Clovers appear in all of the Pikmin games but serve a purpose only in Pikmin 3, in which they act as a hiding place for Pikmin. In Pikmin 3 Clovers are quite rare, only appearing in the Garden of Hope in story mode and in Mission mode they appear in the Tropical Forest and Shaded Garden .
Dandelions and Seeding Dandelions are found in Pikmin 2 and Pikmin 3. They serve no purpose aside from asthetics. Seeding Dandelions' seeds can be dispersed by running into the flower. Oddly enough, despite being common in real life, Dandelions are rare in Pikmin.
A Burgeoning Spiderwort as it appears in Pikmin 3.
Blue Pikmin attacking an Ultra Bitter Burgeoning Spiderwort.
A Burgeoning Spiderwort as it appears in Pikmin 2.
Burgeoning Spiderworts are found in Pikmin 2 and Pikmin 3. They serve the purpose of obtaining sprays. In Pikmin 2, they come in three types, Ultra Spicy, Ultra Bitter, and a rarer Mix of the previously mentioned. In Pikmin 3, the only type is Ultra Spicy.
Common Glowcaps appear in every Pikmin game and are always used for lighting. In Pikmin 3 some Glowcaps have to be activated. It is most-likely that Blue Common Glowcaps are based on Pixie's Parasol mushrooms.
Unidentified Plants & Fungi are plants and fungi that are never given a name in-game and therefore are usually refered to by a nickname, brief description, or are called what the actual item is in real life.
Yellow Wood Sorrels only appear in Pikmin 3 and are very common. Although they don't directly serve any purpose, there is a patch that marks where an Iridescent Flint Beetle is, this is located in the Garden of Hope.