Candypop buds are a species of flowers related to the onions, it is suspected that they are in fact, juvenile onions, but there´s not enough information to confirm this. 
They can be found randomly around the world during the day, they come in different colors, which match the colors of existing Pikmin types, if you throw a Pikmin inside them, they´ll take it and transform it into a seed of the Pikmin type that matches their color, after doing this with 5 Pikmin, they wither away.

Candypop buds can be an easy way to access Pikmin types you don´t have onions for, especially the rare white and purple Pikmin.