Pellet Posies are a species of plant that produces crystallized nectar in the form of pellets, which are high in nutrients and can be consumed by onions to produce more Pikmin seeds

You can find non-mature pellet posies all over the world, They start unmature looking like some kind of fern, and after being replanted (breaking and placing the block again) they´ll become able to either start growing on their own, or being bonemealed by the player. 
Once they reach their first growth stage, they'll grow a small flower with a small pellet bearing the number one, given more time the pellet will grow more in size and change its number to a 5, in either stage you can throw a Pikmin of the same color at it to harvest the pellet, trying breaking the block or harvest the pellet yourself will fail.