Onions are Pikmin nests found in every entry of the Pikmin series. They come in a variety of colors, each different Pikmin color having their own, excluding Purple Pikmin and White Pikmin, who in the course of Pikmin 2 will sleep within Olimar and Louie's ship. Within Onions the Pikmin remain safe. More Pikmin can be created by bringing enemies and pellets to these nests, which will have the Onions suddenly release seeds which will turn into new Pikmin. They are named so because of the resemblance to an Onion on Olimar's own planet of Hocotate.
When a group of Pikmin bring an item to the ship in order to produce more members of their species, seeds will be ejected from the top of the onion and will be planted in the ground almost immediately. If there are already 100 Pikmin on the ground, then the seeds will remain within the Onion and turn into Pikmin automatically when Olimar wishes to take out more Pikmin. If all of a certain type of Pikmin have been killed on the field, a single seed of that colored Pikmin will be released the following day in order for that certain species not to become extinct (and for the sake of gameplay not end the entire game).
When Olimar finds Red, Yellow, and Blue Pikmin in the games for the first time, they will always be accompanied by an onion of their color. When he finds the onion, they will be on on the ground without their legs extended, and their colour will be slightly faded. After finding them, their legs will shoot out and they will turn to their normal color.
The tops of onions slightly resemble the vegetable their named after, though are of the colour of the Pikmin that rests within them. At the top of each onion is a flower, which isn't there when Olimar first finds them (the flower will prevent the player from seeing how it exactly resembles an onion). There are three entry ways into the onion that each have extended legs shooting from beneath it. These legs are used for the Pikmin to climb up and get inside the onion. Beneath each Onion is a beam that will bring in items that the Pikmin brought to it. Once inside, it will allow the Onion to produce more Pikmin, which are planted around the Onion. Along the bottom part of the main portion of the Onion are a series of triangles that serve no real purpose whatsoever. The Onion's three legs are, according to Captain Olimar, covered with tiny hair, probably allowing the Pikmin to more easily climb up it.
Super Smash Bros. Brawl
The Onions also make an appearance in Super Smash Bros. Brawl for the Wii on the Pikmin themed Distant Planet stage. They will appear randomly in their red, blue and yellow colors and will land somewhere on the stage. If the player tosses pellets that drop from flowers into the onions, they'll be sucked in and will release items that can be used against your opponents. If you throw a pellet of the same color as the Onion into the structure, it'll release more items, similar to how in the series it will release more Pikmin seeds.
"Items considered to be the places where Pikmin are born. As to the actual biology of the process, much is still unclear. Onions absorb the prey that Pikmin capture and use it to produce new Pikmin. In addition, Onions can take in Pikmin and later release them. These organisms are crowned with distinctive propeller-like sets of leaves that spin around, enabling them to fly."