Common tansy (Tanacetum vulgare) can be difficult to control. Originally from Eurasia, the plant has a prolific seeding habit and spreading nature that quickly allowed it to escape cultivation in American gardens. It is considered an invasive plant or noxious weed in many regions, particularly in the Pacific Northwest. Common tansy is hardy in U.S. Department of Agriculture plant hardiness zones 4 through 8. It often can be killed using a combination of mechanical and chemical control methods.
Pull small or young common tansy plants with gloved hands before the plants produce flowers, which usually occurs from July through September. Common tansy foliage contains an oil that can be toxic if absorbed through skin. Wear garden gloves, closed-toe shoes, a long-sleeved shirt and long pants when hand-pulling the plants. Moistening the soil makes it easier to pull entire plants, including all of their roots, from the soil. Pulling plants from dry soil often leads to breakage, with some parts left in the soil; common tansy can quickly regenerates from even little root fragments. Place all the pulled plant parts inside a plastic trash bag. Put the bag in a trashcan, and place the lid on the trashcan while waiting to dispose of the plant parts.
Spray larger common tansy plants with a glyphosate-based herbicide before they form flowering seed heads, thoroughly moistening each target plant's foliage with the herbicide. Herbicide chemicals can cause skin or eye irritation. Avoid exposure by wearing goggles and a face mask in addition to garden gloves, closed-toe shoes, a long-sleeved shirt and long pants. One glyphosate-based herbicide's label directs users to mix about 5 tablespoons of the herbicide with 1 gallon of water in a hand-held garden sprayer, but follow the specific mixing instructions on your herbicide's label because directions vary. Spray the herbicide on only a sunny day when the temperature is above 60 degrees Fahrenheit, no wind is present and rain is not forecast for 24 hours. Those sunlight and temperature conditions allow the herbicide to produce faster results than during cloudy, cooler conditions. Wind blows the spray onto other locations, including onto desirable plants. The herbicide needs to be reapplied if rain falls within 24 hours of its application.
Repeat the herbicide application in about four weeks if another treatment is necessary to kill the common tansy.