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Tanacetum vulgare
Someone suggested marigold, but these don't look like marigolds to me. They are only about the size of a small marble. If they are marigolds, can you give a specific variety that is this small?
There is a huge patch if this on the side of our barn. My husband wants to mow it down, but knowing the lady who lived here before us, it's probably useful for something and she probably planted it there for a reason.
They smell very sharp, almost like strong chamomile. They do attract a lot of butterflies, and that is a reason as good as any to let them grow there, but on the other hand, you can let your husband have that win... If he takes them down now, they will be back again next year :)
Here are a lot of text about what it can be used for http://en.wikipedia.org/wiki/Tansy, very interesting reading. That was new to me as well.
Are these marigolds though? They are so tiny. About the size of a small marble. And the flowers are very compact. The stalks these flowers are on are about 5 feet high. And, though they have a fairly strong smell, they don't have the typical marigold smell to them.