A couple of years ago, I did not know this plant, but now it is a regular visitor to my garden (and the rocks beside my driveway). I’m not sure how it got here, but it might have hitched a ride home with me after some roadside botanizing. When it shows up, I admire it and let it hang out for a while, but once it starts to make its tiny marigold-like flowers, it has to go — I pull it up.
This is spanish needles (Bidens bipinnata). The seedheads are the real issue — if only it didn’t do such a great job at reproduction! The flowerheads form balls like dandelions, but instead of soft fluffy down, each seed has two sharp teeth at its tip. (Bidens, its scientific name, actually means “two teeth.”) These teeth help the seeds cling to animal fur or human skin or clothes, and we become its dispersers.
But before it sets seed, the ferny leaves are pretty and fresh, at a time when the perennials are beginning to look a little ragged (July, August). Several species of caterpillars feed on the leaves, which is good enough reason to leave a few, I think. And it’s edible and medicinal.