Weed of the Week: Tansy Ragwort

The daisy-like, yellow ray flowers of tansy ragwort (Senecio jacobeae) give it a friendly appearance; but make no mistake, this invasive plant is highly poisonous to livestock, and one plant can produce over 150,000 seeds, allowing it to spread swiftly to new areas.

Tansy ragwort is a problem plant in the Lower Fraser Valley, southern Vancouver Island and the Okanagan. It’s rapid growth and spread is a concern because it takes over more desirable vegetation for livestock, leading to a significant loss of pasture—up to 50 percent! Seeds can lie dormant for four to five years, or even up to 20 years if buried.

This poisonous plant poses a serious health risk to livestock if they ingest small amounts over an extended period of time. Long-term exposure to its toxic alkaloid (which reacts with enzymes in livestock) causes cumulative liver damage and sometimes death, even if livestock have been denied access to the plant after symptoms appear.

Trace amounts of the alkaloids found in milk and honey produced from infested pastures are also a concern, though there is little known risk to people. Livestock avoid consuming this plant unless other forage is available. The highest risk is after the plants have been cut or when mixed in with hay, because the plants are not as bitter then, but just as toxic. Avoid grazing horses, cattle, and goats in pastures that have any more than five percent tansy ragwort, until plants are controlled.

Use methods of control such as hand-pulling rosettes before plants can flower, or chemical (herbicide 2,4-D) or biological methods (flea beetle – contact the Ministry of Agriculture to inquire about their beetle release program).

Tansy ragwort in considered a provincially noxious weed in British Columbia. Plants are identifiable by daisy-like yellow flowers that appear from July to September in a flat-topped cluster, along with dark green, stalkless leaves that have a ‘ragged’ appearance. They can grow up to about a metre tall. Tansy ragwort is a biennial to short-lived perennial plant that is often confused with common tansy (Tanacetum vulgare). Common tansy, however, does not have ray flowers, and has sharply toothed leaves.

To maintain a healthy, competitive forage against tansy ragwort and other invasive plants: seed with appropriate forage grasses and legumes; re-seed areas of soil disturbance from hoof marks or vehicle tracks; fertilize according to soil needs; and graze pastures uniformly and not excessively as this is the leading cause of pasture deterioration and rapid invasion of invasive plants.

Sources: Ministry of Agriculture; Wikipedia; Weeds BC; Nova Scotia Museum; King County Noxious Weeds
Photo: Lisa Scott 

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