Never Turn Your Back on Japanese Knotweed

[Pompeii]

Never Turn Your Back on Japanese Knotweed

Cheryl J. Shenkle

September 2022

It’s beautiful when in bloom and quickly fills empty garden spaces with a huge, exotic green presence.  Do some people actually like it?  Yes, but eventually they find it is an invasive terrorist that quickly takes over whole acres of land.  It causes erosion on riverbanks, destroys native plant species and spreads cross country via wind, water, animals, humans and as a contaminant in soil that is moved to a different location. Japanese Knotweed freezes to the ground in winter but pops back up in increasingly large areas in the spring.  It is almost always found in roadside ditches, along streams and in soggy soils, and will even work its way through concrete walls with small cracks, sidewalks, bridges, roads, floor boards and buildings. 

Can you simply cut it down and or use toxic sprays and be rid of it? Absolutely not!  Without a minimum two year plan and lots of time and money just to get rid of the worst of it, it will happily continue to gobble up land and destroy native plant species in its path without mercy.  If you harbor it in your garden or on your land, your neighbors and entire community may eventually be blessed with this gift of destruction.

The larger the area it covers, the more money and time it will take to bring it under control.  To completely conquer it may take years of constant scouting for new shoots.  A spring attack and another in the fall is normally recommended if you use Glyphosate which can be deadly to all life around it and should never be used near flowing water.  However, that said, I would still monitor the site every week to catch all shoots that emerge and kill them immediately. Digging out the plants is useless because you will probably not get all of the roots, and wherever you put the soil you removed may contain root pieces that could emerge later.

The dense canopy of a stand of Japanese Knotweed begins its damage by restricting the growth of Native Species through lack of sunlight, and a thick layer of old leaves and stems on the surface of the ground containing growth chemicals which stop the sprouting  Native species.  Our native insects and plant diseases have no effect on its leaves although the Japanese Knotweed Psyllid has been identified which can kill it.  Studies must be completed to be sure the insect will not also affect our own native species.

Knotweed spreads through thick underground Rhizomes and roots up to 20 feet long and 6-9 feet deep in the ground.  Those Rhizomes store food that sun and leaves provide through Photosynthesis for the current year and into the next year.  Cutting down the plant one time will deplete some of the food stores but not all of them.  There is usually enough left to grow the entire mass again and to grow new shoots the following spring.  Then, unless you continually cut away all stems and leaves in the next year, it will quickly grow to its previous size and you’re back to square one.

Once cut or sprayed down, the plant will immediately begin to send up all shoots possible in order for the new growth to begin Photosynthesis in the leaves which creates the sugars needed to live and support new growth. The sugars are immediately sent down into the roots to feed the new leaves.  It’s a never ending fight to survive that the Japanese Knotweed will always win without a well-timed and continuing attack on its food production process. It must be starved of sugars for at least two complete years for the roots to die.

Glyphosate is the only chemical found to be partially destructive to the plant, but it also kills every other plant in the same vicinity and cannot be used near a water source.  Its use as a spray also requires protective clothing and is always a safety concern.

Enterprising companies are developing uses for the plant.  Honey and food supplements are just a few.  Additionally, the plant has many uses as food and medicine.  

For anyone who may contemplate harboring this beautiful but invasive, destructive plant, consider the devastation of the Kudzu Vine in the Southern states. 

Known as the vine that ate the south, even taking down powerlines, Kudzu destroys over 120,000 acres every year.  It’s minor considering that Japanese Knotweed will even overtake and kill Kudzu.  Knotweed has been listed officially as the world’s worst invasive species in Asia, Europe and now in North America. Mortgage companies have even refused to finance properties because of the amount of invasive plants presently growing there.

Ignore stands of Japanese Knotweed at your own risk.  Current costs to the United States in eradication efforts alone rise to $50 Million a year and over $100 Million per year in damages, and that amount is growing annually. 

I also am amazed by the beauty of Japanese Knotweed when it blooms in September, but it’s a deadly shrub in disguise, so never turn your back on it!  It will win.

[Zilch1]

I have this weed growing on the side of my driveway. Don't know where it came from. On a hot summer day it can grow 10 to 12 inches. It is impossible to kill and it takes a machete to cut it.

[Petee]

If you read the article, then you know how to kill it.  It has to be starved of all sugars for at least two years.  Ignore it at your own risk.