A recent study featured in the journal Invasive Plant Science and Management suggests it’s time to rethink control of Russian knapweed, an invasive plant classified as a “noxious weed” in 18 U.S. states.

Dense patches of Russian knapweed can survive for decades. They crowd out more valuable forage plants and are toxic to horses. Controlling them is tricky since knapweed spreads two ways – both by seed and by new shoots from root growth. To date it has been hard to determine which method is fueling local invasions across the western U.S.

When researchers analyzed dozens of DNA samples collected from six patches of Russian knapweed, they found the answer. Each patch was largely or entirely composed of a single plant genotype. That means localized patches are spreading from Russian knapweed roots. Each patch studied, though, was genetically distinct. That means seed dispersal is the source of new patches.

Land managers now have important new insights to guide their management decisions – including the best use of three approved biological controls. Both midges and nematodes reduce Russian knapweed seed production and the development of new patches. They are ineffective, though, at controlling existing patches of the weed. Only a third biological control – the gall wasp – is able to stress Russian knapweed plants and reduce their competitive ability.

You can read more about the study in the article “Invasive Russian Knapweed (Acroptilon repens) Creates Large Patches Almost Entirely by Rhizomic Growth,” found in Volume 10, Issue 2 of Invasive Plant Science and Management.

 

Cover Photo: This photograph is from Gaskin and Littlefield (Pp 119–124). It shows discrete Russian knapweed (Acroptilon repens) clones arising from a sea of cheatgrass. Each representing a distinct genotype that is maintained by asexual, vegetative reproduction. Photo taken near Bridger, Carbon County, Montana in July 2015. Photo credit: Jeffrey L. Littlefield.

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