Two Canon 580ex flashes were used for the shot. The main 580ex was modified with a Lastolite Ezybox Hotshoe softbox.
The second 580ex was gelled with a CTO gel and Justin Clamped to part of the tree just behind the model. To spread the light out as much as possible to light both the background and the model's hair, the flash was zoomed out to its widest setting and the diffuser panel pulled over the gel.
Both flashes were set to manual mode, and triggered with Cactus v5 radio triggers.
This is what the tree looked like before we set up the strobes:
And this is the result:
Canon 5D mkII 70-200 f2.8L IS II lens iso 800 1/100 @ f2.8
In the future, I'll have to remember to experiment with other colors of gels (like blue or green) Or maybe even mixing different colors.