Knotweeds (Polygonum spp.) are an economically important problem because they can grow through concrete, streets, and foundations. They grow an extensive thick root system which regenerates plants from cut segments. Digging up those roots may be impossible because they grow under rock and concrete. The plant looks like a bamboo, with jointed hollow stalks growing from a crown, that dry out in fall. The roots can be identified by their red colour. The floppy heart-shaped leaves are also easy to spot. Those in Stanley Park have a concave base with red veins in early spring. Their exact species is debatable, but it does not really matter.
Never mentioned on the web, are the low tussocks of tightly packed leaves that grow without any upright cane. At most they have foot-long trailing stems. These leaves are much more narrow and pointed. Many times there is no single tap root, but a mass of tiny roots (site #6). Other times, along with the mass of tiny roots, there is a single thread leading down to a deep larger rhizome (site #9). Other times they grow directly from a normal crown (site #5). In the hot, dry summer of 2021 a half dozen of these appeared in areas that had been dormant since before I started work in 2017.
The three samples above were collected on the same day, in the same area, one attached to an until-now-dormant crown, and the other two a few feet away but I believe are exploratory plants from that same root. As you can see their form can differ. The only constant is their leaf shape.
............. Work Done by Parks ..................
The 2011 Ecological Action Plan (page 10) for Parks, funded $12,000 for a three year course of chemical herbicide. I believe Parks never did any work. That was followed by outsourcing to the regional organization created to deal with invasives (ISCMV) until 2018. It is my memory that they only worked Stanley Park in 2016 and 2017. I know they never showed in 2018. I found out that Vancouver Parks had re-assumed responsibility back from ISCMV for 2022, but they never showed in Stanley Park.
So in the last decade governments have sprayed in only 2016 and 2017 - and done no other removal work. In spring 2017, when I started work, all sites still had healthy growth. All my contacts with government have resulted in them telling me to stop doing anything.
............. Official Policy for 'How to Clear' ..................
ISCMV produced a Knot On My Property brochure for the public. I believe its advice is very bad. They tell you to attempt nothing yourself. Their only solution is to hire professionals with access to chemicals. Why? (1) Because supposedly you are incapable of collecting and disposing of plant material, and (2) because they imply that all root systems are too large to dig out. I think both arguments are garbage and counter-productive.
Two more recent publications here and here, are just as negative, saying "Manual control will
not completely kill the plants and may encourage their spread" and is not recommended.
With most invasive plants, information on the web is full of doom and gloom, predictions of failure, and excuses for inaction. Example for Knotweed ..."Reproduction can
occur from as little as 0.7 grams of stem or root tissue. Buried rhizomes can regenerate from depths up to 1 m, and are able to remain dormant for many years." Sure that may be true in a very few situations, but not every situation is that dire. I finally found one Canadian article, and another American one backing up my 'yes you can do this' claims.
............. How to Clear ..................
Since I started in 2017, I've had about 50% success by digging up the root with a hand trowel, and 80% success with the rest by repeated knocking-back.
Your first step should be to use a hand cultivator to see just how large the root is, and dig it up if possible. Fifteen minutes and the job is done. About half the plants I have dug up had roots small enough to remove intact. I would not suggest using a spade because the roots are pulpy and easily broken. Even the force from tilting a hand trowel is sufficient to break the root - leaving you to fish around for what was left. But I have used a shovel (Parks won't lend me a spade) when my access to the plants is limited to a 'do it now or it will never get done' situation. Yes there is the possibility that you leave a bit of root behind, but so what?
While digging out dirt, dump it first onto a garbage bag so you can sift it through your fingers looking for root pieces. Using an actual sieve would be safer still, but probably overkill. The Australians say that disposal should involve first drying, and then burning. That is probably fear-mongering overkill. Vancouver's garbage site operators say there is no problem with putting roots and pullings in a bag into the normal garbage (i.e. not going to compost).
I have found many plants that I think of as 'exploratory'. They are single stems rising from a very thin tiny thread of a root that can travel long distances from the nearest established plants. I think these are last ditch attempts by a stressed plant. I saw no reason to not pull up these plants along with as much of the root I had patience to dig. Re-growth is very rare.
For those plants with roots too large to dig up, the second choice is to inject chemicals either directly into the root, or into the hollow base of the bamboo-like shoot. Spraying chemicals is obviously far less effective and more dangerous. It does have an advantage though, when there are dozens of stalks. The application rules limit the chemical per area, so that limit is quickly used-up when there are multiple stalks to inject. Wiping the leaves uses less chemicals/stalk than injection. In Canada, access to and permission to use these chemicals is restricted to those who complete an extensive training in the use of all chemicals. So this method cannot be used by the pubic.
