I uused to pot mine up,in mid to late February,Steve,and they need to be kept inside,till any late frosts have gone,then planted out end of May/beginning of June..You could plant your tubs/baskets up earlier,if they are growing well, and keep them in your greenhouse,putting them outside if weather conditions are favourable..and bringing them back in,if not..

I'd advise starting them in pots or trays of compost in about mid March. It's terribly tempting to start them off too early, then they'll be too advanced when the time comes to plant them out or pot them up. They have no frost resistance once in growth so I agree with Bloomer, you can't put them out til mid May at the earliest, depending where you live.