In the ground
Very large Passiflora plants may need little feeding if they have a large root system & are in fertile soil. Passion flowers generally will benefit from a high potassium fertilizer such as Vitax Q4, Levington's Azalea, Camellia & Rhododendron Food or a tomato feed. Organic alternatives include Maxicrop Organic Seaweed Extract, banana skins or comfrey. Too much nitrogen gives good leaf growth & few flowers but is better for smaller plants that you are wanting to get up to size. A stressed plant will flower better than one that is too comfortable. See a chart re plant nutrients here.
In UK always use clay pots with broken bits of pot & sharp sand or coarse grit in the foot to improve drainage. The clay surface is always negatively charged & the plants can more easily pick up the positive ions, such as calcium, potassium, iron & magnesium, off the clay surface. What this means in practice is that the plant's roots will grow right into the surface of the clay to pick up these ions. You will see this if you transplant a Passiflora out of a clay pot, the roots will have burrowed right into the pot surface. Where I live there is a very heavy layer of clay soil a few feet down, it is noticable that when the bigger Passiflora reach that layer they start to grow really well. Aaron Gilbert from California USA comments however that clay pots are unsuitable for use outdoors there, due to the pots constantly drying out in the heat. Nevertheless broken bits of clay pot can still be put in the base of plastic pots. Plants grown in pots outside over the warmer months are likely to have the nutrients washed out of the soil if there is a lot of rain.
Whether in the pot or the ground I feed them with high potassium feeds with micronutrients. A complete fertilizer provides all three primary macronutrients and some of the secondary and micronutrients. The label of the fertilizer will list numbers, for example 5-10-5 or 6-1-5, which refer to the percent by weight of the primary macronutrients. That is nitrogen, phosphorus & potassium. always in the order N-P-K. Note that confusingly K= potassium, from the Latin Kalium. Passiflora need a ratio with high potassium e.g. 1-1-2 up to 1-1-5. Too much nitrogen & they will grow vigorously but not flower. They also seem to need a fair amount of magnesium in my clay soil. Fertilizer options in UK include:-
Maxicrop Seaweed Organic Growth Stimulant, banana skins or comfrey leaves left to soak. Burying bananas in the pots or ground is also good.
Levington's Azalea, Camellia & Rhododendron Food, Toprose, Tomorite, Vitax Q4, Miracle Grow ( a bit low in Potassium but OK for those in the ground), Phostrogen, Arthur Bowers Rosefood, Wilkinson's Rose Food and Wilkinson's Tomato food which is a very cheap liquid feed & has an excellent formula.
When to put out
I have also observed that although the less hardy Passiflora will survive outside if put out a bit early, some will tend to go into a dormant state whereby it is very difficult or impossible to restart growth. Sometimes the growing tips can drop off followed by the rest of the foliage and they can end up as 'green sticks'- so keep them inside, particularly when young, to the end of April or even later. Foliar feeding with seaweed extract may help.
And when to take in
As leaf area increases on young plants check them daily, as if very vigorous they can quickly dry out especially towards the end of the summer. Bring them indoors by mid October latest as it is my experience that however well protected only the hardiest Passiflora in pots will survive winter outside. Plants in the ground cope much better with the cold. Tacsonia can be left out later to encourage flowering. Once indoors light & moving air is more important than heat & keep the plants far drier than in summer or the roots will rot. An oscillating fan to provide air movement is useful. Bottom heat is useful too if you can manage it.