There are a variety of different species in Star Trek, from all walks of life. Humanoids, insectoids, silicon-based life-forms, the works. However, I can't seem to think of any sentient plant species from across all the Star Trek shows/films. Are there any that exist, throughout all of the canon? Examples from the animated series will also be included, in all its weirdness.
Since the animated series counts, there are the Phylosians:
They kidnap Spock and make a 25-foot tall clone of him.
In the TNG episode Shades of Grey, Riker is attacked by a vine that demonstrates a basic level of sentience. It attacks living things, but not Data, and writhes when they cut off its thorn.
in DS9 Julian mentions that Ensign Vilix'pran is "budding" with twins.
The plants in This SIde of Paradise seem to have some intelligence - they turn to shoot spores into people who aren't already space-hippie-zombies.
They also seem to have some sort of control over people who get sprayed - they want to protect the plants and expand the plants' control over other people. I'm not sure if this fits your definition of Sentience.
KIRK: Spock, is that you?
SPOCK: Yes, Captain. What did you want?
KIRK [OC]: Where are you?
SPOCK: I don't believe I want to tell you.
KIRK: Spock, I don't know what you think you're doing, but this is an order. Report back to me at the settlement in ten minutes. We're evacuating all colonists to Starbase Twenty Seven.
SPOCK: No, I don't think so.
KIRK: You don't think so, what?
SPOCK: I don't think so, sir.