonaceae family. They measure from 2 to 30 cm long and up to 25 cm wide depending on the species. Generally matte green, the leaves can also display very beautiful variegations of light green, white and rosy cream (Fallopia bohemica ‘Spectabilis’ or Fallopia japonica ‘Variegata’) or be entirely golden (Fallopia aubertii ‘Summer Sunshine’). It should be noted that these species with more colourful foliage are generally less invasive.

The beautiful foliage of Fallopia japonica ‘Variegata’ and Fallopia aubertii ‘Summer Sunshine’

The shrubby species generally reach 2 to 3 m in height, but those with a climbing and creeping habit can extend their stems up to 12 m, and therefore require both space and suitable support.

Flowering occurs between summer and autumn. The very small but numerous flowers are grouped in axillary or terminal spikes with a more or less loose habit, 3 to 15 cm long. White with greenish or pinkish hues, they consist of 5 tepals and 6 to 8 stamens with white anthers. Nectariferous and sometimes fragrant, they attract many pollinators. The black seeds are achenes (i.e., dry fruits that contain the true seed, like sunflower seeds for example), trigonous (three-angled), enclosed in the calyx. Knotweeds spread mainly through their powerful rhizomes, capable of extending several metres, and their roots also plunge very deep into the soil. It is this vigour that makes managing this plant difficult, as it is rare to remove all the roots, and just one piece left in place gives rise to a new plant. Japanese knotweed thrives in moist, deep and rather rich soils, but it can cope with poorer and drier soils. It is commonly found at woodland edges, near watercourses, but also along roads. Ferruginous, volcanic and metal-rich soils suit it well, which has led to its occasional use as a depolluting plant. It can be grown in any exposure, with a preference for sun or partial shade, and tolerates drought well once established.

Very hardy, it can withstand temperatures as low as -30°C and is unaffected by diseases or pests.

The climbing species and varieties can quickly and aesthetically conceal an unsightly element in the garden, whether it’s a façade or a fence for example. Installing a rhizome barrier is recommended if you choose to plant this species at home, to limit its expansion as much as possible.

The flowering of Fallopia baldschuanica

Different varieties

Shrub species

Climbing species

Fallopia bohemica 'Spectabilis'

This variety, resulting from hybridization, features foliage with colourful variegations in green, cream-white and pink. Highly vigorous, its cream-white flowering adorns the foliage from September onwards

• Flowering time October, November
• Height at maturity 2 m

See the product sheet

Fallopia japonica Variegata

With its foliage marbled in green and cream, sometimes edged with red, and coral-coloured stems, this variety is particularly decorative!

• Flowering time July to October
• Height at maturity 1,20 m

See the product sheet

Fallopia sachalinensis

Its broad green foliage, borne on sturdy stems, forms a clump capable of reaching 3 m. At the end of the season, its spikes of white flowers are a feast for pollinators.

• Flowering time October to December
• Height at maturity 2,50 m

See the product sheet

Fallopia aubertii

A climbing Knotweed with exceptional vigour, capable of covering walls, fences or old trunks up to nearly 12 metres. Its abundant white flowering creates, between September and the first frosts, a plant tapestry of great beauty!

• Flowering time September to November
• Height at maturity 8 m

See the product sheet

Fallopia aubertii Summer Sunshine

A less vigorous climbing variety, which can still reach 7 metres in height. Its light green to golden foliage is particularly luminous, and serves as a setting for a mossy and lightly scented flowering.

• Flowering time September to November
• Height at maturity 7 m

See the product sheet

 

Planting

Where to plant?

• Fallopia species are not very demanding regarding soil type, with a preference for light and deep ground. In fresh and moist soil, their invasive character becomes more pronounced. They also enjoy sunny and partially shaded exposures but can also grow in shadier spots.
• Their size and tendency to spread require careful attention, as they can quickly smother and outcompete more fragile neighbouring plants. Climbing varieties need a support that matches their growth—large and sturdy enough.

When to plant?

Most perennials are planted in autumn, but spring planting is also possible.

How to plant?

First and foremost, determine the space you are willing to allocate to your Fallopia.

1. Install a root barrier around the perimeter of the designated area, to a depth of about 70 cm, leaving 4 or 5 cm of the barrier protruding above the soil.
2. Soak the root ball in water to saturate the substrate.
3. Dig a hole in the centre of your area, slightly larger than the size of the root ball, and loosen the bottom.
4. Remove your Knotweed from its pot and place it in the centre.
5. Water and mulch with your chosen material.

