I had my bonfires in Feb, when the weather was a bit nicer than it is now (snowing again

))
I light mine on the veg garden, and have a large open fire rather than a small contained one, that way the ground beneath does not get too baked.
I then rotavate a couple of days later and spread the pot ash around. I am only burning cardboard, and garden waste, branches and brambles. Don't burn anything else if you want to have the fire on the veg plot.
Start small and get a hot fire going then you can add 'green' branches and freshly cut brambles and the like, and rake the centre of the fire out so that you are covering a bigger area with hot ash. This prevents those intense hot ash fires, that can flare up again 2 or 3 days later if the wind catches the hot embers. Not something that you want to happen when you are not around keeping an eye on it.
Bonfire is the way to go to get rid of the prunings as well as providing potash for use afterwards. If you can burn and spread where you intend to plant potatoes - they especially love it, and it helps to kill off any residual nasties in the earth too.
Monika