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Invasive Japanese knotweed is not our friend

  • Martha Proctor
  • With its ability to grow through asphalt and structural material, survive up to 20 years in total darkness and uproot the foundation of homes, Japanese knotweed (Fallopia japonica) could be the villain in a horror movie. The plant’s damage potential and the fact that this plant is listed as one of the top 10 invasive weeds in the world, has heightened concern over the discovery in 2011 of patches of it beginning to take hold in both the San Geronimo and Lagunitas Creek watersheds.

    Japanese knotweed is an aggressive, herbaceous perennial that emerges early and grows rapidly (8 feet in seven months), thus enabling it to easily outcompete native vegetation. Its preferred habitat is wet areas such as riparian corridors, springs and even septic leach areas. Small fragments of rhizomes the size of a fingernail can float along in streams or be transported in soil, thus spreading infestations to new sites along waterways.

    Once it gets a foothold, it can quickly infest large areas. Rhizomes have been shown to extend 23 feet horizontally and 10 feet deep making removal of this pest extremely difficult. Its allelopathic properties prevent native tree seedlings from establishing themselves. Japanese knotweed itself is native to Japan, China and Korea, where it colonizes volcanic geology and soils.

    For identification purposes, the stem looks like red or pink asparagus in spring. In summer the red-speckled stems have a zigzag pattern, which bear white drooping flowers with leaves that are shield-shaped with a flat base. By fall, the plant begins to die back and its brown, hollow stems remain standing until spring. Various common names include false bamboo and Japanese fleece flower.

    The number and size of known patches of Japanese knotweed growing along the San Geronimo and Lagunitas Creek watersheds are still small. Given the fact that manual removal encourages its spread and the ease with which minute plant pieces can reestablish in new sites, it is critical that control and long-term eradication of this weed be carried out by experts in an expedited fashion.

    The Marin Knotweed Action Team (MKAT) is a coalition of local, state and federal agencies, including UC Cooperative Extension, working to educate community members, and identify unknown patches of the weed and treat them before new infestations can arise. MKAT was formed to block the spread of this invasive pest before it is too late.

    The MKAT Team is reaching out to creekside landowners in the San Geronimo Creek watershed asking to survey their property for the presence of Japanese knotweed. If found, a licensed vegetation management business treats infestations with aquatically approved herbicides in late summer and fall when the plant is bringing resources back down to the rhizomes. If treated, the number of stems can be reduced by up to 90% by the end of the first year but it usually takes three to five years of ongoing management to kill the rhizomes. If left unmanaged, this knotweed will spread and dramatically affect the health of the region’s creek and salmon habitat, and could eventually migrate into the Tomales Bay watershed.

    If you think you have Japanese knotweed on your property or know of a suspect plant in San Geronimo Valley, take a photo of the plant and email it to MKAT at acdirkse@ucanr.edu. Include your contact information and the location of the plant. Trying to manage it yourself is not recommended — cutting, tarping, pulling or burning do not work and actually stimulate the weed to spread. For more information, go to ucanr.edu/sites/MarinKnotweedActionTeam. This plant is of serious concern — in the U.K., you can’t get a mortgage if you have knotweed on your property.