Prolific Japanese Knotweed on the move

Japanese Knotweed

Dr. Bob Gilbert – Columnist

It is September and currently the Japanese Knotweed is blooming and spreading rapidly above and below ground.  It belongs to the genus Polygonum. It is very difficult to tell it apart from its various other family members. Japanese Knot Weed has large triangular shaped leaves and purple segmented knots or nodules on their hollow stems. Many in the Knotweed family share common names such as Japanese Knotweed, Smartweed, Japanese Bamboo, Mexican Bamboo, Fleece Flower, Hancock’s Curse, Outhouse Weed, Sally Rhubarb, Peashooter Plant, Crimson Beauty and Monkey Weed.  It is sort of fun to use the common names to construct in your mind’s eye what the plant looks like.  Its segmented stems are like bamboo and are hollow. Its white blooms are soft and silky.  There is a variety that has red blooms and others that open white and turn pink as they mature. 

According to Alan Armitage, the genus Polygonum and its many species have been  “split, lumped and dumped so that identification is even more confusing.”  

Many Knotweeds look similar.  The Latin genus name is Polygonum (po-lig-o-num). I am not sure but I think that means many gonads.  The plant certainly is prolific.  Its Latin plant name should not be confused with Polygonaatum (Poly-go-natum) – Solomon’s Seal, a completely different wildflower.

 Japanese Knotweed was introduced into the U.S .in the 1800s from Japan as an ornamental.  Reportedly it is rampant in Northeastern United States oddly spreading faster in the North than the South. 

Late summer and early fall these shrubs are very noticeable and are alarmingly common.  They grow in full sun along road sides and cut-over areas.  They are invasive by producing lots of seeds and by spreading long roots. There are more underground roots (stems) than above ground.   It has been rated as one of the world’s 100 worse invasive species.  Its name Hancock’s Curse comes from England where a Hancock family planted some in their back yard and it escaped, populating their entire neighborhood.

What is curious is that one of the Southeast’s most reputable nurseries, Plant Delights, offers this plant for sale stating their selection “does not run and is not invasive.”  I would not risk it.  On the other hand, there are some positive attributes to these plants. Its blooms are a good pollen source for bees.  In the fall, Sourwood honey is very popular but there could be new fall honey such as Outhouse honey or Monkey Honey.  I will have to say it is good for erosion control, attractive and edible.  

Sally Rhubarb is a curious common name.  I do not use the word sally often but Webster defines it as meaning as “rush forward,” another way of saying these plants are invasive and have a mild rhubarb taste.

If it is already growing on your grounds it should be eliminated.  Best to cut it to the ground in the spring but any time  will do.   Apply an herbicide to each cut stem.  When and if new shoots emerge, spray them with a herbicide or try horticultural vinegar. You might have to spray more than once.  Remember is it is very active underground. 

One report, trying to make a point, says that it will send seeds back home, all the way to Japan.  We have imported so many plants from Japan that any new introduction automatically becomes suspicious.   Always remember that new imports have no natural predators here.  There is nothing to control their spread whereas in its native environment there is at least one or more controlling predators.  Often our attempts to introduce a predator backfires and gets out of hand.  Asian Lady Beetles are a perfect example.

Dr. Bob Gilbert is cofounder of Smith Gilbert Gardens in Kennesaw, Ga.

Karen Lawrence is a professional nature and wildlife photographer of Franklin.