Ugly weeds of summer: prostrate knotweed

Prostrate knotweed (Polygonum aviculare)

Prostrate knotweed growing along a path June 12, 2015

Prostrate knotweed is a summer annual that grows in hard, compacted soil near footpaths and gate entrances. It emerges early for a summer annual, more than a month before crabgrass, a fellow summer ugly, makes its appearance. New sprouts of prostrate knotweed will continue to emerge throughout the spring and early summer.

Prostrate knotweed emerging among ornamental rocks June 5, 2018

Cotyledons, the seed leaves or first leaves, are very narrow and grass-like. Leaves are alternate, elliptic to lanceolate in shape, and tend to be dull blue-green, although they are more green than blue when young, as in these photos. All descriptions are from Weeds of the Northeast.

Pulled prostrate knotweed showing slender branching stems, and nodes, called ocrea, surrounding stems at base of leaves June 12, 2015

Prostrate knotweed soon develops many slender branches with whitish-brown sheaths, called ocrea, surrounding stems at the base of leaves. Small pinkish-white flowers with sepals but no petals will appear in axils of leaves later in the summer, followed by achenes, or fruits, with seeds inside. Although dead wire-like stems may persist through winter, prostrate knotweed is largely an annual that reproduces by seed.

Pulled prostrate knotweed showing fibrous roots with thin taproot June 5, 2018

Prostrate knotweed is considered a superior indicator weed of compacted soil. Its common names include door-weed, mat-grass, or way-grass, suggesting that it is common to areas with heavy foot traffic. In his book Weeds: in Defense of Nature’s Most Unloved Plants, Richard Mabey points out that the role of such weeds is to fill the empty spaces left by environmental degradation. They are first succession plants, stabilizing the soil, restricting water loss, and providing shelter, thus creating a more hospitable environment for a succession of plants to come.

From the gardener’s perspective, ameliorating the soil herself might make for a speedier succession of more desirable plants. But if the footpath is a necessary pathway, maybe she should just not worry about it. One homeowner summed up her pragmatic response to prostrate knotweed last summer by remarking, Well, anyway, they’re green. Just leave ’em be.

It may have occurred to some readers that we could use a herbicide, like RoundUp. First off, it’s the case that, once established, prostrate knotweed is difficult to remove with herbicides, according to the Department of Crop and Soil Sciences at University of Michigan website. Secondly, I don’t use herbicides by choice, green weeds being preferable to the ecological harms and health hazards of glyphosate or whatever. It’s also the case that broad-spectrum herbicides will kill clover, and that is totally unacceptable to me.

Prostrate knotweed sprayed with vinegar June 10, 2015

A few years ago, I tried spraying prostrate knotweed with vinegar. Vinegar can be used in areas where there are no desirable plants, such as along a compacted footpath. The vinegar experiment seemed to work for awhile, but soon new grass-like cotyledon leaves could be seen re-emerging, as in the photo above.

Last summer, a member of our family, who was convalescing from serious illness, decided to eradicate prostrate knotweed along the footpath by pulling it out. In between throwing balls for our dogs and enjoying the spring sunshine, he pulled prostrate knotweed, dropping it into handy yard-waste trash cans stationed strategically around the area. He filled many trash cans full of prostrate knotweed and sent them curbside for municipal pickup. His fingers soon became proficient in finding the little knot just below the soil line and pulling up the entire plant with taproot intact. He started noticing prostrate knotweed wherever he went. Alongside the road at a summer cottage. In the pastures of a local sheep farm. A friend had to remind him, in a good-humored sort of way, to look up and watch the sheep they were herding with their dogs, and not look down at weeds.

Prostrate knotweed emerging in grassy area June 13, 2018

Pulling prostrate knotweed was tedious work when the knotweed was small, as can be imagined by contemplating pulling knotweed like that in the photo above. Reward time came when more mature plants grew large and spread out, as seen in the photos below. Pulling one of them yields a huge biomass of green stuff and results in a big empty space devoid of weeds. We had more rain than usual last summer, so on most days the taproots could be pulled out easily without breaking off.

The best part of this prostrate knotweed eradication project was getting the weeder out into the spring sunshine for extended periods of time, which may have aided his convalescence, but it’s also true that it worked to get rid of the weeds. By the middle of July there were no emerging prostrate knotweed to be found. And all the mature plants had been pulled. Eliot Coleman advises that the most effective weeding is done early while weeds are small. And before they go to seed. Will the prostrate knotweed be back this spring. Well, probably, since there may be residual seeds in the soil. But not many plants went to seed last summer, so at least there may be fewer this year. We’ll see what happens.

Prostrate knotweed in a drainage ditch with a more upright growth habit July 13, 2015

One Comment on “Ugly weeds of summer: prostrate knotweed

  1. I never bothered to consider that it is related to knotweed (Polygonum capitatum), but it makes sense now. Fortunately for us, it is somehow not a serious weed. It is difficult to remove where it is established, but it does not spread far. If I remember correctly, it is more invasive in chaparral regions.

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