This had me stumped, so I asked other master gardeners to help identify your plant. It is most likely Persicaria amplexicaulis, commonly called the red bistort or mountain fleece. Here is a link to a photo of a variety that looks very much like your plant – from the Royal Horticultural Society, Persicaria amplexicaulis ‘Firetail’. The Missouri Botanical Garden has more detailed information on the plant. It is a perennial that will spread steadily, but is not considered invasive.
To illustrate how challenging it can be to identify plants from a photo, consider the following, which were also suggested as being possibilities. Some of these belong to the same genus as your plant, Persicaria, so indeed are related!):
- Persicaria virginiana (Variegata Group) ‘Painter’s Palette’ (commonly called knotweed) also looks very similar to your plant. Some varieties do not have variegated leaves. [This plant is not overly invasive]
- A type of Japanese knotweed (which is VERY invasive) – Fallopia japonica. Here’s an overview on this weed, from Ontario’s Ministry of Agriculture, Food & Rural Affairs: Ontario Weeds: Japanese knotweed. So, although it looks like a knotweed, there are some inconsistencies: your plant’s leaf shape look more heart-shaped than leaves of many knotweeds. As well, the manner in which your plant’s flowers are borne (the clusters are at the tip of the stem) seems to differ from knotweeds. See also A guide to the identification and control of exotic invasive species in Ontario’s hardwood forests , which provides helpful instructions in how to identify these invasives. The Missouri Botanical Garden has helpful information about a compact variety of Japanese knotweed; take a look at the “comments” link, where several people highlight the invasive nature of the plant.
- Persicaria affinis or Polygonum affine – Himalayan knotweed or fleeceflower(this is also VERY invasive). Here’s a link to the Royal Horticultural Society’s Persicaria affinis ‘Superba’, which includes some information on the plant.The BC Ministry of Forests and Range posted the Key to Identification of Invasive Knotweeds in British Columbia.
- Amaranthus, also called Love-lies-bleeding, or tassel-flower, which is somewhat invasive. See the Missouri Botanical Garden’s profile of one such variety, Amaranthus caudatus. There are many different Amaranthus varieties.
So you see that identifying a plant is often quite challenging, and this is complicated by the fact that sometimes a few plants may have the same common name, and that different members of a genus (i.e., Persicaria, which has 66 “accepted” species names, many of which are commonly called “knotweed”) can look so similar, although they may have very different characteristics (e.g., invasive versus not).
Finally – you have a lovely, non-invasive perennial in your garden. It is not a native plant in Ontario.