I have a Saison on the go (bottling tonight). From what I've read Saison yeasts like to free rise, so start at 21°C and let the temperature rise with fermentation.
As I did mine in a temperature controlled fridge I started at 22°C, fermented at that for three days before stepping up 0.5°C every 12 hours or so. After getting to over 28°C and 10 days it was pretty much finished. That was a lot faster than expected as some Saison yeasts supposedly take a month to ferment out.
In James Morton's Brew book he likes to let the yeast do its thing and rise freely to over 30°C to get the most character out of it.
I'm interested to know how the rhubarb flavour comes through.