For climbing species, position your plant near the intended support.

Always install a root barrier when planting Fallopia

Maintenance

• Maintaining Knotweed mainly involves controlling its vigour.
• For climbing species, don’t hesitate to prune stems that stray beyond the boundaries you’ve set.
• At the end of winter, cut dry stems back to the base.
• As a precaution, and to avoid uncontrolled spread, do not place cut stems on the compost heap, but dispose of them with waste destined for incineration.

Propagate

Propagation is mainly done by taking offsets and roots.

In autumn or spring, simply take a piece of rootstock a few centimetres long and replant it in the desired spot, then water to obtain a new young plant.

Pair

Knotweed isn’t always the easiest plant to pair, as its vigorous growth leaves little room for less robust neighbours. Its tall, numerous stems and the size of its flowers also create shade that can deprive other plants of the light they need.

However, when grown under certain conditions (in a pot, for example, or contained by a root barrier), you can pair it with other plants, choosing those vigorous enough to hold their own. Trees and bushes will have a better chance of thriving. For spring, Judas Tree, Japanese Quince, Hawthorn, Forsythia or Photinia brighten the start of the season as the young shoots of Knotweed develop. To complement their airy flowering, consider Abelia, Buddleia, Elaeagnus (whose flowers are subtle but highly fragrant!), Crape Myrtle or even Sorbaria. For perennials, Knotweeds create a contrast in inflorescence shape (upright spikes) as well as in colour (pink, red…).

For climbing varieties, a Dutchman’s Pipe, a Wisteria, a large-flowered clematis like Clematis montana ‘Freda’ or Clematis rehderiana, a Hop (some of which have stunning variegated or golden foliage), an Ivy (where green is just one of many shades this easy climber offers), a Pileostegia (its creamy-white flowering coincides with Knotweed’s), or an Ornamental Vine with fiery autumn hues can keep them company. To dress their base, use ornamental grasses (Miscanthus, Panicum, Pennisetum), and floriferous perennials like Geranium, Aster, Japanese Anemones or tall Bellflowers. If the exposure allows, a few clumps of ferns add striking texture, their finely cut fronds contrasting with Knotweed’s large, entire leaves. The green screen formed by climbing Knotweeds provides a backdrop against which the flowering of roses stands out beautifully.

With a variety like Fallopia ‘Summer Sunshine’, with golden foliage, pair plants with dark foliage such as Physocarpus ‘Little Devil’, a Heuchera ‘Black Pearl’, a Chervil ‘Ravenswing’ with fern-like purple foliage and delicate white flowers, a Knotweed ‘Red Dragon’, a Cimicifuga ‘Brunette’, a Thalictrum ‘Ghent Ebony’, or a Phormium ‘Purpureum’ (in mild regions).

With the golden foliage of Fallopia aubertii Summer Sunshine, a striking contrast: Physocarpus ‘Diablo’, Chervil ‘Ravenswing’ and Knotweed ‘Red Dragon’

Did you know?

• Knotweed is an edible plant, to be consumed in moderation

The young shoots of many species can be eaten, either raw or cooked like asparagus or rhubarb, and even in pies, after peeling off the outer layer which is more fibrous. However, Fallopia tends to grow in soils rich in metals, which are then ingested when consuming the plant. It’s therefore best to be certain of the harvesting location.

Moreover, this perennial is rich in oxalic acid, a compound also present in the human body and in many other foods including rhubarb, spinach, sorrel, chocolate and numerous fruits, to name but a few. Ingesting small quantities poses no problem, particularly if you’re in good health. However, excessive consumption over a prolonged period should be avoided by people with joint problems or bladder/kidney stones, as it may aggravate these conditions. Oxalic acid also has the ability to bind certain minerals, which are then excreted from the body in greater quantities, potentially leading to certain deficiencies.

• The world’s most widespread vegetative reproduction…

Several studies have been conducted worldwide, particularly in Europe and America, with surprising conclusions! Indeed, except in Japan, the plant appears to reproduce solely through vegetative means via its underground rootstocks. All Japanese knotweed plants found outside their native country may therefore be clones of a single original plant that has multiplied endlessly – whether accidentally or through plant purchases/exchanges – making this potentially the most widespread root propagation by cuttings on the planet…