Ooo this could be fun!
For me, when I'm developing a magic system I like to consider the thematics behind powers, as well as their functions. The great thing about plants, and flowers specifically, is that they hold a lot of traditional and mythological meaning. It could be really fun to consider, and would add a level of depth to the world building alongside being a cool magic system.
Some thoughts along that vein:
-Healers having powers based around herbs that are well known as medicinal. Maybe, they ingest their herb, magically amplify it within themselves, and then transfer the amplified affects to a patient. An example from off the top of my head would be a lavender mage. Lavender is known to help people sleep, and give them good dreams. Maybe our lavender mage uses the plant as a magical medium for putting people to sleep (this could be for medicinal purposes, or could be used on enemies as well!). Or, going further, they could access the 'dreamscape' and interfere with people's dreams using lavender.
-Several different kinds of flowers mean love. Maybe, love potions from different flower mages could have different affects. A rose mage's love potion would give the 'victim' romantic love, a forget me not mage's potion would focus on devotion, a baby's breath mage's love potion would be more innocent, etc.
-A poppy mage being feared as the grim reaper because of their association with death. (whether or not they really have power over that could be up to you)
-The flower Amaranth symbolizes immortality. Does the flower really grant people immortality? Do mages who use the flower gain immortality? Maybe the flower is rare, and people search for it because they believe it will give them long life.
-Himalayan blue poppy's are associated with psychic skills, which could obviously be used for telepathy and empathy type powers.
This wiki page has a lot of good meanings for flowers, but is lacking for other plants. Research for this sort of thing isn't hard, and generally you can find good stuff with just a quick google.
Obviously, this is just one way of taking things--and the way I personally would do things. I focused more on the 'separate plants' side of things, but you could also do different biomes, animals, natural phenomena, weather, etc. Nature is a really good topic because you can do so much with it!