Japanese Knotweed, as a lot of people know, is not just an invasive plant and a weed, it's a terrible plant. When it grows in the United States it has a zero-tolerance policy for other plants. It can take damp, nutrient rich earth, like the rare rich fen that my backyard is (the fen actually covers about 5 acres, but I have about 1/50 of an acre of it, and some of its deepest pools) and turn it into nutrient-free, salt-filled, baked clay that is highly basic (my soil pH where the worst knotweed WAS is over 9, despite being chemically acidified and in some places removed and replaced, my normal fen soil ranges from 7.9 to 8.75 (more basic than baking soda, causes chemical burns if you don't wash it off, dangerous to breathe when dry...which is never is, wetter gets more Calcium Carbonate and drier does not.)
When we first moved into Chez MacMorgan, we removed about 300lbs of Japanese Knotweed from the property. This was bagged, sacked, labeled as "invasive plant, do not compost" and sent out to be burned, including a lot of the dirt around it. We had rhizomes, growing against the neighbor's foundation, that were over 50lbs in weight, over 10ft long, and took over 2 years to remove, by a combination of frost heaving, pulling every rosy red sprout we saw, and painting (not spraying) exposed roots with FULL STRENGTH concentrated Roundup (an off-label use, and should be done carefully.) After a few years of treating the rhizomes (they look like logs) the frost-heave cycle lifted them out of the ground, and I picked them up and bagged them for burning. Where this happened, not a single piece of knotweed has returned. We used a similar technique to kill invasive privet, but did not paint the privet stems with full-strength roundup, we used burning, cutting and painting the stems with roundup as diluted on the package. There are very few people who hate herbicides more than me, so understand that the fact that I resorted to herbicides (after speaking to a chemist, etc.,) was not an easy choice...I literally cried over it, and felt like my biologist chops were being busted, etc. One of my favorite organic people had to pat me on the back and go 'but it's knotweed,' to make it better for me.
Last year we removed less than a pound of Japanese Knotweed, most of it newly planted, from flooding, from bits dropped by neighbors who mowed it (they know better now) and the rest. Where we found a new growth (which will usually be round, growing from a small, central piece) we dig out a full spade of earth around the plant, threw it out dirt and all, bagged it and labeled it for burning, more than 90% of the time, nothing came back in these spots.
When attacking an ESTABLISHED MOUND of knotweed, where you cannot access the rhizomes, such as that growing along a city-planted chain-link fence, like that at one edge of my property, the going is slower. You'll note I said that 3-4 years were needed to remove the ones where I could have access to the rhizomes....and this is slower.
First, and this will make you nuts, allow the knotweed you cannot reach the bottom of to grow to a minimum of 18 inches in height. You need good needle skills for this, so wear gloves and take precautions. Take a small syringe, (you can probably get one from someone who is diabetic,) and pop a hole about 1 inch above the third or forth "knot" in the plant from the ground. These are the breaks in the plants that look like the rings in bamboo. Draw out as much water as you can get from this hole, using the syringe, pointing the needle down into the plant, about a 45 degree angle. Suck until you can't get any more out. If it GUSHES when you poke it initially, allow it to drain (you can put a small hole at the top of the section to help.) If you wait for a couple of dry days, it will probably not gush.
The knotweed is holding this water in reserve for later. Your job now is to replace that water with your full-strength, non-diluted, Roundup concentrate (I used 4 different compounds, and only this worked.) If you removed 3-4 syringes full of water, add 3-4 of roundup. Remember, it is going to get diluted by the plant.
Within about a week, the stems you 'got' will begin to die very dramatically. Feel free to cut every inch above your 'targeted' hole. That's already dead. Use this time to get the stems you missed. After about one week more, the stems you got the first week will die to the ground. Pull them out, cut them as low as you can get, bag the debris. Do not compost, and if you live in an area where it may be composted, label it invasive. HELL, LABEL IT MEDICAL WASTE! Just don't let it back into the ground as plant material. Get it burned, do anything.
Eventually, the rhizomes will stop sending out these long canes. Instead, you will get twisted rosettes of leaves near the rhizomes. Twist them off, then spray or, preferably, drip or paint more weed killer into the holes they leave behind. In time, frost action, flooding and draining and the local plants will pull out the damaged rhizomes. Blackberries, feral roses and currants compete happily with these damaged rhizomes, and will 'win,' against them, although I don't recommend eating them until the poisoned rhizomes are gone.
If you have a stand like the one in the picture, like the transit worker up there is trying to kill, doing the one-stem at a time thing can be difficult. Easier (though not as effective) is to cut all the stems to about 1-1.5ft in height, place 1-2oz of Roundup in the cut area, wait two weeks, pull up those dead stems and repeat as needed. You will need to be on top of it, lifting out roots as they die, killing rosettes, etc. IT CAN BE DONE, however. This 'kill them all and let god sort them out," view is hard, but it does make a difference, and if you stay on it, killing the new growth is pretty easy.
