I don't know about anyone already writing one (although it's more than likely someone has), but the real question is:

Do you need permission from the copyright holders to do so, and if so, have you got it?

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Charles Dodgson (Lewis Carroll) died in 1898 so his work now lies in the public domain. That means you can adapt his prose work into a play if you wish without permission. Copy right ends 75 years after the death of the author.
There have to my knowledge been a number of adaptations of Alice both "In wonderland" and "Through the Looking Glass" which are of course different books. This doesn't mean that you should not continue with your work. I don't remember seeing the talking flowers on stage in a production although my wife who is looking over my shoulder says she has. The Alice stories are wonderful and never seem to lose their charm, each generation rediscovers them for themselves. Go for it.

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Hey Luke,

There are a couple stage adaptations that I am aware of (and very probably many more that I am not aware of). Maverick Musicals holds the rights to one that I personally like - it's a musical version and is great for junior performers.

As far as the talking flowers scene goes - I haven't seen a stage show with that particular scene - but once again, that doesn't mean one doesn't exist.

On another note, Alice in Wonderland is one of my favourite tales and I love to read the different interpretation's of Dodgson's work - if you need someone to edit/offer feedback, I'd be more than happy to - just email me if you would like

Good luck with the writing
Darren