Naughty Knotweed

Penticton Herald - - HOME GARDEN & - LISA SCOTT

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 Okana­ganSim­ilka­meen 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.

Spe­cial to the Herald

The po­ten­tial for knotweed to ex­pand in the Okana­gan is some­thing that de­serves at­ten­tion.

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