What is the meaning of Old_man's_beard?

Chionanthus virginicus, a tree.

Clematis aristata, an Australian climbing plant.

Clematis vitalba, a climbing plant.

Tillandsia usneoides, a bromeliad; Spanish moss.

Usnea, a type of lichen.

Used other than figuratively or idiomatically: see old, man, -'s, beard.

Source: wiktionary.org