Polystyrene and hessian
If you have cold winters a very useful tip is to take a square of polystyrene (see above) say 5"x5"x1/2" or even much bigger if your winters are very severe. Cut a thin slot the width of the stem to the middle - remove the sliver & slide the polystyrene at ground level towards the plant until the stem is in the middle (it should be a tight fit) & then replace the sliver. Put this on before winter frosts & cover with a mulch of grass cuttings plus fleece or even better hessian protection for perhaps the first 3'-4'. This will make a big difference to the amount of cold the plants can take. Others use a large plastic water container as a cloche, slit right up the side with the bottom cut off. It can then be flexed out & over the plant to protect the base.
As an example of what good protection can achieve, a P. 'Adularia' which survived -5°c intact, died back to the ground overnight at -6°c. It then regrew from the well protected roots to my surprise. Pushing the plants too hard is not recommended though as it can take them the whole season to recover -if it recovers at all. The hardiest Passiflora are P. lutea, P. caerulea & some of it's hybrids & P. incarnata. All of them especially P. lutea & P. incarnata are prone to root rot & the dampness of the soil is more critical than temperature.