The optics of a single drop is sufficient to describe the rainbow and a natural rainbow is just the accumulated effect of billions of individual drops.
A water drop on a blade of grass is slightly distorted but can still show rainbow features when viewed out of focus.
Some additional effects caused by the distorted surfaces are also visible.
Most of the examples here are actually guttation drops where the grass is exuding water under the humid conditions in the lawn.
This seems to give better results than wet grass after rain for example.