Plants were a group of generally non-sentient lifeforms with the exception of the Phylosians, which were sentient lifeforms. Plants included trees, flowers, and many others. The edible plants (or parts thereof) included herbs, vegetables, fruits, and seeds. (TAS: "The Infinite Vulcan") Many plants, such as Oblissian cabbage, reproduced via pollination. (VOY: "Elogium") The plant life in a specific region was named flora.
The study of plants was known as botany. On Earth (and most class M planets), plants contained chlorophyll, giving their stems and leaves a green color. On planets like Nibiru and Pahvo, the plants had red and blue coloration, respectively. (DS9: "A Man Alone", "Paradise", "Rules of Engagement"; Star Trek Into Darkness; DIS: "Si Vis Pacem, Para Bellum")
Constable Odo told Commander Benjamin Sisko that he could not see any sense in sacrificing various plants to serve as tokens of affection when talking about Human mating rituals. (DS9: "The Forsaken")