Options vary for invasive species: Smother knotweed or maybe eat it

I seldom make it to the parking lot after finishing a talk on invasive species without at least one desperate gardener asking how to eradicate Japanese knotweed. Also called Mexican bamboo and Japanese bamboo – actually related more to rhubarb than to bamboo -Japanese knotweed (Fallopia japonica), native to Japan, Korea, Taiwan and China, is now firmly established in 39 states and Canada as the most noxious of weeds.

It was first introduced to Europe in the mid-18th century by a Bavarian physician, Phillipp Franz Balthasar von Siebold. Stationed in Japan with the Dutch East Indies Army, Siebold smuggled two shipments of Asian plants home to Europe. Among these plants was F. japonica. By the late 18th century, Japanese knotweed plants were being sold as ornamentals in nursery catalogs in the United States.

A member of the buckwheat family, Japanese knotweed is an upright, shrubby, semiwoody perennial that can grow to more than 10 feet in height. The stems are hollow, smooth, stout, and swollen at the nodes – they do resemble the canes of bamboo -and they rise each spring from coarse spreading underground stems (rhizomes) to form dense leafy thickets. The stems die each winter but persist, new shoots emerging in spring from beneath a canopy of dead woody canes. Year by year the thicket grows in size, claiming more and more of the garden’s space.

Gardeners who battle this weed in Maine look weary and defeated as they speak of waging war each growing season, of futile attempts to restrain the monster’s spread, of living in fear that it will spread by seed or underground stem to take over the entire garden. They talk about campaigns to dig it out, burn it out, or annihilate it with herbicides.

It keeps coming back. Digging propagates the rhizomes by division. Rhizomes and seeds are imported in soil used as fill and in topsoil purchased for the garden. Any open space with moist soil is subject to invasion, even forest clearings.

My recommendation is to smother the knotweed with overlapping tarps that extend well beyond the colony’s current borders. Put down the tarps in early spring after cutting the previous year’s canes flush with the ground, placing large stones along the edges and seams to keep the tarps in place.

The new shoots will start to grow in spring, lifting the tarp like tent poles. Unable to reach the light, they will quickly weaken and can be trampled down by foot. Keep the tarp in place for several years to ensure that all of the rhizomes have starved to death. Better yet, construct raised beds on top of the tarps and continue gardening!

If any shoots should manage to emerge around the perimeter of the tarp, consider eating them. Yes, Japanese knotweed stems are edible, the young shoots (cut when 6 to 8 inches tall) reported to be tangy and tart like rhubarb, only better. Don’t eat the leaves, only the stems.

Like rhubarb, knotweed stems can be used to offset the sweetness of fruits in pies and jams. For example, one Internet recipe for Apple and Knotweed Pie calls for a ratio of nine parts apples to one part sliced knotweed shoots. I don’t know about this – I think I would make sure the tarps eliminate the need for this form of weed control.

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