Prostrate Knotweed

Last fall we had some cement work done on the sidewalk leading to the deck. The ground had settled significantly in the 20 years since the house was built, and the cement sidewalk settled with it. This project was to correct that. In the process of completing the work, the ground next to the sidewalk was disturbed and heavily compacted. No grass was growing there this spring. But nature abhors bare dirt, so she sent in the cavalry—prostrate knotweed (Polygonum aviculare).

And the cavalry was ready. Millions of knotweed seeds have lain dormant in the soil on Owl Acres for years. Iowa winters are nothing to them—the seeds remain viable as far north as zone 2 and as far south as zone 10. In short, they are everywhere, just waiting for their chance to shine. So when they find some nice compacted clay, a little moisture and early spring sunshine, they’re ready to charge. The first thing they do is send up their little first-leaves. These leaves look like grass and raise hopes that perhaps grass is going to grow here this year after all. But no—it’s just a trick. The knotweed gets ahead of the real grass (and the crabgrass) and starts colonizing the space, crowding out the grass.

Knotweed goes by a long list of other names, none of them particularly complimentary. They include prostrate knotgrass, birdweed, doorweed, armstrong, sanguinaria, centinode, yard knotweed, matgrass, stonegrass, waygrass, wiregrass, wireweed, lowgrass and pigweed. Most of these names refer to the way the plant grows. Its wiry stems lie on the ground, and its roots go deep into compacted soil, making it hard to pull out and remove. It forms dense mats on the ground. 

Polygonum, the first part of its scientific name, means “many knees,” referring to the little knobby nodes occurring at the base of the rather sparse leaves. The second part, Aviculare, means “little bird,” referring to the shape of the fruit, which reminded somebody of a bird’s beak.

The mature plant sends out long runners in all directions which branch and reach two to three feet along the ground. The nodes on the runners are covered in a papery sheath, and they don’t root. The stems just tangle into mats with the wiry stems growing over each other. The leaves on the stems are spaced about an inch apart and are blue-green in color and oval in shape.

From June through October, clusters of tiny white flowers (less than a tenth of an inch across) develop at the base of the leaf stems. Beginning in early summer, the flowers develop into tiny three-sided fruits containing hundreds of seeds. An individual plant can manufacture 15,000 seeds in a season. If you were very patient and dedicated—or very hungry—you could gather these tiny fruits and grind them into a flour. They are edible. If the seeds escape your culinary efforts, they will need light and at least 40 degrees of soil temperature to sprout. But if your main concern is crabgrass, note that knotweed comes up first.

Prostrate knotweed is an annual invasive originating in Eurasia. Because of its preference for poor, compacted soil, its ground-hugging tendency, and its tolerance for drought and being trampled, it is an effective ground cover for disturbed soil. With that in mind, we’ll leave it this year while new grass gets established.

Photo by Author. Alt text: You might think this an ordinary weed, climbing over the sidewalk, but it’s not. It’s knotweed.