Monsters of the Garden: Japanese Knotweed

A menacing grove of Japanese Knotweed

There is a monster that lurks under your garden. It is part hydra: whenever one head is removed, two more form in its place. It is part hive mind: a complex city that lives just out of sight. The monster is impossible to tame or kill. Gardeners can only hope to keep its tendrils at bay in an endless war for backyard dominance. The name of this deadly creature? Japanese knotweed!

I have been waging war against this botanical beast ever since we found a grove on our farm. Year after year we poison, cut, and dispose of this wily weed, but are unable to vanquish it from are yard. Japanese knotweed has taken over a woodpile at the edge of the forest and threatens to extend its tendrils into our fields we use to grow flowers. But where did this monster come from? How does it grow so fast and continue to survive year after year of poisoning and cutting? Do we stand any chance at all against this monster?

Before Japanese knotweed (scientific name Reynoutria japonica) was the tyrant of farmers, gardeners, and preservationists, it was the prized plant of Dutch explorer and entrepreneur Philipp Franz von Siebold. In 1850, Siebold brought the plant to Europe from Japan and sold it to noblemen in Germany as ground cover. Japanese knotweed has been engulfing the European country side ever since. The plant was introduced to America, where it continued its conquest. Today, Japanese knotweed has claimed over 200 acres of land in New York. The plant is so prolific that it is a crime in Great Britain to transport knotweed uncovered. In 2013, lab assistant Kenneth McRae killed his wife and then himself over fear of Japanese knotweed. McRae's suicide note stated that he killed himself over fears of the Japanese knotweed adjacent to his house spreading into his yard. Clearly, something needs to be done about this great nemesis.

A full stem and node of Japanese Knotweed. The node can weigh up to 10 pounds for a large cluster

In order to defeat our foe, we must understand how our foe fights. Japanese knotweed overruns native species by smothering them with a canopy of bamboo like stalks which sprout large palm-like leaves. knotweed stalks can grow to be over 7 feet tall with leaves as large as a square foot in area. The leaves create a barrier over smaller shrubs and grasses, restricting the sunlight available to the smaller plants. The smaller plants are suffocated by the taller Japanese knotweed. Once the plants are dead, they are replaced by the growing field of knotweed.

The knotweed's offensive against your garden is twofold. Japanese knotweed spokes are much more efficient at gathering nutrients from the surrounding soil than most plants native to Europe and North America. Japanese knotweed's native environment of Southeast Asia has soil that is less rich in nutrients like nitrogen than European soil. The agricultural revolution in Europe and the United States during the 1700s and 1800s brought about the widespread use of soil fertilizers to facilitate larger yields for crops like corn and wheat. The same fertilizers were only used in Asia after knotweed was brought to Europe by Siebold.

vertical cross section of a Knotweed stem that shows
 the internal membrane system 

knotweed has a particular structure that facilitates the diffusion of nutrients throughout the plant's stalks. knotweed stalks have a series of semipermeable membranes that travel up and down the otherwise hollow stalk. These membranes allow the plant to exploit concentration differences along the plant to suck water (and other small non-polar molecules like nitrogen) from one end of the plant to the other. The membranes create a series of separate compartments in the stalk, each with a different concentration. Japanese knotweed exploits this difference in concentration to pull resources from one side of the stem to the other at a frightening rate. When one side of the plant is filled with an impermeable substance like sugar, water from the other side of the plant travels up the stem to return the concentration to an equilibrium position. The membrane system allows the plant to work like a vacuum cleaner. When the plant produces glucose, water from the roots will move upward into the top of the plant, carrying nitrogen with it. Japanese knotweed can suck soil of its nutrients, leaving none for the plants that live below it.

The most terrifying aspect of Japanese knotweed is its resilience. knotweed can be cut, mowed, poisoned, and starved and continue to ravage your garden. The key to its resilience is its hidden enormous body. The Japanese knotweed you see above the surface is not 100 individual organisms, but one massive leviathan with a labyrinth of roots connecting nodes and stalks to a central hive mind that controls the whole plant. Below the surface of each group of four or five Japanese knotweed stalks lies a node. The node collects the food made by the stalk and disperses it throughout the plant. A cluster of knotweed can have over a mile of roots underneath the ground. The ability of Japanese knotweed to disperse resources allows the plant to survive if some of its stalks are cut or poisoned. The body can even send out tendrils that invade neighboring areas, spreading the plant.

A Japanese Knotweed root (my hand for scale). Over a mile of roots can exist underneath a large grove of Knotweed.

The root system of the Japanese knotweed is as resilient as it is immense. The wooden roots can grow up to half a foot in diameter. The outer area of the root is covered in a fibrous material that protects the inner portion of the root. 

A sample of the internal and external layers of a Japanese Knotweed root.

The inner layer is made up of material that is similar to a tree root rather than the fibrous outer layer or stalks. These arteries make up the body of the knotweed plant. Even if every stalk was cut down, the interior plant can survive off of the resources it stores throughout the year. 

The Japanese knotweed is certainly a formidable foe for any gardener, but it can be beaten. In order to kill Japanese knotweed, you must be patient, relentless, and willing to get more than a little dirty. The summer before you begin your counteroffensive, make sure to map out where your Japanese knotweed is growing and mark it out with garden flags. In the following spring, clip and dispose of any knotweed stalk that grows every two weeks. The plant will continue to expend its energy growing stalks and will stop expanding its territory. Make sure you dispose all clippings in plastic bags because knotweed clippings will regrow and reattach to the mother plant. In late Summer, apply a generous amount of herbicide to the cut end of the knotweed. As the knotweed retreats for the winter, it will pull the poison into the roots of the plant. Do not be discouraged when the knotweed reemerges the following year. It can take up to four years to fully dispose of Japanese knotweed.

With this information, you are empowered as garden warriors to hunt and kill your nemesis. Go forth and reclaim your gardens from this most monstrous enemy.