Knotweed

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 low tussocks of tightly packed leaves that grow without any upright cane, not apparently attached to any established root. These leaves are much more narrow and pointed. Many times, there is no single tap root, but a mass of tiny roots. Other times, along with the mass of tiny roots, there is a single thread leading down to a deep larger rhizome. Other times they grow directly from a normal crown. 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.

Stanley Park invasive plants - Japanese Knotweed Tussock

The three tussock 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 Government ..................

The 2011 Ecological Action Plan (page 10) for Stanley Park, 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 first worked Stanley Park in 2023, although they did not treat all the sites.

So since planning in 2011 governments have applied chemicals in only 2016, 2017 and 2023 - and done no other removal work. In spring 2017, when I started work, all sites still had healthy growth. All my contacts with government bodies have resulted in them telling me to stop doing anything.

............. Official Policy on 'How to Clear' - Don't ..................

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".

I finally found one Canadian article, and another American one backing up my 'yes you can' claim. It turns out that what 'might' happen, rarely does. Roots are most likely small. Nothing I have ever done has resulted in a plant spreading. Simple sifting the soil with your fingers will prevent re-burying small pieces of root you dig up. Apply chemicals is just as likely to make a root dormant (not dead) as repeated knocking back.

............. How to Clear ..................

Digging up the root is the only sure way to get the job done once and for all. 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 remaining by repeated knocking-back. Six years later; only laying landscape fabric will work for one site, one plant requires chemicals because its root runs under a rock and blackberries prevent knocking back, and a handful of individual plants still require knocking back. I have made (very roughly) 40 plants dormant.

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). Their representative made this assurance before the ISCMV board.

For those plants with roots too large to dig up, the second choice is to apply chemicals. In Canada, access to and permission to use these chemicals is restricted to those who complete an extensive training. So this method cannot be used by the pubic.

ISCMV's two years of chemicals left a number of dead crowns, indicating it had worked at some sites. But even some of these sites started sending 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 continue to check regularly for a least a few years. The 2023 chemical application failed to even wilt 80% of the plants.

Around sites where chemicals were used, and resulted in 'dead' crowns, I have found plants that I think of as 'exploratory'. They are either tussocks (pictured above), or are single stems rising from a very thin tiny thread of a root that can travel long distances from the nearest established root. I think these are last ditch attempts by a stressed plant. Just pull it up with as much of the root as you have patience to dig. Re-growth is very rare, but watch for it exploring elsewhere.

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 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.

............. Specific Sites ..................

SPES's recently published claim that knotweed is 'under control' is pure poppycock. They refuse to do this work and tell me to STOP doing it. Heck no experts even know that exploratory plants exist.

Status in fall 2023: Site#

Monitor monthly at least in summer to dig up rare exploratory plants.
All roots dug up before site washed away.
Monitor monthly at least in summer to dig up very rare exploratory plants.
Live roots still sprouting. Needs weekly summer knocking back.
Dig up weekly new exploratory plants in summer.
Only landscape fabric will work. Parks refuses.
Root in cliff produces offshoots needing pulling. Exploratory plants in front flats need the buried root piece dug up weekly.
One plant needs chemical application
Dig up weekly rare exploratory plant in summer.
Never found - assume cleared.
Needs access to dig up plants outside the fabric and knockback growth through fabric. Staff will not tell me when they weed-whack the blackberries to allow that access.

This area is the circle created by the equestrian bridge ramp. It was a huge infestation at the start. Chemical applications 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 root only about half the time. By 2020-2021-2022 only exploratory plants appeared on the edges of the circle, and across the pathway. 2023 has had no growth.
This is a marshy area with only speckled sun just beyond the trail paving. I believe it was created in the first place by its stream flooding in some wet winter. A mud 'flat' was created with small pieces of knotweed that must somehow have come from site #1. In the first years I dug up three crowns.
In later years sprouts appeared, scattered over the flats, each from a 6-inch piece of root buried only about 2 inched deep. The heavy rains and wind of winter 2021-22 scoured away most of the flats. Hopefully any remaining root pieces will not regain vigour wherever the flooding leaves them this time.
Behind the left bench, in front of a tree, one plant grew 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, 2022 or 2023.
This was originally an extensive area ... 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 by asphalt inside the bank (one on each side) are sending up shoots. Weekly knocking back in spring is required but much less frequently by summer.
This area is between the first (1bench / 2bench) combo west from the point. I now consider this site to be the most problematic. I have dug up three crowns and one large isolated piece of root, over the years. One crown remains at the west end of the grass, back into the woods ... protected by the roots of a tree. It was 'killed' by the original chemical applications, but recently started sending out many exploratory plants into the grassy edge. (At least I think that is their source.) That started after the heat of 2021 and the following winter king-tide flooding. The cause ??
This wet grotto is a problem, but the city's problem. My knocking back has cleared the front-facing bank. The inside of the area is dripping wet all year round (or was until the droughts of 2022 and 2023). Chemicals are pointless (and off label) unless injected, but ISCMV still wiped in 2017 and 2023 without success. I have tried the other manual methods without success.
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. They say 'fabric does not work' ... in spite of fabric working very well at site #11.
This spot is just SE of the steps, under the cliff, in the bay around the corner from site #6. The original chemicals had killed two crowns in front when I took over. I dug up an active third one in between. 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, and disconnected its root outside the crack.
The high tide flooding of 2022 scoured the earth from the front root crowns. Scattered tussocks started appearing and continues. Digging showed their attachment to the prior-dormant crowns and to individual small pieces of root that came from who-knows-where.

In winter 2023, Parks directed the rock scalers to excavate the bottom of the cliff where the root lived in the cracks. They had no one on site to supervise or even collect the pieces of root left scattered. There is still root left in the rock. They did not use the opportunity to dig down into the front flats to unearth the now-buried crowns. All they accomplished was to compromise the sandstone cliff's integrity, threatening a slide.

I continue digging up the exploratory plants out front, and clearing the growth from the root still in the crack.
This spot had about 20 plants scattered behind the seawall at 3rd Beach's south end. In spring 2020 Parks weed-whacked the blackberries, giving access to the knotweed. I dug up about half with a hand spade. In 2022 they weed-whacked again. This time I had a shovel, so I believe I have removed the remaining roots except one. The roots of that plant dive below a rock so I can not dig it up. It will need chemicals, but they missed it in 2023.
Just around the corner from the north end of 3rd beach is a grassy area with plants throughout. Initially I dug up about 7 plants successfully. Another 8 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 edge. In 2023 I dug out the north root, but a tussock appeared in the middle.
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 was 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 2023.

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. In 2022 I waded through the blackberries to dig up the roots growing overtop the fabric.

Spot C was also covered in landscape fabric in the past but 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 had 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.