Nectar weed

Because this subject has no official name, the name "Nectar weed" is conjectural.

Some White Pikmin plucking a patch of Nectar Weed.

Nectar Weed is found in a number of overground areas in both Pikmin games, and resembles a patch of long grass. Its purpose for Pikmin is as a source of Nectar: they can extract pools of Nectar from it seemingly by plucking out single leaves of grass. There is no need to swarm the Pikmin to the patch, as they will attempt to use it whenever they walk over it. It is found only in above ground areas, Awakening Wood and Perplexing Pool in Pikmin 2.

As for Pikmin 1, these are located in every area.