ISCMV's two years of spraying left quite a few dead crowns indicating it had worked. But even some of these sites sent up new growth in the heat of 2021, and again after the winter 2021-22 king tides opened up sites to the sun. Don't presume that any of your success has 'killed' the plant. Presume it has only gone 'dormant', so you continue to check regularly for a least a few years.
There is plenty of web advice saying that weekly knocking back of new growth will eventually exhaust the plant. This is mostly true. For me it worked within a year about a quarter of the time, and maybe another quarter stopped after the second year. But be prepared for a 3 or 4 year program. If an unattended plant is allowed to grow and replenish its strength, then presume you must start the process all over again. Check weekly in early spring, extending the the schedule in later August.
I have been leaving rock salt (from winter road de-icing) in the hole, to stress the remaining root. (Documentation has shown some success from flooding plants with sea water.) I'd say it definitely helps. I leave any root partially dug up exposed to the sun in order to create stress and dry it. This allows me to spot new buds of growth immediately. I am guessing that the only thing accomplished by breaking off the root would be to trigger the remaining plant to expand in another direction. I found that slicing off the top of the crown delayed re-growth, but I worry that by doing so the root is more likely to sprout elsewhere.
Covering large areas with a tarp, landscape fabric, or old carpet, accomplishes the same thing as knocking back - it limits the plant's ability to photosynthesize with leaves. Parks has refused my request for landscape fabric at spot #6. The one spot they previously used fabric (#11) was never maintained for maybe a decade. After I cleared the limited growth through the fabric, there has been only very limited re-growth. So I consider this method to be very effective for the limited work involved.
............. Work at Specific Sites ..................
SPES's published claims that knotweed is under control, is pure poppycock. Site #6 is going strong because Parks will not put down landscape fabric. Site #11, with its historical landscape fabric, needs periodic removal of plants growing through the fabric. Site #7 has a self-sustaining root growing through the cracks in the rock wall. After the winter 2021-22 king tides that scoured some sites and flooded others, exploratory plants galore sprang up in sites #5, 7, and 9, even in areas free from growth since I started.
This area is the circle created by the equestrian bridge ramp. It was a huge infestation at the start. Spraying chemicals had killed most of it by the time I took over. In the first couple of years there were about a dozen different attempts at re-growth, different spots in different years. I could dig up the roots about half the time. There were no exploratory plants in 2020 and 2021, but two in 2022; one on the north edge where I had never before had sprouts, and another beside the path leading off from the bridge.
The major problem now is that the plot has been colonized by Bindweed, which is just as much a problem. Spot #1 is right in the middle of a huge wild area (both east and west of the causeway) that is otherwise clear of Bindweed. So this point source of further contamination is a major problem.
In 2020 I stopped SPES volunteers from clearing this area of bindweed. The knotweed is only dormant and not yet dead. The other invasives compete and help kill it. IMO killing the knotweed is the priority for now. In 2021 Parks sent in machinery to cut back the blackberries before I could stop them.
- This is a marshy area with only speckled sun just beyond the trail paving. It spends many winter months under water. In the first years I found sprouts attached to roots never more than 6 inches deep. Each seemed to be isolated - short lengths as if they had broken off from some other larger root mass sometime in the past; possibly arriving in a flood. There were two individual crowns which I dug up. I believe the site is now cleared.
The heavy rains and wind of 2021-22 winter scoured away most of the bank where these plants had been showing up. Hopefully remaining root pieces will not regain vigour wherever the flooding leaves them.
- Behind the left bench, in front of a tree, one plant is growing from a root that I believe travels under the tree root to the right. Originally there were exploratory shoots every year, off in every direction. There were no shoots in 2021 or 2022.
- This area was huge ... extensive roots growing up the gully, into the bank, and alongside the pavement. Originally I cleared two plants from up the gully and another two from the bottom. But other roots were either too big to dig up or inaccessible underneath old asphalt slabs. I left as much root exposed as possible, knocked back growth weekly, and left salt. Now only the roots protected inside the bank are sending up shoots. By 2020 and 2021, knocking them back once or twice in spring was enough. But 2022 had regrowth weekly into September.
- This area is between the first (1bench - 2bench) combo. In 2017 I dug up a crown halfway between the benches and a plant at the east end. The chemicals had worked on the two crowns at the west end (one at either end of the grassy patch). One re-sprouted in 2019 and I dug it up. In the heat of 2021 at the west end, MANY new plants grew from a root about 2 yards long along the back edge of the grass. I knocked back and salted. The added sunlight in 2022 has caused sprouting further back in the trees.
I now consider this site to be the most risky / problematic. The sand makes for easy digging, so I think some major spade work is required to dig up the back edge of the grass. I'll see what happens in a 'normal' year.
