I've seen two great ideas for making plastic flowers recently. One in the Harvey Nichols window in Edinburgh - alongside the animals, and another in New York - in the Anthropologie window.
The New York flowers are made out of plastic drinks bottles. They've used the top bit of the bottle, chopped it off, and cut out petals which they've then bent backwards and sprayed into myriad colours. You wrap wire around the little round hole where you used to pour the drink from, and hey presto.
The Edinburgh method is to use those plastic rip fastners. Make a flower stem out of wire and finish it with a little loop. Attach lots of the rip fastners to the loop and again, sray them different colours. They'd look fantastic planted outside!