Hybridization is a rather easy thing, which will be performed by any gardener with some patience and a few different species ; the more species you have the more hybrids you can create. There are many good reason for engaging into hybridization ; trying to create a colourful and hardy plant, a good tasting fruit, an early season and perfumed flowering vine etc...
Let us remind of a little morphology now ; as you all know, the reproductive apparatus of passionflowers is called an androgynophore, bearing both the male part (five anthers with pollen) and the female part (three stigmas leading to the ovary). The job will consist in taking pollen from plant A (the father) and bringing it onto the stigmas of plant B (the mother). This can be done using a small brush or by tearing off the anthers. Of course it will be easier if you grow species which flower at the same time of the year, otherwise you will have to harvest pollen, dry it and store it in the fridge until needed.
An important thing when performing hybridization is to castrate flower B so that it would not self-fertilize ; this has to be done as soon as the flower opens and is to be followed by tying an hermetic bag or packet around the flower to avoid unwanted pollen. The bag will be removed to allow the pollen hand deposit and then will be tied again until the flower closes back (usually the following morning). Of course the pollinated flower has to be clearly identified with some kind of marking, saying what pollen was used. Now, you have time to wander around the garden and enjoy your plants. After a few days the flower may fall off which means it simply did not work ! But it may also stay on and wilt while the ovary will start growing, obvious sign of victory which is perceptible only 48 hours after pollination (for a trained eye). You then have more time to enjoy your garden as it will take between 60 to 100 days before the fruit is fully ripe. Of course it may very well fall before ripening, abort, rot or get eaten by some predators...Say you are lucky, the fruit ripens, you pick it and open it. Well sometimes you get an empty fruit, sometimes you get non viable seeds. Let us stay positive, you got nice seeds, you sow them in your special secret sowing mix. They may just stay there and not germinate, or germinate and be so weak they don't make it. But they may also very well grow into a very healthy plant which will take over your greenhouse or back-garden. Still, this is not the end...some hybrids grow like mad plants, produce masses of nice healthy foliage, hundreds of flower buds all of which abort before opening (this happened to me when crossing P.coccinea with P.flavicarpa and P.alata, rather deceiving). It is best to start with hybrids that have been previously done so you know it is a possible cross and though you may get a result very similar to registered hybrids you may also produce some new plant. You should also consider the fact that hybridization will not be successful if you use species belonging to different sub-species (such as trying to cross a Decaloba with a Tacsonia).
In case you succeed in raising a nice new plant, you have now the right to name it, provided the name is not longer than 11 letters and has not been used before. The hybrid name will preferably be followed by the names of the parents, the mother (pollen receptor) always mentioned first, this is an international botanical rule, nothing to do with gallantry. Genus (Passiflora) and species (caerulea) are always written in italics, the genus name starts with a capital and species name with a lower case but hybrids names are not written in italic, start with a capital letter and are between comas. If you don't manage to get there, you sure will have gained a lot of patience in the process...
Let us see now a few more or less known hybrids ;
P. ‘Kewensis' (P. caerulea x raddiana). This one was named after the famous botanical garden at Kew, England. The rather thick leaves have two or three lobes and look like they are varnished, the flowers have delicious pink petals and sepals onto which the pure white corona filaments show off. Those flowers open only in the afternoon.
P. ‘Purple Haze' (P. caerulea x amethystina). A rather recent one produced by Cor Laurens, curator of the Dutch national passiflora collection. The deep green leaves have three lobes and the nicely perfumed flowers are a delicate pale purple with a showy corona banded deep purple/white/violet. The hardiness of its parentage should provide it with a good cold resistance.
P. ‘Empress Eugenie' (P. alata x caerulea). Created in 1824 by the William Masters nursery (Canterbury) this plant is now widely grown in Europe as well as in the USA and Australia. It has light green trilobed leaves and big scented flowers with thick petals. Those flowers are light pink with a corona made of five rows, banded white/deep purple/pink. It is also known and sold under other names such as ; P. x belotii, P. ‘Impératrice Eugénie', P. ‘Kaiserin Eugenia'. This is a recommended cross for the beginner, both species are rather easy to obtain from nurseries or seeds dealers and not too fussy to grow provided you supply enough heat to P. alata.
P. ‘Adularia' (P. sanguinolenta x citrina). Created by John Vanderplank (curator of the national British passiflora collection) this one has the particularity of being the first officially registered hybrid which parents belong to the Decaloba subgenus Xerogona section. The unusual shape of the leaves (deeply bilobed) and of the flowers (tubular with a reduced corona) are typical of this subgenus. It is a very free flowering plant, blooming all year round in subtropical climate. It may be grown as climber, ground cover or in suspended baskets. It will take temperatures down to 2°C.
P.'Cannelle' (P. coccinea x incarnata). Similar to plants obtained by the same cross (P. ‘Lady Margaret' or P. ‘Red Inca') this one was done by the author of this little article and named after my elder daughter...The beautiful bright red of P. coccinea has given birth here to a charming deep pink flower with corona filaments banded white and purple. The hardiness of P. incarnata allows this hybrid to take temperatures down to 7°C and to survive to 4°C for short periods.
Now, try your chance, good luck !