- This wet grotto is a major problem. The inside of the area is dripping wet all year round. Chemicals are pointless (and off label) unless injected but ISCMV still sprayed in 2017. I tried digging out roots one year without any success even though I toted away many bags worth.
Landscape fabric must be used - all other methods have been tried. I requested that Parks drop off a bolt of fabric for me to install. They refused. Their excuse was that 'fabric does not work' ... in spite of fabric working very well at site 11. There would have to be some pulling away of cliff rocks where roots have found protection. That might compromise the cliff.
- This spot is just south of the steps, up against the cliff, around the corner from #6. The chemicals had killed two crowns in front when I took over. I dug up an active third one in between the two. In July 2017 ISCMV sprayed a fourth plant growing from a crack in the rock wall. In spring 2018 there was healthy new growth from the crack. I knocked a lot of the root out of the crack, but not all of it. I packed the crack with landscape fabric to cut off sunlight.
The high tides of 2022 scoured away the earth and root crowns from the front. They exposed the root end of the plant living in the rock cracks. I dug it up, but the growth within the rock cracks is still alive. I don't know if it will survive without a root into real ground. Weekly, I am pulling out spindly growth trying to get through the fabric.
2022 also saw four or five tussocks, widely spaced, on the scoured front bench, and from one of the original crowns that chemicals had made dormant.
- This spot has about 20 plants scattered behind the seawall at 3rd Beach - just north of the bike rack. In spring 2020 Parks weed-whacked the blackberries back a few yards, giving access to the knotweed. I dug up about half complete with roots. Parks refused my request for a spade for the rest. In 2022 they weed-whacked again. This time I had a shovel that worked well enough, so I believe I have removed the remaining roots.
There is just one plant (flagged #7) where the root dives below a rock and I could not dig it up. This is about half way along the stretch, right at the transition from flats to the slope. I had tried to dig it up back in 2020 (a flag already existed) so this will have been my second attempt. Next time, dig from the NorthEast about a foot from the plant.
- Just around the corner from the top of 3rd beach is a grassy area with plants throughout. Initially I dug up about 7 plants successfully. Another 6 had roots too big to dig, so I repeatedly knocked-back new growth and used rock salt. That had reduced new growth to about one attempt per summer. But the hot, dry summer of 2021 saw a half dozen tussocks appear in new areas in the north and central back edges ... along with a few growths of canes from roots too large to be dug up. There was no re-growth from the one knockback. 2022 resulted in two new spots at the north end.
- I could find no remaining evidence of this spot near the lookout above the gun tower. The chemicals must have worked. My request to Parks for confirmation the site had been cleared was ignored.
- There are three sites here. Site A is a large grove of about a dozen mature plants at least 10 feet tall, back from the path up the hill and inaccessible behind blackberries. ISCMV sprayed in 2017. There has been no regrowth visible as of 2022.
Spot B was covered in landscape fabric at some time in the past. In 2021 I noticed for the first time canes growing farther back toward the big tree. I waded through the blackberries in 2022 to dig up the roots growing overtop the fabric.
Spot C was also covered in landscape fabric in the past. But it was not maintained. The Knotweed grew through the fabric, leaves fell and created new top soil, grasses and blackberries colonized the new soil. There was also Knotweed growing from crowns in the new soil, sending out roots sideways because they cannot go down. The blackberries then prevented any maintenance.
In 2018 Parks weed-whacked about half of the blackberries. By chance I noticed. I cleared all the plants living in the new soil and the accessible plants outside two edges. Unlike any other spots, the plants growing above the fabric are connected, not by rhizomes, but by old hollow stems. This might be because the area was used to pile salvageable logs during the windstorm clean up. Those logs may have pushed stems down into the ground, prompting them to root themselves. That would mean the fabric was laid before the 2006 storm.
In 2019 Parks weed-whacked the complete area of landscape fabric. I cleared plants in the new area but there was no regrowth where I cleared the year before. I conclude that the landscape fabric did an excellent job. It was only the failure to do even minimal follow-up that prevented complete success. There were plants outside the fabric that I also cleared.
In 2022 Parks weed-whacked at knee-height the full fabric area, plus a large adjacent area. But they did not remove the cut canes, making access almost impossible. I could see one site growing in the new area, and about four areas of regrowth in the 2019 fabric area. So I pushed in to clear the plants using a shovel to open a path.
There remains at least one plant I expect to regrow. Outside the 2019 fabric's edge, at the front/north, I could not clear a root with hand tools because it was too large. So I made a cardboard barrier to prevent regrowth. There had been no regrowth in 2020 or 2021 - that I could see from the road. I found regrowth in 2022. But now it was so tightly hemmed in by blackberries I could not remove the cardboard.