She wants it to repel ants. Tansy, Tanacetum vulgare, is native to Europe, and is heat and drought tolerant.
Some folks use it as a companion plant. It's a natural, organic insect repellent. The leaves give off a stinky, strongly pungent odor that the bugs just don't like. Lots of people don't like it much, either.
It blossoms with clusters of flat, knob-shaped, yellow flowers in mid-summer. It blooms in July-August above ferny, but firm leaves.
I've even heard of people who like the foliage so well, they tolerate the persistence of the plant, and simply clip off the flowers when it blooms. They really do just look like yellow knobs.