Several years ago, while leafing through the catalog of a German perennial nursery, I came across a photograph that gave me palpitations. The label read “Polygonum polymorphum—Knöterich (nicht wuchernd).” It was a strapping plant that looked to be about four or five feet tall, densely clothed in large, tapered, deep green leaves and practically smothered in panicles of tiny, creamy white flowers, like an oversize astilbe or a pumped-up goatsbeard. I wanted it very badly. Unfortunately, it was a Knöterich—a knotweed—and knotweeds being what they are, I figured it was bound to be one of those horrors that would swallow up the garden in a single season and then proceed to colonize the entire continent, resulting in complete ecological collapse. Still, there was that enigmatic phrase, nicht wuchernd, so I hauled out the bilingual dictionary, where I discovered that wuchern means “to grow rampant.” So this knotweed wasn’t a thug after all.
That was enough of a green light for me, and soon thereafter I managed to acquire the plant, which has since become readily available in this country. (Like many other former polygonums, has lately been reborn as a persicaria.) I planted the large, knobby rootstock in a part of the border that gets about a half-day of sun and where the soil is reasonably moist. It settled in quickly, making about three feet of growth its first season. When it began blooming, I felt an even greater thrill than when I saw the photograph—it had the heft and stature of a small shrub, and it looked good with practically everything: delphiniums, daylilies, roses, verbascums, grasses, you name it. Best of all, it kept on blooming all summer; not until about the middle of September did it finally call it quits. I confess I was still a little nervous about its territorial ambitions, but the next year, although it increased in height and girth by about a foot, it stayed firmly where I had planted it; moreover, I haven’t seen a single volunteer seedling.
There is, however, a mystery surrounding P. polymorpha: how can so well-behaved and gardenworthy a plant have escaped notice for so long? Although it bears similarities to certain central Asian species of Persicaria—it shares, for example, the tall stature of P. coriaria and the long bloom period of P. divaricata—it cannot be considered synonymous with any of them. The answer may be that the plant is actually a hybrid of fairly recent origin; in fact, the authoritative German magazine Gartenpraxis claimed a few years back that the name P. polymorpha is erroneous. Mysterious or not, this splendid perennial has quickly proven itself to be indispensable. And remember—it doesn’t wuchern a bit.
A version of this article first appeared in Horticulture magazine, vol. 98, no. 5 (June 2001).