Removing Japanese Knotweed by Glyphosate Herbicide Injection

Japanese knotweed stalks before and two weeks after glyphosate herbicide injection

David Beaulieu

Those of you who have attempted Japanese knotweed removal in your yards can testify to the fact that the antagonist in this backyard battle is no ordinary weed. Japanese knotweed removal is nightmarish enough without being in the dark as to the options available to you. One option for removing Japanese knotweed (Polygonum cuspidatum) is not widely known: herbicide injection.

Japanese Knotweed Removal by Herbicide Injection

You may be aware that the herbicide, glyphosate is often used to remove Japanese knotweed. Glyphosate, widely sold under the brand names, Roundup and Rodeo, can be applied as a foliar spray (i.e., you spray it on the leaves) to stands of Japanese knotweed, using a pressurized garden sprayer. But did you know that glyphosate can also be injected into Japanese knotweed canes? Call it "death by lethal injection" for Japanese knotweed.

"What would be the advantage of herbicide injection over a foliar spray?" perhaps you ask. Well, glyphosate is a non-selective herbicide. In other words, it wipes out everything in its path! If you are attempting Japanese knotweed removal in the proximity of desirable plants, an errant foliar spray of glyphosate could wreak considerable havoc upon them. Enter herbicide injection as an alternative method for Japanese knotweed removal.

Hit It Right Between the Nodes

An herbicide injection gun puts herbicide directly into hollow stem plants, such as Japanese knotweed. If you inspect a cane of Japanese knotweed closely, you'll see that it is segmented on both dead canes and new shoots of growth. Where one segment ends and another begins, you will find a node. Between the nodes is a hollow tube, as one finds with bamboo. Indeed, "Mexican bamboo" is one of the nicknames for Japanese knotweed.

So what is a "node?" Nodes are the places on a plant's stem from which leaves or branches grow. Likewise, on the branches themselves, the places from which leaves, buds, or other branches grow are called "nodes." Pruning instructions sometimes exploit these nodes as signposts that one can go by to prune a plant. By making your cut above them, you do not lose out on the buds, etc.

Note also that many plants put down additional roots where these nodes touch the ground. An example is woodland phlox. Thus you will often see "rooting at the nodes" as a description of one way in which a plant can spread to propagate itself. The nodes are very easy to detect on Japanese knotweed: They are the horizontal lines that you see cutting across the stalk.

How to Remove the Weed by Glyphosate Injection

Although it's certainly possible to use the injection method at other times of the year, it is best to wait till late summer or early autumn, when the Japanese knotweed canes are a half inch or more in width between the first and second nodes (up from the bottom). Japanese knotweed plants are herbaceous perennials. Since their leaves die back in winter, it is through their roots that the plants live on. At the end of Japanese knotweed's growing season, nutrients are transferred from the leaves down through the canes and on down to the rhizomes. This transfer, which continues until the first killing frost, presents you with an opportunity to hit Japanese knotweed where it really hurts! Glyphosate injected during this time will "go along for the ride," following the same path down to the rhizomes as the nutrients. And it is the rhizomes which must ultimately be destroyed if you are to be successful at Japanese knotweed removal.

The herbicide injector is easy to use. It is comprised of three parts: the gun, the reservoir that holds the glyphosate and a needle that is inserted into the tip of the gun. Once you take the tool out of its packaging, here's how to proceed:

  1. Assemble the three parts of the injection tool.
  2. Fill the reservoir with herbicide (follow any mixing directions that may be provided on your herbicide label).
  3. Inject the needle between the first and second nodes (up from the bottom) of the Japanese knotweed cane.
  4. The herbicide squirts out of a small hole in the needle. After thrusting the needle into the Japanese knotweed cane, make sure this hole is positioned within the hollow area of the cane.
  5. Pull the trigger to release the glyphosate.

Repeat injections of glyphosate in future years as necessary. are tenacious, therefore Japanese knotweed removal requires tenacity on your part. Be persistent!

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  1. Japanese Knotweed. WVU Extension.