Polygonaceae on emsenn.net

Identifying Japanese Knotweed

Thu, 02 Apr 2026 00:00:00 +0000

This guide teaches you to identify Japanese knotweed (Reynoutria japonica) in the field, in any season, and to distinguish it from the two other members of the knotweed complex: giant knotweed (R. sachalinensis) and Bohemian knotweed (R. × bohemica). You do not need any botanical training. You need your eyes, ideally a hand lens (10×), and the willingness to look closely at leaves.

Why identification matters

Correct identification determines:

Whether you have a legal obligation (in the UK, knotweed-contaminated soil is controlled waste)
Which management approach will work (the three species respond differently to treatment)
Whether the plant can produce seed (Japanese knotweed alone usually cannot; the hybrid can)
Whether a property survey will flag the infestation

Misidentification in either direction is costly. Confusing knotweed with a harmless plant means a growing infestation goes untreated. Confusing a harmless plant with knotweed means unnecessary alarm, expense, and potentially aborted property transactions.

Polygonaceae

Thu, 02 Apr 2026 00:00:00 +0000

Polygonaceae (the buckwheat family) is a family of flowering plants in the order Caryophyllales, comprising approximately 1,200 species in 48 genera distributed across all continents except Antarctica. The family includes plants of major economic importance (buckwheat, rhubarb, sorrel), ecological significance (the invasive knotweeds), and pharmaceutical value (Japanese knotweed as a source of resveratrol).

The ochrea: defining character

The single most reliable diagnostic feature of Polygonaceae is the ochrea (plural: ochreae) — a tubular or funnel-shaped sheath formed from fused stipules that encircles the stem above each node. The ochrea is present in nearly all members of the family (reduced or absent in a few genera of subfamily Eriogonoideae) and is found in no other angiosperm family. When identifying an unknown plant, the presence of an ochrea at the stem nodes is strong evidence for Polygonaceae.

Reynoutria

Thu, 02 Apr 2026 00:00:00 +0000

Reynoutria Houtt. is a small genus of large, rhizomatous herbaceous perennials in the family Polygonaceae, native to East Asia. The genus contains the three species collectively known as the “knotweed complex” — Japanese knotweed (R. japonica), giant knotweed (R. sachalinensis), and their hybrid Bohemian knotweed (R. × bohemica) — which together constitute one of the most damaging groups of invasive plants in the temperate world.

Taxonomic history

The genus name Reynoutria was established by Maarten Houttuyn in 1777 for the species now known as Japanese knotweed. The subsequent taxonomic history is among the most confusing in angiosperm botany, involving at least three genera and dozens of name changes: