Most obnoxious plant is a big category and if this becomes a contest there could get a lot of entries, but what the hey. Our first nomination comes from TPP's old friend the FlyGuy. Apparently he had a run-in with this plant and then spent a leisurely hour or so picking the achenes, not seeds, but fruit and seed, from his pants and socks. Here's his description. This plant "looks innocuous enough when young. Nice fancy leaves. Not too bad when it blooms, but the flowers are a little underwhelming. But it reaches its true evilness when the seeds mature and become dry. They are long, thin with " hooks" on the distal ends that catch in everything that touches them. In the photo you can see immature clusters of seeds in the upper left and upper right corners."
This is without doubt a species of Bidens (maybe B. pilosa), and like Joe, it is truly obnoxious. The apically barbed achenes, the one-seeded dry fruits, are sometimes called beggar ticks. In some places people have called them stick tights, but this common name is more usually associated with some other hitchhiker fruits technically called zoochorous. This is a mechanism for seed dispersal and us humans conveniently provide some nice artificial pelts for them to attach to. Other nominations are welcome.
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