Don’t be wooed by pretty but destructive knotweed

When you hear about terrible, even evil, plants, one wonders how they can be allowed to spread so prolifically. And then you look along a roadside or on the edge of wooded area and you notice a gorgeous shrub with fluffy white clusters of spire-like flowers and you know. They woo us with their beauty.

Japanese knotweed (Polygonum cuspidatum) and its cousin giant knotweed (Reynoutria sachalinensis) and their child Bohemian knotweed (Reynoutria x behmica) are objectively beautiful at this time of year. When you see it in the wild — and if you pay any attention to the roadsides now, you will — you’ll probably think: That would be a great plant to have in my garden.

And that’s how you’ll inadvertently invite what is perhaps the most destructive plant growing in Wisconsin into your yard.

There are several invasive plants in contention for that title — Canada thistle, creeping bellflower, buckthorn, reed canary grass and purple loosestrife are serious contenders — but knotweed takes the cake because of its potential for physical harm.

While we’re busy admiring its pretty flowers and attractive foliage, underground rhizomes are reaching deep and wide — up to 65 feet wide and 10 feet deep — displacing native plants and growing up and through cracks in asphalt, concrete, drain pipes and even foundations.

Knotweed does a good job of spreading itself around with rhizomes, but in many cases it gets started in an area by well-meaning but misguided plant lovers who move a small plant to their yards. Masquerading as an innocent bamboo-like plant, it is misidentified as a harmless native plant by some.

And it doesn’t take long for one small plant to turn into a fast-spreading colony, particularly in riparian areas where it thrives. In many cases, it’s not until it grows to a large size that property owners begin to get concerned.

The Wisconsin Department of Natural Resources and many other agencies offer research-based instructions for managing knotweed. The most effective methods call for herbicides, and although triclopyr, picloram, glyphosate and imazapyr can all be used, the first two should not be used near wetlands or water.

While foliar spray applications are OK where possible, the most effective method is a two-step system that involves cutting stems between the first and second internode, then injecting stems with 5 milliliters of glyphosate or triclopyr. Research shows this may be even more effective if stems are cut in June and injected at least eight weeks later.

Regardless of the method, multiple applications over several years are necessary for eradication.

Any knotweed that is cut must be removed and either bagged and thrown away or thoroughly dried on pavement before disposal because any cut material can resprout, making tilling and mowing disastrous management methods.

If you have knotweed on your property, you’ll have to manage it, but if you don’t, you have one important job: Ignore the siren song of that beautiful plant you see blooming now.