This is growing at the base of one of my Iris. It is a self starter. If it is a weed, I need to just pull it out. ( photo / image / picture from Tina's Garden )
Does it kind of have a funny smell when you touch it?? If it does it might be a very aggressive little daisy type plant, chamomile I think :-? I picked up a teeny tiny piece in some plants my friend gave me and it pops up everywhere I have spent the last 7 or 8 summers pulling it out of every flower bed, sidewalk crack, gravel driveway...
It is Feverfew Tina. Tanacetum parthenium (Feverfew) It does spread quickly but isn't hard to control. I like to keep a good sized clump in my garden because the blooms are pretty and bloom all year.
I agree with Netty. I think it is Feverfew, (which I thought was a chrysanthemum relative, although the latin name is different, so maybe I am wrong about the Chrysanthemum part.) Anyway, I grow it in my garden & it self sows readily, but is not aggressive. I agree with Netty that the flowers are pretty.