Weed of the Week: nodding thistle

Unlike some of its cousins, such as Canada thistle, nodding thistle nearly always begins from a seed, which grows to the height of a person — one with an agreeable personality because once it gets its purple hairdo, it starts nodding.

Each one of those big purple flowers can contain up to 250 seeds, and the plant’s ability to stick them way up in the air allows for their wide wind spread on those feathery pappus.

While the nodding thistle isn’t a big concern in cropped land, it is in pastures and hay. The plants quickly take over, and using their aggressive rooting structure, and likely a chemical tool that curbs plants close to them, removes all the nutrient and water resources in their immediate area.

Flowering of the carduus nutans takes place in June and early July with uncontrolled plants generating up to 20,000 seeds. Often seen at roadsides and other areas where grassland is disturbed, if left uncontrolled the pests rapidly take over. It is found across North America and can appear as a winter annual or a spring germinated weed. Other than producing some nectar for bees, the plant has no other beneficial uses.

The nodding thistle is considered an invasive plant and noxious weed in Alberta, Saskatchewan, Manitoba, Ontario and Quebec, as well as 20 American states. The federal seeds act considers it a Class One weed.

Mowing just prior to flowering is known to be effective in controlling it, but there may need to be several attempts at this because a patch of nodding thistle might have several growth stages in it.

The biennial weed began life in Europe and Africa and likely arrived in ships’ ballasts. In Saskatchewan it is suspected to have shown up in canola seed lots and found a home along the rail lines, say researchers.

It can be controlled when it is young using herbicides such as 2,4-D, MCPA, clopyralid, dicamba and picloram. Repeated application of phenoxy herbicides to populations over several decades has led to resistant types developing in New Zealand.

Rhinocyllus conicus, the thistle head weevil, and another weevil, trichosirocalus mortadelo, have been effectively used in Canada on thistles, including the nodding kind. Urophora solstitialis, the nodding thistle gall fly, can also be effective and is found in the region. Fungal controls developed by Agriculture Canada researcher Karen Bailey and others might one day have a role in controlling the pest. Canada-based Premier Tech has an agreement with Agriculture Canada to develop those tools.

The nodding thistle has poor resistance to steel, so tillage remains an option, although possibly an undesirable one in pastures and native grasses.

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