These small yellow flowers look innocent, but after they're fertilized the central disk grows longer and develops into hard, brown seeds.
The seeds splay out as they dry. Each one is topped by a tiny pitchfork of two to four spikes with downward-facing barbs.
The needle-like seeds detach easily from the plant...
... and stick to my sweater.
That's when I noticed the plant.
The Spanish Needles plant (Bidens bipinnata) is so annoying I was sure it was an alien invasive. Not! It's a native annual that's very adaptable, willing to grow in disturbed soil in vacant lots. These seeds grabbed me on Winthrop Street in Oakland.
Bidens bipinnata has many hitchhiker relatives in the Bidens genus. I identified this one by its lobed leaves and needle-like seeds.
(photos by Kate St. John)