Japanese Knotweed: Accurate Identification

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- The invasive plant Japanese knotweed, an herbaceous perennial member of the buckwheat family, was first introduced from East Asia in the late 1800s as an ornamental, and to stabilize streambanks.

Now, Japanese knotweed is a highly-successful invader across a wide geographic range, most commonly colonizing stream and river corridors, forest edges, roadsides and drainage ditches.

Similar in appearance, ecology, behavior and control requirements, are this plant's close relatives giant knotweed, and bohemian knotweed, a hybrid of Japanese and giant knotweed.

Knotweed infestations decrease biodiversity in both plant and animal communities.

Degrade soil and water quality, and can damage human infrastructure.

The presence of knotweed on your property may also decrease its value.

Training yourself to positively identify knotweed is the first step in being able to control this tenacious invasive plant.

Let's examine the identification characteristics of both Japanese and giant knotweed.

Emerging in early spring, the young growth of knotweed is often bright-red or purple, and tipped with many furled leaves which are distinctly triangular.

Mature knotweed has many alternately arranged spade or heart-shaped leaves, emerging from nodes along the stem.

The lower leaves are often shed as the plant grows, leaving the bamboo-like stem bare.

Japanese knotweed leaves can be up to six inches long, and have a squared leaf base.

Giant or hybrid knotweed leaves will grow much larger up to a foot in length and have a rounded leaf base.

In late summer, white or pale-green flower clusters sprout from the nodes.

These upright finger-like clusters consist of several dozen tiny five-petaled aromatic flowers.

The stems are jointed and hollow with red or purple nodes where the leaves attach.

They're otherwise smooth, bright-green and often covered with darker spots or streaks.

Portions of the upper stem appear to zigzag from node to node.

The dense, low canopy formed by their thicket of tangled stems and large leaves, creates a monoculture, excluding nearly all other vegetation.

Compared to native streamside vegetation, knotweed is much less effective at controlling erosion and its presence along streams gradually degrades aquatic habitat, and water quality.

Be aware that knotweed is sometimes confused with another invasive plant, bamboo.

But bamboo has slender, papery leaves which persist year-round.

Bamboo stems are also jointed and hollow, but are tough and woody, while living knotweed stems are herbaceous and will appear wet when cut.

The first step in controlling knotweed on your property is positive identification.

To review, be on the lookout for bright-red or purple young growth of knotweed in early spring.

And alternately arranged, sometimes zigzagging spade or heart-shaped leaves in mature stems.

In late summer, knotweed will have white or pale-green flower clusters sprouting from the nodes.

Once you have positively identified Japanese knotweed or its close relatives, it is time to start control measures against this aggressive invasive plant.