Japanese Knotweed Identification: Stems, Leaves, Roots

Japanese knotweed (Reynoutria japonica) is identified by three features together: hollow, bamboo-like stems with purple speckles, flat-based spade-shaped leaves arranged in a zigzag, and orange-centered rhizomes. Any one alone can mislead; all three together are reliable.

• Stems. Cornell's weed ID page describes them as hollow, stout and green to purple, with prominent joints where the leaves emerge. The Invasive Species Centre gives stems up to 2.5 cm in diameter with reddish-brown nodes wrapped in papery sheaths. The RHS calls them bamboo-like.
• Leaves. Alternate and broadly egg-shaped with a flat base and pointed tip. PBA Solutions compares them to a spade on a playing card. Cornell gives 8 to 15 cm long by 5 to 10 cm wide; the Invasive Species Centre gives 10 to 17 cm by 7 to 10 cm.
• Zigzag. The stem changes direction slightly at each leaf, so the leaves line up in a zigzag. PBA Solutions calls this one of the most distinctive features.
• Rhizome. The RHS describes rhizomes extending more than 1 m deep, dark brown outside and orange underneath. PBA Solutions notes the orange or yellow color shows when a rhizome is cut.

What you see depends on the month, which is why people miss it in spring and winter.

• Spring: the RHS describes reddish-purple shoots emerging. The Invasive Species Centre records stems growing up to 8 cm a day.
• Summer: dense stands 1 to 3 m tall (Invasive Species Centre; the RHS gives typically 2 m, and PBA Solutions up to 3 m) with the zigzag leaves fully out. The Invasive Species Centre says the thickets cut sunlight by more than 90%, so almost nothing grows underneath.
• Late summer to autumn: sprays of small creamy-white flowers about 10 cm long (PBA Solutions); Cornell notes they attract honeybees. PBA Solutions gives flowering from late August into September; the Invasive Species Centre gives late July and August in Ontario.
• Winter: the canes die back, turn brown and brittle, and keep the zigzag shape, so a patch can be identified from last year's dead stems (PBA Solutions).

Two groups of look-alikes, with different consequences.

Other knotweeds. Cornell lists giant knotweed, Bohemian knotweed (a hybrid of giant and Japanese) and Himalayan knotweed as sharing similar traits and needing the same management. For practical purposes, if you have one of these, treat it the same way.

Unrelated plants. The RHS names Russian vine, Himalayan honeysuckle and various Persicaria species as common look-alikes, and bamboo is the comparison most people reach for because the RHS itself calls knotweed stems bamboo-like.

Rather than learning each look-alike, confirm the full knotweed set on the plant in front of you:

• Alternate leaves, one per joint, with a flat base
• A zigzag from joint to joint
• A hollow stem with purple speckles and papery sheaths at the nodes
• A rhizome that is orange or yellow when cut

A plant that fails any one of these is probably not Japanese knotweed. A plant that passes all four almost certainly is, or is one of its close relatives.

Yes, you should deal with it, and it takes years. Why it matters:

• Spread from fragments. Cornell notes any piece containing a root or stem joint can regenerate. The Invasive Species Centre says the root system can spread up to 10 m from the parent plant.
• Damage. The Invasive Species Centre records it growing through concrete and asphalt up to 8 cm thick and through building foundations, and destabilizing stream banks.

What control looks like, from Cornell: for small patches, dig out the entire root system and monitor for three years; for large patches, smother under an opaque tarp for five years, including a buffer zone; with herbicide, cut in June, collect every piece, and treat regrowth in late summer, repeated over several years. The RHS says glyphosate treatment typically takes at least three growing seasons. Follow the product label and ask your local extension office or invasive species program before you start.

Never compost or dump it, and do not mow it and leave the cuttings, since fragments start new plants. In the UK, the RHS notes it is not illegal to have knotweed on your land, but it is an offence under Schedule 9 of the Wildlife and Countryside Act 1981 to cause it to grow in the wild, and it counts as controlled waste under the Environmental Protection Act 1990.

In its native East Asia, knotweed is held back by insects and fungi that do not exist where it has invaded. The RHS says biological control research is investigating sap-sucking insects and fungal pathogens from that native range, and Cornell reports biocontrol research under way that is not yet publicly available. There is no product you can buy that kills it naturally.

None of the sources used here describe Japanese knotweed as a skin hazard, but that is not the same as saying it is harmless to handle. Wear gloves when digging, and bag every fragment. If a child or pet has eaten any plant you cannot identify, call Poison Control at 1-800-222-1222 in the US, or your vet.

NatureLens gives free scans every day, so you can photograph the plant or sign and get a suggested name in seconds, then confirm it with the checks on this page.