Sounds like wormwood, a type of artemisia. Does it have a smell when you pull it? If it's in a small bed try pulling what you can, then cover the bed with newspaper and then a mulch of somekind over that. I've keep my larger areas weed free like that. I do always find the wormwood popping up here and there though.
chaz

Thanks chaz,
I googled Wormwood, and my weed looks sort of like Artemisia Absinthium, but I have never noticed any flowers on it. It looks almost exactly like Chrysantheum. Is there a wild one perhaps? I hate to admit it, but I think I may have actually planted it years ago! My MIL (who isn't a gardener) gave me a couple cuttings of something that was supposed to have blue flowers on it, but never did. I will try to post a picture if I can figure out how to do it. Thanks again!!

~Jackie

The flowers just look like part of the foliage. If you are sharp eyed you may be able to see the stamens.

If you like the plant do not, do not, fertilize. I have killed several by forgetting

mugwort?

Yes. Chrysanthemum like leaves sounds like Mugwort, Artemisia vulgaris.