See a comprehensive list of minimum temperatures for both Passiflora species and hybrids by Alain Clavet, to whom many thanks, here. See also my species & hybrids A-Z lists for temperature range for some Passiflora. Figures may vary bewtween the two. Hardiness zones for Europe and USA are here. I have been growing P. caerulea, P. actinia & many Passiflora hybrids in heavy clay soil with winter temperatures down to -8°C/18°F for many years. For how to overwinter the hardier Passiflora see here and the more more delicate Tacsonia outside see here. Please note that the tips below refer specifically to growing in UK climate zone 8b, though they should also be of general use.
Note also that you can graft some Passiflora onto better rootstocks to help with hardiness, disease resistance or to prevent wilt in for example heat sensitive Tacsonia. There is a very good illustrated tutorial here by Ethan Nielsen of the Utah Passiflora Society.
The clay soil gives two problems, that young plants find it very hard to push their fine roots down through the clay & then over winter the poor drainage rots the roots. I have lost many plants in earlier years through poor preparation of the soil. Once established however the clay surface is always negatively charged & the plants can easily pick up the nutritious positive ions off the negatively charged clay surface. To maximize success:-
Put new plants outside in 5'' progressing to 10" pots - the large pots will encourage root growth. For more information see soil recipes. Generally all Passiflora love sun but some Tacsonia can overheat. Bring them inside for the first winter & plant them as below in the ground next year. This will give best survival rates.
Look for microclimates in your garden, even when it's very cold there will always be some bits warmer than others.Put your plants in close to a wall or fence if possible, in beds that are South facing, dry, well drained & ideally hard against brickwork with overhanging eaves. They like a sheltered location without too much wind. The combination of cold winds from the North or East and frosts over winter can cause severe damage. Add sharp sand if necessary to improve drainage & help plants establish roots. All Passiflora hate having wet cold roots. Train them up a diamond shaped wooden trellis or similar. If the soil is very high in clay it may be worth digging down the whole bed two spade depths or more & partly filling with sharp sand & rubble to improve drainage. Vanderplank also suggests raising the beds. I have found that most plants with small root balls struggle to establish in soil outdoors.
Even if you don't prepare the whole bed, dig individual plants in with plenty of sharp sand suitable for garden use so soil is perhaps 30% sand - you could also mix in a little compost. It might also be worth using Pearlite or Vermiculite to keep roots warm as well as to aid draining. Smaller plants can particularly struggle to establish themselves, being under attack by pests & diseases, with leaves looking a bit yellow or crinkly with feeble flowers. Generally be very sparing with watering & feeding to force the young plants to make deep roots quickly. Once the plants are established they should grow really well but if the winter is too wet you will lose some - resist watering next year until they are visibly growing.