Weekly Weeder: Giant and Japanese Knotweed

          When a plant has “weed” in its common name, it is a warning to all gardeners: plant or allow it at your own risk.  This goes double for knotweeds, particularly Japanese knotweed, Fallopia japonica, and its relatives such as Giant knotweed, Fallopia sachalinensis.

          This is a plant that grows up to about 13 feet in crowded situations, but in my yard topped out at 6 feet.  It has heart-shaped leaves around 6 inches long, and stems around an inch thick, knotty and hollow like bamboo, but too soft to make anything with and they die back to the ground in the winter.  They have oxalic acid which, according to Euell Gibbons, author of several foraging books, makes it a rhubarb substitute, but in my opinion, not a good one.  It also has resveratrol and thus has some medicinal uses.  It has large, rhizome roots and little white flowers in triangular clusters that smell like common privet, very sweet, and bees love it.  It is a good source of nectar at a handy time of year, making a light “buckwheat” honey.

          Yes, I transplanted it from the wild into my yard.  At least I did this to my yard alone.  I planted it in a long, curving row as a windbreak, and also in one corner of the yard, with some Arrow and palmate bamboo—another big mistake, one that I’m still paying for.  It grew well and made a great summer windbreak, except that I had to keep it from spreading, mainly by roots, but I also got the occasional seedling.  At least it does not readily make seed, although knotweeds are part of the polygonium family, literally, “many seeds.”  There is a hybrid of Japanese and Giant knotweed that makes many seeds that is taking over parts of Canada.

Blooming Japanese knotweeds, with spent flowers (deadheads) in the foreground

          The real trouble came when I tried to get rid of that curving bed several years later as I changed the arrangement of my garden beds.  The root mass had grown too large and hard for me to dig it out.  Wikipedia says that its roots can grow 23 feet wide and 10 feet deep, making it nearly impossible to control by digging.  I sprayed it with Roundup, but the root was too big for the poison to make an impression on it, not even producing yellow, stunted growth.  It took me 2 or 3 years to kill it out by cutting the sprouts as I saw them.  I probably only killed it that fast because it was relatively young.  I regret that I didn’t go after the stuff in the corner at that time; I was cutting it for the second time the other day.  It had sprung up 4 to 5 feet in the month since the first cutting, while the blackberries around it grew only 18 inches.

          It scares banks so much that many UK banks would not lend on a property where it is present until recently.  Now they look more closely at the situation, but still may deny a loan because of it. 

                             

Rycke Brown, Natural Gardener          541-955-9040        rycke@gardener.com