Knotweed: The naughty and the not

To be honest, most knotweeds are at least a little naughty. They can spreaaaaaaad. That might be a good trait in a ground cover. But some, notably the invasive and hard to eradicate Japanese knotweed (Reynoutria japonica, syn. Fallopia japonica, syn. Polygonum cuspidatum), are very, very naughty indeed. This PDF from the Ontario Invasive Plant Council explains.

Others, like our covergirl, Persicaria amplexicaulis ‘Firetail’ seen at the Toronto Botanical Garden, can make handsome garden plants, for flowers, or foliage, or both.

Knotweeds (in the diverse buckwheat or Polygonaceae family) were the plants I said were used well in June Blake’s garden. And they’d also work here in Canada – in the right spot, and with a bit of caution. Let’s look at examples.

Don’t know the cultivar of this knotweed in Blake’s garden – anyone? Fatter and puffier than Persicaria ‘Firetail’, the red spikes make a pretty picture with the Rudbeckia and purple Lobelia.

Cultivated knotweeds that used to be Polygonum have been subdivided into other genera, including Persicaria and Fallopia. You might see them labelled with one of a chain of aliases, like the Japanese knotweed near the beginning of the post. Looks like the most garden-worthy ones fall under Persicaria. But there are exceptions, as you’ll see.

Massed red on pink Persicaria in the June Blake garden.
Blake marks the edge of her steps with a cascade of groundcover knotweed (Polygonum affine syn. Persicaria affinis)

I first saw Polygonum affine used as a groundcover at the International Garden Festival at Jardins de Métis in Quebec, then more recently in a garden in Toronto’s Bluffs. So I know it’s cold-hardy. It makes short, abundant flower spikes that deepen in colour as they age, giving it a two-toned quality. The cold weather brings out a strong red in the leaves. But the flower spikes are shorter than the other species we’ve looked at – less than 25 cm/10″.

Shot in closeup from way down at ground level, this cultivar might be P. affine ‘Darjeeling Red’.

Knotweeds tend to have spiky flowers like these, and have common names such as fleeceflower. Again, there are exceptions, such as the fluffy panicles of silver lace vine (Fallopia aubertii syn. Polygonum aubertii). But many knotweeds are also grown for their attractively marked foliage.

V-shaped markings on an unknown Persicaria make a foliage statement in June Blake’s entry bed.
Unsure if the red is the fall colour coming through? Blake had reddish Persicaria in the ground as well as in this container, which probably helps curb its wandering ways.

The two photos below were taken on Toronto garden tours. At left is P. ‘Painter’s Palette’ (Persicaria virginiana syn. Polygonum virginianum), which has the double beauty of white variegation and red V-markings. If you click any picture to see the slideshow, you’ll get a clearer image of the red stems of the white-variegated one on the right (at the time, the Toronto Botanical Garden’s Paul Zammit told me it was a Fallopia, but I don’t know the species or cultivar).

The three below are from the garden of D.C. Fling organizer, Tammy Schmitt, and two were labelled. Left is Persicaria microcephala syn. P. runcinata ‘Purple Fantasy’. Tammy was growing it in part-shade, which might not bring out the full purple colouration. Still, they’re handsome, smaller, pointy leaves with strong markings. At right, it’s Persicaria microcephala ‘Red Dragon’, which might just be the cultivar in Blake’s garden, both the potted red specimen and the one in her entry bed. These newer species might be borderline hardy in Canadian Z6/USDA Z5 with reliable winter snowcover or mulching – and our cold winters could reduce the spreadage.

The centre plant was unlabelled; possibly another variegated Fallopia? I include it because growing these vigorous plants in pots would be a smart strategy for small gardens. (Perhaps you have space in an unheated garage?)

Speaking of small, back to a Toronto East York garden in June. This large display of Persicaria bistorta ‘Superba’ begged to be photographed. I like the colour echo with the falls of the yellow irises.

Persicaria bistorta (syn. Polygonum bistorta) ‘Superba’

Then back to where we began at the Toronto Botanical Garden, with P. ‘Firetail’ (left, below) looking just fabulous (yes, that’s the technical term) with grasses. And in an east-end garden (right, below), the tall, showy giant fleeceflower (Persicaria polymorpha) illustrating the dramatic range of form.

All members of this family prefer soil that retains a bit of moisture, but might survive some dryness once established – though they probably won’t look their best.

In case you needed reminding about the very naughty knotweed, here’s what Japanese knotweed looks like when it tries to take over the world in Toronto’s wild spaces. As much as you might like its cousins for your garden, don’t let this one take root anywhere near yours. You’ve been warned.

One of a million triffid-like patches of Japanese knotweed in Toronto parks, trails and ravines. Grrrrrrr.

7 comments

  1. Japanese Knotweed is a remedy for Lyme Disease! Nature usually throws up the cure and waits for us to see them.

    Check my blog on N Korea mega volcano that may be activated by testing and or US attack. It’s also a spiritual site for all Koreans. Nuclear winter is a poor time to garden.

    Lovely photos!

    1. Interesting, Kate. I could find a lot of support online from alternative medicine sites for the knotweed/Lyme connection, but nothing that confirms it from a purely scientific perspective. But you never know. Anecdotal evidence seems to be strong.

  2. Planted Persicaria amplexicaulis ‘Firetail’ last year for the 1st time and it came back lovely this year in a part-shade location. Looking forward to next year’s show. Your shot of the Persicaria microcephala syn. P. runcinata ‘Purple Fantasy’ was so pretty I Googlde for purchasing source. Didn’t find any source but found this: https://thebikinggardener.com/2014/05/04/rampant-fantasy/

    1. Yes, I did read that post when researching mine, but I’m guessing that Canadian gardeners (in Toronto’s climate, anyway) might have less of a problem with rampancy than this UK gardener. I guess time will tell.

Leave a Reply

Your email address will not be published. Required fields are marked *

You might also like