Giant Knotweed vs Japanese Knotweed: The Leaf Test

Turn a leaf over and look at where the blade meets the stalk. Japanese knotweed has a squared-off base, as if the bottom of the leaf were cut straight across. Giant knotweed has a heart-shaped base with rounded lobes. Then check the size: Japanese knotweed leaves are 4 to 6 inches long; giant knotweed leaves run 6 to 14 inches.

The leaf-base test

1. Pick a leaf from the middle of a stem, not the tiny leaves near the tip.
2. Look at the base, not the tip. The tip of both leaves narrows to a point. The base is the difference.
3. Trace the edge of the blade where it meets the stalk. A squared base meets the stalk in a straight line. A heart-shaped base curves out into two rounded lobes first.
4. Measure the blade. Under 6 inches points to Japanese knotweed. Over 6 inches points to giant knotweed.
5. Check the underside veins with a hand lens. Wisconsin DNR says the two species can also be separated by the type of hair on the veins underneath the leaf. Use this only as a third check, because the hair type is hard to see in the field.

It is really three plants

The pair is not always a pair. Japanese and giant knotweed hybridize, and the offspring is called Bohemian knotweed. Because it is a hybrid, a plant that does not fit either column may be Bohemian knotweed rather than a badly identified parent.

Wisconsin DNR says all three are equally invasive. That matters because a two-way answer can be wrong even when the leaf test is done correctly. If the plant sits between the two descriptions, call it Bohemian knotweed and treat it as knotweed.

Does the difference change what you do?

For control, not much. All three have deep rhizomes, all three resprout from fragments, and all three need repeated treatment over more than one season. Cut stems and rhizome pieces can start new plants, so do not move soil or leave debris on the ground.

For reporting, it can matter. Wisconsin classifies giant knotweed as Prohibited and Japanese knotweed as Restricted. Those are different legal categories in that state, and the same split may not exist in yours. If the plant is on a property line, near a waterway or subject to a local ordinance, confirm the identification and the local rule before you act.

What not to do

Do not cut it once and walk away. Cutting without follow-up stimulates regrowth, and repeated cutting is needed to exhaust the root reserves. Do not till the area, and do not move soil that may contain rhizomes. A fragment as small as 1 inch of giant knotweed can resprout.

If you use an herbicide, follow the product label; the label is the law. The label for the product you actually bought controls the rate, the timing and whether it can be used near water.

For the full removal sequence, see Japanese knotweed removal. For the species profile, see Japanese knotweed. For the bamboo confusion, see bamboo vs Japanese knotweed. For the other pairs, see look-alikes.