Here's another handy-dandy batch of inspiration for you, this time it't the singing/speaking/snarky and mean flowers that Alice encounters in a Wonderland Garden when she's shrunk to just a few inches high.
When I was in 2nd grade my mom made me an EXCELLENT sunflower costume, and I won several costume contests with it! I should find and scan in a photo - it was basically a hood with a circle where your face stuck out (I wore huge sunglasses for character) and 2" cardboard petals covered in yellow fabric that fanned out around my face like a sun. It was occasionally difficult to make it through doorways being that wide, but it sure was fun! I wore a green tunic and green tights, and there was one big green quilted leaf hanging down the body. Years later I think I heard it was made out of pizza boxes! Yay eco-friendly recycled costumes!
Anyways, until I can find a photo of that one, here are a few more versions flower costumes to inspire you:
First, here are Disney's versions - my favorite is the school-marm Iris
Here's a simple, standard, adult flower costume - felt "petals" frame the face, the rest of the outfit is solid green. I think you can buy headbands like this at a lot of stores off the rack.
Here's the same general idea but with a bit more flourish and detail - satin fabrics, sparkle, more petals...
Now this guy is a lot more dramatic (and oddly creepy)- his looks like a lot more manipulation of the fabrics to become the whole flower, and then there is pink face paint involved. Might be a little Georgia O'Keefe for me, if you get my drift...
Zombie flowers! Love how a little bit of face paint can totally transform a "normal" flower into something twisted:
Feel a bit more daring? Mostly I gag at the "slutty cop, slutty construction worker, slutty whatever else" costumes, but here's a barely there one that would take real balls - those are LIVE flowers!
Not willing to go that bare? Here's another version where you are just draped in flower garlands - sometimes you can even find them at the dollar store:
The "flower" part doesn't have to be around your face, here are some gorgeous lotus flower skirts:
I love the idea of huge flower hats, isn't this gorgeous:
It reminds me of the "Rose Petal Place" dolls I used to have as a kid - I've always wanted to do a costume like one of these dolls wore, they were AWESOME! I'm pretty sure I started a daffodil hat once. They had great dresses AND huge hats with their blooms, either would enough for a great costume:
Here's a drawing of Sunny Sunflower, so you can see more easily how to make her dress & hat:
Well there you go, another huge batch of inspiration, and I've done all the googling for you :) Hope you're finding something to get excited about, I know I sure am...