Japanese knotweed (classified as Fallopia japonica in Europe and Polygonum cuspidatum in North America) has been blooming for weeks around here and will continue to do so until the frosts begin the kill the plants. The inflorescences are ragged creamy-white panicles that spring upward from older growth on the arching branches that are lined with alternate shield-shaped leaves. These are perennial herbaceous plants (they die back to canes in the winter), but they look as large as shrubs, standing six to 10 feet tall in many cases. They are abundant along roadsides at the boundaries between woodland and open areas and also along watercourses.
And they are widely reviled as nightmarish invasives and for their almost supernatural ability to persist in face of efforts to eradicate them.
Their introduction to this country came from Europe, where they had been introduced from their native range in eastern Asia (Japan, Korea, and China). None other than Frederick Law Olmsted, designer of Central Park as well as innumerable other public and private landscapes, was a contributor to the ubiquity of this impossible member of the Polygonaceae or buckwheat family. Like a lot of other late 19th century designers he was impressed by its rapidity of growth—about a meter per month—and its sturdiness—it can grow in almost any soil and tolerate both heat and cold. In addition, by blooming late in the season, knotweed keeps the cavalcade of flowering shrubs going in a designed landscape.
Knotweed emerges in late April and starts growing more than an inch per day. At this stage it is actually edible and is supposed to taste something like rhubarb. The canes, which look a bit like reddish-purple asparagus, should be harvested when they are less than a foot tall. As they get taller the outer layer starts to toughen and needs to be peeled off before eating.
The recipes on the Internet are often variations on what people usually make with rhubarb, a strawberry-knotweed pie, for example. One site warns that knotweed shares with rhubarb not just a flavor, but also a laxative quality, and urges experimenters to try a little bit at first.
By the time it is two feet tall the plant begins to lignify, and at maturity is quite woody. The stems are punctuated by encircling ridges at the leaf nodes that cause it to resemble bamboo (another one of its vernacular names is “false bamboo”). The leaves and petioles die back in the late fall, leaving the naked bamboo-like stems sticking up until the spring.
The root system is extensive and deep (extending down seven feet) and a new plant can spring up from any part of the root. Actually, any part of the plant can be cut off from the rest of the plant and if it is left to lie for several years, it can still start to grow into a new plant if placed in contact with soil.
Some of the comments to one of the “edible knotweed” sites warned against the roadside-to-table approach because cutting it up and bringing it anywhere is a recipe for spreading the pesky plant further. The gourmet adventurers have responded by warning against adding the scraps to your own or municipal compost piles.
One can only imagine the chagrin of 19th century landscape designers who thought that they had found the perfect space-filling, hedge-making, flowering perennial, only to discover they had imported a monster. In my own yard knotweed appeared in two places, but luckily has not spread far. But it was my experience that digging it up and removing it did little but set it back a bit. It happened to be in a place that is now regularly mowed (it had been a meadow, mowed only once a year), and yet the stuff still continues to come up, outpacing the grass in reaching new heights before the mower visits again.