I'm sure you will be able to find some growing wild somewhere Dirt. You can collect some seeds off of them. If you just have a small patch of them in your garden it would be easy to deadhead them like one does with any other plants. The seeds are viable for 5 years.
B. bipinatta might one of the Bidens that grows where you are Dirt.
http://plants.usda.gov/core/pr...
Spanish Needles (Bidens bipinnata)
I'm not exactly sure where you are but I'm just guessing.
