Eradicating Japanese Knotweed along Sligo Creek

Clair Garman
June 25, 2004
July 3, 2004
July 4, 2004
June 14, 2005
Sept 6, 2005
July 17, 2009

Japanese knotweed was introduced to the United Kingdom as an ornamental in 1825, and from there to North America in the 19th Century. Japanese knotweed is also known as Japanese or Mexican bamboo, because the stem becomes woody and has enlarged nodes similar to those of a bamboo fishing pole. It is a very fast grower that can reach a height of 10 feet and overtop native vegetation very quickly. It is Britain's most invasive non-native plant.

There are three locations near the Carroll Avenue bridge across Sligo Creek where Japanese knotweed has become established. Teams of Friends of Sligo Creek members have attacked these stands on multiple occasions, cutting and digging the stems and roots, but the plant has always grown back from the undug roots. Google yielded up an interesting eradication technique from The plan involves cutting the stems back to ground level, digging roots wherever possible and covering the patch with dark plastic sheeting to smother the plants.

Using materials provided by the Weed Warrior program of Maryland Park and Planning, a patch of Japanese knotweed next to the second hiker-biker bridge downstream from the Carroll Avenue bridge was attacked. The knotweed at this location was a danger to navigation on the bike path. When allowed to grow to its full height, this patch obscured the view of the upstream path from bikers crossing the bridge.

This patch had been cut back twice to ground level and the accessible roots had been dug.
Rip-rap rocks line the bank here. The knotweed grows up between the rocks and its roots cannot be dug out.

The stalks have been cut back to ground level.
Every effort is made to ensure that none of the sharp stalk fragments remain to pierce the plastic covering.

A single sheet of 7 mil plastic, 45 feet long and 20 feet wide, is laid over the knotweed patch.
Rocks from the stream are hauled up to anchor the edges of the plastic.

Update July 3, 2004

On Saturday, July 3, 2004, a Friends of Sligo Creek team covered two more areas infested with Japanese Knotweed in section 2. One area was near the first area covered. The other area was upstream of the Carroll Avenue bridge. The original intention was to cover the bottom part of the area on the hillside that leads up to the Washington Adventist Hospital. A nest of wasps convinced the team to work on a nearby area close to the stream. The area covered had not been clipped this year; the stalks were at least 8 feet tall.

The map shows the areas infected with knotweed. Areas 1, 2 and 3 have been covered with plastic. Area 4 lies on Washington Adventist Hospital property, on a steep hillside and has a nest of wasps in the part closest to the path. The plan is to wait until the wasps are inactive in the autumn and then cut and cover the section of Area 4 closest to the hiker-biker path.

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Update July 4, 2004

On July 4, 2004, a severe thunderstorm dumped inches of rain into the Sligo watershed. The creek ran 3-4 feet above normal and concern grew for security of the plastic covering the knotweed areas. All areas were secure even though the bottom foot or so of the plastic was in the raging stream. The map above shows the locations of the numbered areas.

Area 1 looking along the path

Area 1 looking down toward the stream

Area 2

Area 3 viewed from across the creek

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Update - June 14, 2005

After almost a year under black plastic, the Japanese knotweed has not died.
It grows up underneath the plastic, peeking through any holes.
It constantly seeks to grow out from under the edges of the black plastic.

Knotweed grows up through the holes and rips in the black plastic.

Knotweed grows out from the borders of the black plastic.

Knotweed grows up under the black plastic even where there are no holes or rips.
Place your mouse over the photo to see the result of walking over the large hump, thereby squashing the knotweed causing it.

The original plan had been to remove the black plastic in early autumn 2005 and plant native plants.
A "wait and see" strategy will be used for the remainder of 2005.
The plastic will not removed while the knotweed remains active.

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Update Sept 6, 2005

After fifteen months, it is decided to remove the black plastic to measure the success of using black plastic to kill Japanese knotweed. It is estimated that 10% of the area still has knotweed growing. Most of the growing knotweed grew at the periphery where the plastic had be pushed back by rainfll and stormwater in the creek, but amazingly some knotweed survived under the plastic where small holes passed light. The knotweed under the plastic was white and thin and obviously not healthy.

The remaining knotweed will be sprayed with herbicide to complete the removal.

The plant in the upper center survived under the plastic.
The plant in the foreground oozed out from under the edge of the plastic.

The swirl of stems shows the hunting under the plastic for sunlight.

The plastic at the bottom of the slope was often pushed back by stormwater in the creek and the knotweed thrived.

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Update - July 17, 2009

In the spring of 2006 grass seed was sown at the site of the former knotweed infestation. In the more than three years since then, the area has grown up in that grass and native weeds. The trees above the site have grown and shade the area. Knotweed has not reclaimed the site.

Compare the photo above to the same site in June 2004 below.

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