Good luck, my Knotweed warriors!
Knotweed is an indiscriminate killer. It will shade plants, eat all their nutrients, grow a system of roots nearly nothing can grow through. It laughs at being sprayed with most herbicides, and even if you poison it with sprays, you generally kill the stuff under it. It likes to grow in broken areas, so what happens is little bits of these long, stringy roots break off, float away, land in a shaley or sandy bank, and destroy wetlands. It's bad, mmkay.![]()
Above: New York State Worker vs Japanese Knotweed.
When we first moved into Chez MacMorgan, we removed about 300lbs of Japanese Knotweed from the property. This was bagged, sacked, labeled as "invasive plant, do not compost" and sent out to be burned, including a lot of the dirt around it. We had rhizomes, growing against the neighbor's foundation, that were over 50lbs in weight, over 10ft long, and took over 2 years to remove, by a combination of frost heaving, pulling every rosy red sprout we saw, and painting (not spraying) exposed roots with FULL STRENGTH concentrated Roundup (an off-label use, and should be done carefully.) After a few years of treating the rhizomes (they look like logs) the frost-heave cycle lifted them out of the ground, and I picked them up and bagged them for burning. Where this happened, not a single piece of knotweed has returned. We used a similar technique to kill invasive privet, but did not paint the privet stems with full-strength roundup, we used burning, cutting and painting the stems with roundup as diluted on the package. There are very few people who hate herbicides more than me, so understand that the fact that I resorted to herbicides (after speaking to a chemist, etc.,) was not an easy choice...I literally cried over it, and felt like my biologist chops were being busted, etc. One of my favorite organic people had to pat me on the back and go 'but it's knotweed,' to make it better for me.
Last year we removed less than a pound of Japanese Knotweed, most of it newly planted, from flooding, from bits dropped by neighbors who mowed it (they know better now) and the rest. Where we found a new growth (which will usually be round, growing from a small, central piece) we dig out a full spade of earth around the plant, threw it out dirt and all, bagged it and labeled it for burning, more than 90% of the time, nothing came back in these spots.
When attacking an ESTABLISHED MOUND of knotweed, where you cannot access the rhizomes, such as that growing along a city-planted chain-link fence, like that at one edge of my property, the going is slower. You'll note I said that 3-4 years were needed to remove the ones where I could have access to the rhizomes....and this is slower.
First, and this will make you nuts, allow the knotweed you cannot reach the bottom of to grow to a minimum of 18 inches in height. You need good needle skills for this, so wear gloves and take precautions. Take a small syringe, (you can probably get one from someone who is diabetic,) and pop a hole about 1 inch above the third or forth "knot" in the plant from the ground. These are the breaks in the plants that look like the rings in bamboo. Draw out as much water as you can get from this hole, using the syringe, pointing the needle down into the plant, about a 45 degree angle. Suck until you can't get any more out. If it GUSHES when you poke it initially, allow it to drain (you can put a small hole at the top of the section to help.) If you wait for a couple of dry days, it will probably not gush.
The knotweed is holding this water in reserve for later. Your job now is to replace that water with your full-strength, non-diluted, Roundup concentrate (I used 4 different compounds, and only this worked.) If you removed 3-4 syringes full of water, add 3-4 of roundup. Remember, it is going to get diluted by the plant.
Within about a week, the stems you 'got' will begin to die very dramatically. Feel free to cut every inch above your 'targeted' hole. That's already dead. Use this time to get the stems you missed. After about one week more, the stems you got the first week will die to the ground. Pull them out, cut them as low as you can get, bag the debris. Do not compost, and if you live in an area where it may be composted, label it invasive. HELL, LABEL IT MEDICAL WASTE! Just don't let it back into the ground as plant material. Get it burned, do anything.
Eventually, the rhizomes will stop sending out these long canes. Instead, you will get twisted rosettes of leaves near the rhizomes. Twist them off, then spray or, preferably, drip or paint more weed killer into the holes they leave behind. In time, frost action, flooding and draining and the local plants will pull out the damaged rhizomes. Blackberries, feral roses and currants compete happily with these damaged rhizomes, and will 'win,' against them, although I don't recommend eating them until the poisoned rhizomes are gone.
If you have a stand like the one in the picture, like the transit worker up there is trying to kill, doing the one-stem at a time thing can be difficult. Easier (though not as effective) is to cut all the stems to about 1-1.5ft in height, place 1-2oz of Roundup in the cut area, wait two weeks, pull up those dead stems and repeat as needed. You will need to be on top of it, lifting out roots as they die, killing rosettes, etc. IT CAN BE DONE, however. This 'kill them all and let god sort them out," view is hard, but it does make a difference, and if you stay on it, killing the new growth is pretty easy.
Good luck, my Knotweed warriors!