May 20, 2002
County issues tansy ragwort warning
from King County Department of Natural Resources and Parks,
Water and Land Resources Division
The King County Noxious Weed Control Program is asking landowners to be on the lookout for the noxious weed tansy ragwort, particularly in eastern King County.
Now is the ideal time to remove the persistent pasture and roadside weed before it becomes well-established on your property. County staff say the weed is already established in eastern King County.
You probably have seen the bright yellow daisy-like flower heads in fields and along roadsides in late summer. At this time of year, the plants are rosettes of deeply divided and wrinkled leaves low to the ground. This is the ideal stage to remove the weeds from your property.
A big problem for many landowners with tansy ragwort is that it is toxic to livestock, particularly cattle, horses and pigs. The foliage contains pyrrolizidine alkaloids that cause cumulative liver damage. This can lead to photosensitization, jaundice and wasting. Tansy ragwort also out-competes pasture and desirable native plants.
Tansy ragwort can produce incredible amounts of seed. On average, a plant produces 60-70,000 seeds, but a large plant can produce as many as 250,000 seeds. Seeds can remain viable in the soil for at least eight years. This means that preventing seeding of plants is essential for control.
By stopping all seed production and eliminating existing plants, the infestation will decline as the seedbank is depleted. Seeds are spread by wind, water, animals, farm equipment and in agricultural produce, including hay. Take great care that you do not introduce seeds into your clean areas.
When controlling tansy ragwort, it is best to concentrate first on lightly infested areas and work towards areas of higher infestation.
Manual removal works well, especially for lighter infestations and in moist soils. The key here is to remove the entire root, as the plant can regrow from root fragments. Hand pulling often leaves roots behind. Mowing will also not eradicate the weed, as the roots are unaffected. Use of a hoe, mattock or shovel is usually more effective. Tillage can work for large areas when combined with an improved pasture or cropping program, but this may initially expose more seeds for germination.
Chemical control may help control large infestations. Take care to use a herbicide registered for this use and in accordance with the label.
Try to maintain a dense competitive vegetative cover, such as pasture or native vegetation. This will greatly reduce seedling re-establishment and out-compete any new weeds that try to emerge.
None of these methods will affect seeds already in the soil. Follow-up control will be needed for more than one season.
Check for small plants at the rosette stage every spring and remove any found.
It's also a great idea to coordinate your control efforts with those of your neighbors so that larger areas are controlled at the same time. Please contact our office for more information or help with organizing a neighborhood group control project.
Many landowners have shown that with a little persistence and effort, tansy ragwort can be controlled.
For information about tansy ragwort or other noxious weeds, call King County Noxious Weed Control Program at (206) 296-0290, firstname.lastname@example.org, or http://dnr.metrokc.gov/weeds