Lisa Scott's Invasive Species

In an earlier article I wrote about how the spring flooding will allow some invasive plants in our region to thrive.

One of the species I mentioned was knotweed which will unquestionably take advantage of the disturbed shorelines and establish in new locations.

While the Okanagan-Similkameen region currently has limited infestations of invasive knotweed, the potential for this invader to expand is something that deserves

attention.

There are four species of knotweed in B.C, and all of them flourish in roadside ditches, low-lying

areas, irrigation canals and other water drainage systems. They are also found in riparian areas, along stream banks, and in other areas with high soil moisture.

Once established, invasive knotweeds change river flows, choke spawning beds, undermine riverbanks, shorelines and hillsides, punch through roadways and threaten the foundations of homes.

I am writing this article while attending invasive plant meetings in Revelstoke, and itís hard to miss the large patches of knotweed growing along

the roadsides and railway beds.

Japanese knotweed and its close relative Bohemian knotweed both occur in our region, and can be challenging to distinguish.

Most of the knotweed occurrences are on private land where quite often they were purposefully planted.

I am aware of at least one residential property sale in Penticton that did not happen due to the presence of knotweed. The potential buyer feared that the time and cost to control this invasive plant would be horrendous. I have to say that his thinking was not wrong.

This superweed is recognized by

international experts as a “world’s worst species”. In the United Kingdom it is referred to as the “scourge of the suburbs”, feared by

homeowners and gardeners.

Homes in Britain have been

demolished to rid them of an

invasion of Japanese knotweed. Needless to say, an outbreak of this invader is not to be taken lightly.

There is no doubt that invasive knotweeds have an exotic appeal which has contributed to their rapid spread.

The vivid green foliage and

hollow stems with distinct raised nodes give knotweed the

appearance of bamboo, which it is commonly marketed as, although it is not closely related. But don’t be deceived; knotweed is feisty, forceful and resilient to most

control treatments.

Knotweed prefers moist habitats and freshly disturbed soils, but is capable of thriving in gravel and pavement landscapes and in sun or shade. The hollow stem is covered in a reddish-brown speckled papery sheath and will grow 1-5m in height.

Knotweed has small white-greenish or light pinkish flowers in showy plume-like branched clusters which are visible right now.

The leaves of Japanese and Bohemian knotweed are heart or triangle-shaped and zigzag along the arching stems.

A broken piece of knotweed root as small as 2 cm can form a new plant colony with shoots that sprout all along its rhizomes. Think of how many new shoots a plant can have if its roots can grow 3 metres vertically and up to 20 metres horizontally. Therefore, it poses a massive risk of spreading via groundwork and

disturbance.

While Japanese knotweed spreads only by root and stem fragments, recent research has shown that Bohemian knotweed produces viable seeds, adding a new urgency to an already serious issue.

Interestingly, my colleague on Vancouver Island where knotweed is rampant tells me that the dry summer has made

treatment of this invader much more difficult.

There was considerably more seedling growth from the Bohemian knotweed this year and site checks of what they thought were eradicated outbreaks are

revealing young knotweed plants.

Controlling the spread or managing existing knotweeds requires diligent and continuous action. If you have knotweed, an integrated management plan of several

different methods of control may be needed as these invaders are extremely persistent and have such an extensive root system.

Knotweeds will not compost

effectively and must be bagged and buried deep in a landfill — depths of at least 2 metres are

typically recommended.

For information on invasive species go to our website: www.oasiss.ca or contact the Program Coordinator for the Okanagan-Similkameen,

Lisa Scott, at 250-404-0115.