Common Tansy continues to spread in ditches and fence lines. Very little is known about how we can control it on annually cropped land and why it has not already become a major problem. Direct seeders may be ignoring this potential weed problem!
Common tansy was a native European plant introduced to North American as an ornamental. It is an aromatic perennial with stiff stems standing 1.5 to 6 feet tall. Flowers are numerous, yellow, 0.25 to 0.5 inches in diameter, and arranged in flat-topped dense clusters. It is a prolific seed producer and spreads primarily by seeds but can also reproduce from rootstalks. These established rootstalks can become quite long, woody, and hard to kill. Seeds can be spread significantly with snow drifting. It is recognized as a dangerous weed in pastures because some plants produce a substance toxic to cattle and horses.
Clark Brenzil, Weed Control Specialist with Sask. Ag. & Food suggests that it is a weed similar to Scentless Chamomile. It is located in Saskatchewan mostly in the northeast corner. It does not tolerate tillage. Even minimum tillage with one pass a year with sweeps will probably provide adequate control. A small project carried out by Roy Button on a stand of common tansy established to evaluate commercial uses for the pyretherin content indicated that 1 to 3 L/ac of Roundup had very little control of the stand! However, others suggest that it may be more susceptible at the seedling stage.
Competition can certainly reduce the vigor of common tansy. Dan Cole weed specialist with Alberta Agriculture suggests that cropping practises such as fertilizing to soil test recommendations, using competitive crops, and using high seeding rates should all be effective in suppressing common tansy in low disturbance production of annual crops. Cole's research has been done with common tansy in pasture land where he has tried several different herbicides in conjunction with mowing and fertilizing. One of his suggestions to control the spread of common tansy is to mow the ditches where it is prevalent while the flowers are still yellow. The seed will not be viable at this stage. Escort, an industrial herbicide with the same active ingredient as Ally, has a registration for controlling common tansy in ditches.
Some long term direct seeders have been watching this weed for a number of years and there is no report yet of it becoming a major problem. It needs continued surveillance to determine what if anything effects establishment in direct seeded fields. If problem patches develop, options for control need to be tried immediately. There may be a great long term benefit to the use of both Escort and mowing to try to reduce the spread of common tansy.
Several weed control experts are concerned that the potential for it to become a problem weed for direct seeders like scentless chamomile exists.