JAPANESE KNOTWEED AKA  Reynoutria Japonica, Fallopia Japonica

Almost every time I search for or take a photograph of the "invasive plant of the week" I am surprised by the plant's decorative potential!  The first photo is a good depiction of what the plant looks like at its present stage of its growth.  The second photo shows the mature plant in flower.  Although commonly called Japanese knotweed this plant is native to all of Eastern Asia. When in bloom the plant is positively loaded with gorgeous foamy white flower spikes that are attractive to pollinators.  An interesting note is that the young shoots and leaves of this plant are edible.  The shoots have a tart flavor similar to rhubarb and can be used as its substitution in recipes.  Should you be interested, recipes for knotweed, including knotweed wine, sorbet, and chutney can be found on the internet.  If you are choosing to harvest this plant for consumption, be certain you are harvesting the correct plant and that it is not from an area that has been treated with chemicals!

How did this plant arrive on our shores?  Female plants, thought to be sterile, were first introduced in the prestigious Kew Botanical Gardens in England in 1850. During the 1880's it became a desirable exotic plant for inclusion on the property of several expansive Long Island, NY estates.  It appeared as an ornamental plant in nursery catalogs until the latter 1930's.  Even then it was under watchful surveillance as a suspected outlaw and garden escapee.  

How did this plant arrive in Scotland, CT?  One of its adapted uses was for erosion control.  Stands of it were installed on highways and state roads.  This is likely the cause of its present bounty along some of our roadsides in town.  When traveling through Scotland on our town and state roads, you can't help but notice vast colonies of knotweed so dense that nothing else grows among it.  The reason for that is that it, like some other invasive plants, it has an allelopathic superpower.  The plant's roots and other decaying plant parts exude chemical compounds that prevent other plants from growing in its midst.  The plant also has dense lush upper leaf growth, keeping any brave potential competitors from finding the sunlight they require to grow.  This is how Japanese knotweed quickly creates a monoculture.  It crowds out native species that many of our native insects and animals require to thrive. You may wonder, how did only female plants reproduce so prolifically?  There are a couple of answers to that question.  One is that are other several species of knotweed and if one of these happens to be located nearby, there is a potential for cross pollination, which would create viable seed.  This plant, however, spreads primarily through its root system, technically called a rhizome system.  Rhizomes are roots that grow horizontally periodically sending up new growth. In the case of knotweed, these rhizomes are hardy enough to penetrate through any small cracks in a house foundation or any pavement potentially causing severe damage.  In some areas, the presence of this invasive plant is actually having a detrimental effect on real estate values.  Although the plant goes dormant in the winter months and temporarily dies back when there is a spring freeze, the rhizomes remain viable underground.  Even if plant growth is suppressed and no additional nutrients are added, it is said that the rhizomes are capable of remaining viable for up to 20 years!  Any small bits of this powerful rhizome system accidentally transported by a snowplow, brush cutter or unintentionally by a garden shovel will readily form a new ever-increasing colony.  If this plant colonizes along our waterways, the diverted water flow caused by dense stands of Japanese knotweed can cause flooding and potential bank erosion.  As riverbanks erode, segments of the plant's rhizomes could be transported downstream.  Rivers brooks and streams are a virtual highway for the plant to follow. Elimination of this plant should definitely be a priority!

HOW TO REMOVE

Take a deep breath!  This is going to be a difficult but not impossible task.  It is one that will likely involve a combination of techniques possibly including the use of herbicides.

If you want to try a method that does not use herbicides the present recommended method is to cut the plant back three times during its growing period.  Once in June, again in July, and last in late August.  The repeated cuttings are meant to keep the plant from flowering.  Cut as low to the ground as possible.  The stem has what look like joints or notches which are called nodes.  You want to cut the plant beneath the plant's lowest node.  Cutting the plant numerous times also helps to reduce the plants energy transfer to its rhizomes.  After cutting, the knotweed patch can be covered with black plastic carefully weighted around the edges.  The cut stems should be placed in contractor bags and disposed of in regular trash.  Do not compost them, as any pieces of attached rhizome could sprout in your compost pile! This method of removal will need to be repeated for at least three years.  Continue to monitor the area as this plant can be very persistent. 

If you are comfortable using herbicides, The recommended method is to cut the plant back to 2 to 3" in late May or June, and then let it grow until August.  Spray before the plant flowers using glyphosate with an added surfactant or with triclopyr.  Many pollinators feed on this plant so spraying either before flowers form or after the flowering stage is complete is essential so as not to harm them. Cutting the plant back in the spring reduces the height of the plant, therefore the amount of chemical needed for coverage.  The same protocol will need to be repeated the following year.  After the second year of treatment, plant the area with some native species to reduce the growth of Knotweed.  You will likely need to continue treatment every two years following the second treatment.

Biological Control:  An insect called Japanese Knotweed Psyllid or Aphalora itadori was introduced in 2020 as a possible biological control method.  The insect is host specific to Invasive knotweed.  During all of its life stages, the insect consumes the sap from the leaves and stem.  This in theory weakens the plant.  In the United States in 2020, several releases were done in areas including Rhode Island and Massachusetts.  Other releases in the UK and Canada done back in 2010 have resulted in populations of the Knotweed Psyllid having established a presence in that area. It remains to be determined how successful the release of this insect in the United States will be.