There are a number of passionflower species that are hardy and in recent years some very smart breeding work has been carried out with polyploid hybrids principally by Myles Irvine, a now retired gym owner, who has dedicated a lot of his time to creating hybrids that have big, colourful flowers and are, above all, hardy. They have recently become available at mainstream nurseries and will, I’m sure, mark the start of a new era in passionflower cultivation in the UK and around the world. The hardiest are P. caerulea, P. ‘Snow Queen’, P ‘Betty Myles Young’, P ‘Damsel’s Delight’ and P x colvilii. If growing them outside dig a nice deep hole, use micorrhizal fungi, water with a whole can once a week for first season and protect with fleece first winter. If your plant is very small then grow on under cover and plant out in year two. These precautions should help the less hardy ones settle, and give the really hardies a great start.