Tansy
The small gold buttons of this wildflower are beautiful. In the middle ages, tansy was used as a strewing herb, spread over rushes to make floors more fragrant for sleeping on.
Tansy was introduced to North America in the 1600s, but the province of Alberta has now designated it a noxious invasive alien species.
Dense clumps of tansy contain enough toxic alkaloids to threaten livestock, at least in theory. Fortunately, actual cases of poisoning are rare. Cattle must eat large amounts to be affected, and they don't appear to find the plant tasty.
Like other "noxious" plants, it has medicinal properties. Strongly aromatic tansy leaves can be used as an insect repellant.
According to Mrs. M. Grieve, the name probably comes from the Greek word for immortal. It was likely given because the plant was used in embalming the dead. Also, it was supposed to have have been fed to Ganymede to make him immortal.