Hi John,
Tansy is a species that can be controlled via herbicide application...Typically, Tansy is sprayed in the spring before it begins to flower, and there are a wide variety of herbicides that can be used to control it (Glyphosate,2,4-D, Aminopyralid, etc.) all of which should be available over the counter by their trade names...although it looks as though it might be too late for it now, something to consider in conjunction with/as an alternative to spraying is pulling/digging up as many of the plants as you can next year, before it goes into flower...a combination of manual, chemical, and (if they are present) biological controls, sustained over several years, should help you get a grip on the infestation!
-Jackson Morgan, Polk SWCD

Polk SWCD
July 27, 2020, 2:15 a.m.

If it hasn't gone to seed yet, I don't see any reason to not include it in the weeds that you are currently treating, that being said, if you were able and willing to delay your treatment until later in the fall, the efficacy of your treatments would be greatly improved!
-Jackson Morgan, Polk SWCD

Polk SWCD
July 27, 2020, 2:46 a.m.