csk's Journal

After finding two separate occurrences of giant knotweed in the Cuyahoga River watershed, I started to wonder if this species might be more common than we think. At a casual glance, it looks very similar to the more commonly known Japanese knotweed. Japanese knotweed is so common and easy to identify, why would anyone bother looking at it closely? So if there are more populations of giant knotweed in the area, they may well be going unnoticed, assumed to be Japanese knotweed.

The good news is that anyone familiar with Japanese knotweed can very easily learn to tell the difference between giant and Japanese knotweed.

The bad news is that the two species hybridize. The hybrid, Bohemian knotweed, has intermediate characteristics. Recently, I've been looking more closely at every stand of knotweed I encounter, asking, "is it Bohemian?" Here are some distinguishing characteristics you can use to answer this question, in descending order of usefulness.

Distinguishing Characteristic #1: Leaf base shape

Japanese knotweed leaf bases are truncate. The base of the leaf is straight, with distinct corners where the base meets the curved edge. It's a really distinct shape, and one that makes Japanese knotweed a delightfully easy plant for a beginner to learn.
Leaf base flat with corners = Japanese knotweed

Giant knotweed leaf bases are deeply cordate - they have lobes.
Leaf base lobed without corners = giant knotweed

Bohemian knotweed has both shapes of leaf base, and every possible variation in between. Shape varies from deeply cordate to slightly cordate to essentially truncate. Sometimes on the same stem, often in the same stand.
lobes and corners on the same plant or in the same stand = probably Bohemian

Distinguishing Characteristic #2: Leaf length to width ratio

Japanese knotweed leaves are only slightly longer than their width (their length:width ratio ranges from 1:1 to 1.5:1).

Giant knotweed leaves are much longer than their width (their length:width ratio is about 2:1).

So if you find yourself looking at some knotweed and thinking, "Those leaves look unusually long and skinny for Japanese knotweed," it just might be giant knotweed.

This characteristic is not useful for distinguishing Bohemian knotweed.

When using leaves to identify knotweeds, always look at mid-stem leaves - that is, leaves growing near the middle of a stem, not right at the tip.

Distinguishing Characteristic #3: Hairs on the underside of the leaf
If you have a good hand lens, supposedly you can tell them apart by the hairs on the underside of the leaf. I don't, and I can say from experience that you can't just zoom in on a photograph and see the hairs. Or maybe you can - zoom in on photographs 3 and 4 of this observation and tell me if you think there are hairs on the underside of that leaf. The information below is solely from Zika and Jacobson (2003), and not at all based on my personal experience.

According to Zika and Jacobson (2003), this is the best distinguishing characteristic. The hairs are supposed to be easiest to see early in the growing season, June through mid-September. Look for hairs on "veins on the basal half of the leaf underside." Any hairs on the top or sides of the leaf don't count.

Hairs on the underside of Japanese knotweed leaves are blunt, broad-based scabers or knobs. They look more like little bumps than hairs. May require 30-40X magnification to see.

Bohemian knotweed has broad-based, stout, single celled hairs. They look more like thorns than hairs.

Giant knotweed has narrow-based, long, multicellular hairs on the underside of the leaf. They look like the common definition of a hair, eg like arm hair. Should be visible with 10-15X magnification.

Miscellaneous Characteristics

Leaf Thickness:
Giant knotweed leaves are thin. Japanese and Bohemian knotweed leaves are thicker and tougher.

Leaf Length:
Japanese knotweed mid-stem leaves are less than 7 inches long. Giant knotweed mid-stem leaves are often more than 12 inches long. Bohemian knotweed leaves vary between those two extremes.

Leaf Tip Shape:
Japanese knotweed leaves tips are "abruptly cuspidate and long acuminate." "Abruptly cuspidate" - there is an abrupt transition from the wider part of the leaf to the narrow tip. "Long acuminate" - the leaf tip is much narrower than the main part of the leaf, and the narrow tip is fairly long (usually about 1/4 inch in my experience).
Giant knotweed leaves taper more gently from the wide part of the leaf to the narrow tip. The tip is typically blunter than Japanese knotweed.
Bohemian knotweed leaf tips are intermediate, "generally long acuminate but not cuspidate." So, they have a long narrow tip, but the transition from the leaf blade is not abrupt.

Branching and Plant Height:
Japanese knotweed - shorter (1.5-2m, occasionally up to 2.5m), branches profusely
Bohemian knotweed - intermediate height
Giant knotweed - much taller (to 5.3m), branches infrequently

Relative size of inflorescence and its subtending leaf
Japanese knotweed - inflorescence typically longer than leaf
Bohemian knotweed - variable
Giant knotweed - inflorescence much shorter than leaf

For further information, see:

Zika, Peter F., and Arthur L. Jacobson. "AN OVERLOOKED HYBRID JAPANESE KNOTWEED (POLYGONUM CUSPIDATUM × SACHALINENSE; POLYGONACEAE) IN NORTH AMERICA." Rhodora 105, no. 922 (2003): 143-52. http://www.jstor.org/stable/23313523.

Identification and control factsheet from the Minnesota Departments of Agriculture, Natural Resources and Transportation

Invasive species compendium datasheets for Japanese knotweed , Bohemian knotweed and giant knotweed

There's a decent comparative diagram at the end of this guide from the UK.

GoBotany has a dichotomous key for distinguishing Japanese knotweed from Giant knotweed.

Extreme differences in population structure and genetic diversity for three invasive congeners: knotweeds in western North America by John F. Gaskin, Mark Schwarzländer, Fritzi S. Grevstad, Marijka A. Haverhals, Robert S. Bourchier and Timothy W. Miller, Biological Invasions, October 2014.

Cytological and morphological variation of Fallopia sect. Reynoutria taxa (Polygonaceae) in the Krivánska Malá Fatra Mountains (Slovakia) by Mereďa, Pavol, Jr, Zuzana Koláriková and Iva Hodálová, Biologia, March 2019.