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Eradicating Japanese Knotweed along Sligo Creek |
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Clair Garman June 25, 2004 |
Update July 3, 2004 |
Update July 4, 2004 |
Update June 14, 2005 |
Update Sept 6, 2005 |
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 about.com. 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.
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.
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.
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.
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.
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.
