Dealing with Japanese knotweed

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This time of year, with our rolling hills flushing out so quickly in varying shades of green, the serviceberries blooming with pops of white here and there, it’s easy to miss the not-so-nice botanical banes of a native plant’s existence: invasive species. If you dial back the spring rush a bit, to maybe mid-April, you’ll see a different story.

Invasive species are often some of the first to leaf out in spring, beating out their native counterparts by a good two weeks, allowing them to get a jumpstart on their goal of taking over the world. Not really, but it does give them a leg up. Invasive honeysuckles are very easy to spot this way in our woodland understory.

Then in early May, another invasive rears its ugly head in damp, sunny areas. Growing incredibly fast from absolute nothingness into a seven-foot-tall (or more) monstrosity is Japanese knotweed. Calls start coming into the Master Gardener Volunteer helpline (in Upstate New York at 315.684.3001 ext. 119 or asking, “What is it?” and “How do I get rid of it?”

Before I dive into the details, let’s talk about what makes a plant invasive. It’s not always easy to define, but in general, it’s a plant that is not indigenous and competes with native plants for resources until natives get ousted by sheer volume – literally squeezed out – or the invasive species changes the environment enough that the area where natives once thrived becomes inhospitable.

Invasive species can sometimes change the very composition of the soil in our woodlands. Couple that with invasives lacking natural predators, the insects and diseases that kept their growth in check in their homeland, and you have a recipe for disaster.

Luckily, many of our invasive plants have been here long enough that universities, environmental agencies, botanic gardens and local groups have figured out ways to if not eradicate things like Japanese knotweed then at least work with homeowners and municipalities to get it as gone as it can be when the alert is raised.

How do you get rid of Japanese knotweed? One thing to remember about invasive species in general is that they require persistence to remove them and constant vigilance to ensure they remain gone. Japanese knotweed has an extensive network of underground modified stems called rhizomes. These rhizomes creep just under the soil, functioning as roots and stems combined, putting down roots below ground and sprouting shoots above ground, rooting into larger and larger patches at a speed that seems supernatural.

According to the USDA Forest Service, small stands of knotweed can be killed by repeated cutting, which will eventually exhaust the rhizomes. Any piece of the plant that is greater than a half-inch long has the capacity to root itself, so plant material should be incinerated, buried deeper than five feet or composted (using chopped plants in smaller than half-inch pieces). Removal of plant material from the site is not recommended, so some sort of disposal should happen on site. Mostly this plant moves around because people don’t realize a tiny bit of root is all it takes to start a new colony. It often grows next to streams and bodies of water, and pieces that fall into moving water can be carried downstream to root and start new colonies quite easily.

Smothering small patches with heavy grade plastic for three to five years can eventually kill the plants as well. Finally, for larger patches, a combination approach of breaking/cutting the stems in early summer, then applying a glyphosate herbicide to the regrowing shoots in late summer before they reach three feet in height, can be effective over time. Injecting glyphosate into the broken stems is more effective than foliar sprays, but more time-consuming and labor intensive. Always check label instructions when applying pesticides!

by Patty Stimmel, Ag/Garden Educator/Horticulturist, MGV Coordinator

Featured photo courtesy of Randy Westbrooks, Invasive Plant Control, Inc.,

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