Nectar weed is found in a number of overground areas in the first two 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. Unlike eggs, these will only ever contain nectar, not spray drops. The amount of nectar contained inside a patch is random, and it is possible to have no drops of nectar appear. Only five Pikmin can be pulling a patch of nectar weed at a time.
There is no need to swarm the Pikmin to the patch, as they will attempt to use it whenever they walk near it. Given how mundane and camouflaged these leaves of grass are, it is likely that Pikmin were programmed to interact with them without the player's consent. Otherwise, the player would only realize that this particular type of grass is useful by pure accident, in the chance that they were to accidentally command a Pikmin over a patch of the plant. This does however have the drawback of distracting Pikmin when the player doesn't want it.
Ideally, it is best to have five flower Pikmin work on the leaves, and, with all leaf and bud Pikmin, use group swarming to keep the Pikmin in a tight circle around the patch of grass. By using an overhead camera angle, it's possible to get a better view on the location of the newly-uncovered nectar drops.
Nectar weed can be found in every area of Pikmin, but only in the Awakening Wood and Perplexing Pool in Pikmin 2, and never underground. It should be noted that in Pikmin 2, Purple Pikmin appear to stomp in mid-air, if thrown above a patch of nectar weed.
In Pikmin, when a blade of grass gets plucked, there is an 8% chance that it will produce a blob of nectar.
Rubble is found in small piles in the first two Pikmin games, and its use is exactly the same as that of the nectar weed. When Pikmin pass over a stockpile of rubble, whether directed to by the leader in charge or not, they will begin attacking the pebbles, causing them to randomly give out pools of nectar, until all pebbles have been destroyed.