Knotweed in my raised vegetable beds

9 years ago

This is my first year gardening. The spring in Coastal BC was brutal this year with the entire month of April and half of May being extremely cold and wet.

We built the raised beds. I lined the beds with food grade plastic since we opted to build them from Douglas Fir rather than cedar. I lined the bottom of the bed with landscaping cloth which at the time I felt to be excessive given that the beds are nearly 2 ft tall. In late March we filled the beds with soil so it experienced about 6 heavy weeks of rain. I wasn't very happy with the soil blend I ordered....it seems to have a lot of un-rotted an fairly large wood chunks and is a bit sandy. I laid a layer of organic potting soil over the top of the beds to add some higher quality soil to the mix.
In the middle of May, I notice a strange red tipped weed poking up through the garden bed. I was sort of shocked that the plant was able to grow 2 ft through the soil. As the weeks go on, I find a red growing tips popping up in different areas of the garden bed. Only about a week ago did we finally bother to identify the weed and have discovered that it's the invasive "Japanese Knotweed." It's growing under our fence from a neighbor's yard under the ground and up 2 feet into my veggie garden!

I have been frustrated by my garden's performance this year and have been blaming the poor weather and the soil I bought. Finally I google "allelopathy knotweed" to see if this invasive plant is poisoning the soil similar to a walnut tree. I did find some results on studies done on native plants that do demonstrate it may have some allelopathic tendencies....but I'm wondering if the few shoots I keep finding can have that affect on my entire garden?
I started 9 tomato plants indoors and 2 weeks ago I planted 4 in the garden bed. They received adequate sun and water and seem healthy enough but aren't thriving. I planted two of the tomato seedlings in pots on my front balcony and 3 in a raised bed out by the front door-all in the same soil as the backyard bed. Those 5 plants are getting significantly less sun and for some reason are all looking a lot more robust than the ones in the veggie garden.

Does anyone have any experience dealing with knotweed in their vegetable garden? Is it possible that it's poisoning the soil?

Sort by:Oldest

Comments (5)

floral_uk z.8/9 SW UK
9 years ago
Allelopathy is not the main issue with Japanese knotweed. This is a plant which can grow 6 to 8 feet tall, root down twice that depth and spread to cover acres of allowed. It is the number 1 worst possible invasive species in th UK. I would stop even trying to grow vegetables until the JK has been totally eradicated. If necessary dig up the entire garden! I had it in my woods and it took 3 years to remove by a professional company applying industrial herbicides 3x per year.

Unfortunately if it's coming from the neighbour's you are pretty much scuppered unless they control theirs too. This stuff is so awful that it affects property prices over here and there are criminal charges brought against those transporting or dumping it.
User
9 years ago
I'm not sure if it will work or not, but I had an invasive called greenbrier, which forms extensive underground root systems. When the bit growing out of the ground was thick enough I would take a stem tube - the kind with a cap on them with a hole for the stem (purchased at a craft store that sells diy bridal stuff), and fill it with gly. I would then cut the end off the invasive and stick it in the tube and let it sit for a few weeks. The plant sucks it into the root system and starts to kill it. You have to be persistent but it definitely worked for me!
Related Professionals
Jackson Landscape Architects & Landscape Designers · Houston Landscape Architects & Landscape Designers · Harrisonburg Landscape Architects & Landscape Designers · Wausau Landscape Architects & Landscape Designers · Bronx Landscape Architects & Landscape Designers · Cumberland Landscape Contractors · Tupelo Landscape Contractors · Greenville Landscape Contractors · Bartlesville Landscape Contractors · Florence Landscape Contractors · Seattle Landscape Contractors · Lake Charles General Contractors · Martinsburg General Contractors · Traverse City General Contractors · Plattsburgh General Contractors
Paul MI
9 years ago
"Gly" = glycophosphate (Roundup), I'm assuming? If yes, then I would add that the concentrate is even more effective. With hollow stemmed plants, after cutting the stem one can use a syringe to inject gly into the stem cavity. Cap the end with something like tape, wax, small piece of plastic wrap, et cetera.

I don't use gly often, but there are some plants for which I make exceptions.
mary_rockland
8 years ago
I got that cursed plant from a fellow gardener who called it "Mexican Bamboo". It seemed like a great way to make a quick privacy hedge. She said she just mows over where she doesn't want it. I only planted it in the lawn. I live in the country and have lots of space, but when it started coming up 30 feet away in a flower bed I'd had enough. I had only let it go a few years and then decided it was too greedy. It took about 5 years to get it "under control" and even 20 years later I still find it coming up in my lawn. We used round up for a few years and then after that every time we saw a darned red shoot coming up I cut it back and still do every year. Good luck.
Sandra Abrams
5 years ago
Did you ever get an answer to your question? I just bought a home with 3 raised beds I planned to use for veggies. I see japanese knotweed shoots popping up in the beds. The neighbor's yard is full of the shoots so they have spread to my yard and raised beds. I'm afraid to plant veggies in the beds if the knotweed is going to kill them.