Japanese Knotweed (Reynoutria japonica)

How much light does a Japanese Knotweed need?

Sun exposure for Japanese knotweed runs from full sun to part shade. Full sun is the preferred setting for this plant. Full sun here means 6 or more hours of direct sunlight a day. That 6-hour figure is the minimum sun hours recorded for it. Part shade is also listed as a suitable condition.

• Light: Full sun and part shade (full sun preferred)¹
• Direct sun: 6 h/day¹

How often to water a Japanese Knotweed

Water needs for Japanese knotweed span the range from dry to medium up to wet. It thrives on most sites that are at least seasonally wet. Soil drainage is recorded as moist and occasionally wet. Dry to medium moisture marks the low end of its range, and wet marks the high end.

• Water: Dry to wet²

Best soil for a Japanese Knotweed

Soil drainage for Japanese knotweed is recorded as moist and occasionally wet. The plant thrives on most sites that are at least seasonally wet. Its water range runs from dry to medium at the low end to wet at the high end. No soil pH range is documented for this page. Soil texture and amendments are also not documented. Because only drainage is recorded, this page states nothing more about the soil.

• Soil drainage: Moist and occasionally wet²

Hardiness and temperature

The hardiness zones recorded for this plant are 5a to 5b. It is listed as cold-hardy to USDA Zone 5. No minimum temperature is documented, and humidity is not covered either. Without a stated minimum temperature, the zone listing is the only cold guide this page has.

• Hardiness: Zones 5a–5b³

Pruning a Japanese Knotweed

Hard renewal pruning is the recorded method for this plant. The timing noted is June. Cutting in June results in shortened regrowth of 2 to 5 ft (0.6–1.5 m). Plant height at maturity is recorded at 10 to 13 ft (3.05–3.96 m), so the cut regrowth stays well below it.

• Pruning: Hard renewal pruning (Jun)²

How to plant a Japanese Knotweed

Zones 5a–5b are the hardiness range recorded for this species. Allow for a width of 3 to 20 ft (0.91–6.1 m) when you choose a spot. Plants reach 10 to 13 ft (3.05–3.96 m) tall. Pick a site with at least 6 hours of direct sun a day, and full sun is preferred.

• Hardiness: Zones 5a–5b³

Winter care

Cold hardiness is recorded as USDA Zone 5, with zones 5a–5b listed. No minimum temperature is documented for this plant. Winter protection, mulching and dormancy habits are not documented either. It is a herbaceous perennial. The zone listing is the only cold-weather fact on file for this page.

• Hardiness: Zones 5a–5b³

How to propagate a Japanese Knotweed

Propagation methods recorded for Japanese knotweed are seed, division and stem cuttings. Some populations also reproduce via seed. Colonies are usually formed from an interconnected, underground system of horizontal roots called rhizomes. Seed is the less usual route for colonies than rhizomes. Stem cuttings are the third listed method. Division is also on the list of recorded methods.

• Propagation: Seed, division and stem cuttings²

Common problems

Recorded pests for this plant include japanese beetle and Pochazia shantungensis. Xylella fastidiosa is listed as a disease. No other pests or diseases are documented on this page. Pest and disease records are limited to the names above. Symptoms and control methods are not covered.

• Common pests: japanese beetle and pochazia shantungensis⁴
• Common diseases: xylella fastidiosa⁴

Questions and answers

How much sun does Japanese knotweed need?

Japanese knotweed prefers full sun, which is 6 or more hours of direct sunlight a day. Part shade is also listed as a condition it takes.

How tall does Japanese knotweed grow?

It reaches 10 to 13 ft (3.05–3.96 m) tall at maturity. Its growth rate is rapid, and its width ranges from 3 to 20 ft (0.91–6.1 m).

Is Japanese knotweed hardy in cold climates?

It is listed as cold-hardy to USDA Zone 5, with zones 5a–5b recorded. No minimum temperature is documented, so the zone listing is the only cold guide on file.