The 'flowers' are actually inflorescences--white sterile ray florets surround the yellow fertile disk florets. Each tiny disk floret produces copious amounts of nectar and that's why gardeners love them. In my garden, they attract zebra longwings, gulf fritillaries, monarchs, numerous skippers, and honeybees.

Bidens are named for their achenes (dry fruit). Each disk floret produces an achene containing a seed. The top of the achene has two barbed prongs--bi (two) and dens (tooth). The achenes readily break off and attach to any passerby that brushes against the plant. Both the English common name, beggarticks, and the Spanish common name, romerillo (pilgrim, I've been told with connotations of one who begs), are references to the hitchhiking ability of the achenes.

A single beggarticks will produce around 1,500 seeds in its short lifetime. They attach to cats, dogs, birds, and clothing and this is why gardeners hate them. Picking the achenes out of clothing before laundering is truly one of the more aggravating necessities in life. One uncontrolled plant gives rise to many plants. Pulling them is close to impossible, the brittle stems snap at the ground line and without getting the root, the plant sends up new shoots a few days.

Although classified as a facultative wetland species, they are not particular where they germinate. Bidens in drier parts of my garden do not seem to live as long as those in the wetter parts, but they still produce a few seeds. They manage to survive year-to-year in the more chaotic parts of my flower beds...but, at least, I have lots and lots of butterflies.