Invasives-Schminvasives, Part 4
Japanese Knotweed
(It can Tie you in KNOTS)
This stuff is not to be trifled with
A few key points to kick us off, and to ensure you’re invested in reading this: Japanese knotweed is tenacious. It can *grow through asphalt and concrete.* It can damage your home’s foundation. If you have it, you should be concerned, and, if you value your property, you should get serious soon about controlling it. If you’ve been reading this summer’s invasives posts, you know that this is nothing I’ve said about any of the other invasives I’ve discussed.
For these reasons, I’ve had Japanese knotweed on my hit list for this series since I first decided I wanted to share information about invasives. I thank my lucky stars every day that I have not yet seen it popping up in our neighborhood, but there are large stands of it within a mile, so I just keep crossing my fingers that it never finds its way here.
It goes by several scientific names, including Fallopia japonica and Polygonum cuspidatum; due to widespread natural hybridization with its relative, Giant knotweed, a new taxonomic genus has been assigned to both: Reynoutria, so, in the most updated science, it’s known as Reynoutria japonica.
The key thing to know about Japanese knotweed is that it spreads vegetatively, that is, via its root system, which can grow 7 – 10 feet deep (yes, you read that correctly), as well as through any small pieces of root material or even plant cuttings that are left in the ground or in contact with moist soil. Japanese knotweed’s root system is comprised of rhizomes, which are fleshy and capable of holding significant energy stores, adding to its ability to send out copious new growth.
It is exceedingly difficult to control, especially if done without understanding how important it is to tackle it in the correct way at the correct time(s) in its growth cycle. To attempt to control it otherwise only causes it to spread more.
Once you recognize it, sadly, you’ll start seeing it along roadsides almost anywhere you go.
How to identify it
Because I’m so lucky as to not have this monster on or (relatively) near my property, I don’t have any of my own photographs of it. I was frustrated to see what I believe to be a State crew out recently cutting it (along with other overgrowth) along the main road a bit under a mile from here, as I had been planning to take some photos of those areas. More important than the scuttling of my photo and blog plans, however, was the fact that, in cutting it now, they were, I’m sure inadvertently, most likely contributing to its spread.
Therefore, rather than scrambling to find photos (and proper attribution) online to include here, I’m going to point you to this article (published in 2020, prior to the introduction of the Reynoutria genus) by the Penn State Extension, which includes several excellent, detailed photos of Japanese knotweed at various stages in its lifecycle, so you can more easily identify it.
Control
Also included in the above Penn State Extension article is a strong set of control recommendations. Rather than including a lengthy description of the control methods here, I believe it’s more efficient to direct you to simply read that article.
I have also read that research has identified an insect predator to Japanese knotweed, a psyllid called Aphalara itadori. After extensive research (since at least 2000), it was finally approved for controlled release in several states in the U.S. in 2020. I am not aware of its approval/availability yet in CT, so for now, I think we have to soldier on with other methods until we can get our hands on the little suckers (literally).
Returning to the PSE article, being a member of several invasive species groups, I will offer a bit of clarification to the PSE instruction on “late season” foliar application of herbicides, based upon the extensive experiences of the experts in the groups I belong to:
1) The best and safest timing for “late season” application is *after* flowering, but before first frost, known as “the window.” Not only does this appear to maximize uptake of the herbicide into the rhizomes, but it protects pollinators. In areas where Japanese knotweed infestation is significant, unfortunately, there’s often little other available late-season nectar for them;
2) Glyphosate appears to be the most effective herbicide for this daunting job, BUT, keep concentrations to 2%. Much stronger, based upon my readings, and it can burn the leaves and send the plant into “protect”/regeneration mode, making the spread worse.
3) Do not expect to eradicate it in one fell-swoop, and be very suspicious of anyone who suggests they can. Apply the 2% glyphosate concentration in “the window” (see #1 above), ensuring it thoroughly hits the underside of the leaves as well as the tops, followed by repeat application in the spring after emergence, and again in the next “window,” or cutting back in June, then waiting until the following “window” to treat again. At this point, this appears to be the most successful (and least expensive/invasive) approach for homeowners. You can expect the need to repeat the process for at least two seasons; more if you’re dealing with a particularly mature stand.
If you’re considering non-chemical alternatives, good luck to you (unless you can get your hands on some Aphalara itadori). Understand that this plant evolved on the slopes of volcanoes, and the rhizomes can go dormant for years, waiting for the right conditions to re-emerge. As someone in one of my groups noted, “The plant is quite well-adapted to Hell, which is where it belongs.”
Post-Control
Equally important to controlling Japanese knotweed is what to do with the cleared area once you do have it under control. Replanting the area with native plants appropriate to the conditions (especially with vigorous native woody shrubs and trees) is key to ensuring you’re helping to establish a more resilient ecosystem in place of the knotweed, and one which can actually also help keep it at bay going forward.
Questions?
Ask away!! I’m happy to help in any way that I can!!
**NOTE: The image featured at the top of this post is attributed to Tom Heutte, USDA Forest Service, Bugwood.org
