Fighting Back Against Noxious Invaders

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They creep in unannounced, stealthily putting down roots in Alberta s most precious grasslands, and their numbers are increasing.

Leafy spurge and spotted knapweed are perhaps two of the five major rangeland weeds in the Great Plains of North America and they have caused millions of dollars of lost revenue, said Don Battiste, program director of the Alberta Invasive Plants Council.

Both plants are primarily found in southern Alberta, but spotted knapweed is starting to be found in the Banff through Calgary corridor, around Wainwright and in other sporadic patches in the province.

Spotted knapweed (Centaurea maculosa), is native to Eastern Europe, prefers well-drained soils, and is intolerant of dense shade and constant moisture. The weed can self-pollinate and is also cross-pollinated by insects. Perhaps the plant s most interesting trait is the ability of its roots to exude a chemical that actually inhibits the root growth of other plants.

Chris Neeser, a weed scientist with Alberta Agriculture, says knapweed is mainly a problem on rangeland and roadsides rather than cultivated land. Knapweed is regulated as a prohibited noxious weed, which means that it has to be destroyed.

There are quite a few herbicide options available, so it s not a technical challenge to go about controlling it, it s more finding the resources to do so, said Neeser.

Are we making progress? I would say we have in some counties, said Neeser, adding that the full picture of the plant s distribution won t be clear until all the reporting by counties is done.

In the winter, the weed s skeleton can break off and blow away, spreading seed as it travels. One individual plant can produce more than 140,000 seeds per year. However, Neeser believes ultimately, the people will beat the plant. We re confident that this one can be eradicated. It s not going to happen overnight, because with weeds you always have seed and they tend to come back for a few years, but we think for this one in particular, it s going to be a challenge, but not an insurmountable one, he said.

Spurge a bigger challenge

Leafy spurge (Euphorbia esula) is a noxious weed, native to Europe and Asia, that was introduced in the early 1800s as an ornamental, or perhaps a crop seed contaminant. It spreads by re-sprouting from its large and creeping root system, but it can also reproduce by seed. Like knapweed, it s mainly a problem in rangeland. Its roots can extend nearly 15 feet laterally from the plant, and almost 30 feet deep.

Leafy spurge is a little bit more of a challenge in terms of controlling. The herbicide options are a little more limited there, said Neeser.

Asked if progress is made, Neeser said I wouldn t venture to say that, I think the best we can hope for at this stage is to confine it to where it is occurring now and these are usually extensive patches in a variety of areas.

It s harder to get rid of to begin with, it s far more widespread so we have deemed it s beyond the point of it being practical to eradicate it, in terms of how many acres are infested, Neeser said.

Leafy spurge is a noxious weed, not a prohibited noxious weed as is spotted knapweed, and therefore, landowners are not legally required to try and eradicate it. There are incentives from a rangeland perspective to do something about it because it doesn t help the quality of your pasture, but it s not as much of a legal incentive, Neeser said. More information at

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