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There are two P. edulis species. Passiflora edulis Sims f. edulis, which has smaller sweeter purple fruit, and Passiflora edulis f. flavicarpa syn. Passiflora verrucifera*, which has larger acidic yellow fruit.
Shown above Passiflora edulis ‘Nancy Garrison’. A hardy selection of Passiflora edulis but not hardy by UK standards and lost in a very mild winter. While it can be fun growing P. edulis from seed from fruit you have bought, in UK even with a heated greenhouse you will struggle both to set fruit & produce fully ripened fruit. Better to just enjoy the fruit that you buy and grow decorative Passiflora like the Riverside Passiflora.
Wise words from John Edmiston of Tropical Britain (online specialists in British Grown hardy exotic plants) re the difficulties of growing P. edulis in the UK:-
‘Native to southern Brazil, Paraguay and northern Argentina, Passiflora edulis, the Edible Passionfruit is something of a challenge in the UK even for experienced Passiflora growers. It is not hardy and must be kept frost-free. A plant that is so easy to grow in the warm humid temperate climate of – say New Zealand – becomes an infuriating prima donna in the cool temperate climate of the UK. When it is young and still relatively small it can be overwintered in a well-lit kitchen or bathroom although care should be taken to make sure it is not over-watered at this time – in fact, any watering at all during the dark winter months is risky and the plants should be given a good last water in November then kept dry-ish until the early Spring. Once Spring growth recommences it is usually vigourous and the vines need good support in a well-lit but not overly dry environment. Sun-rooms and conservatories are often too dry and you may need to move this temperamental Goldilocks plant around until you find the right spot for it. This of course is not exactly practical for a climbing plant so in the early first years a largish pot with its own small trellis or support canes is often a good solution. Sun and humidity are the ideal combination. When in full growth, water and feed it well. Regular applications of seaweed and liquid potassium-rich fertiliser will steer it towards the Holy Grail: flowers and perhaps…maybe… one day… fruit!”
To differentiate between the two species see the table below. Various selections of them, or hybrids between them, are the most widely grown Passiflora worldwide commercially with sales of both edible fruit and juice. Read more about commercially grown Passionfruit.
In the Passiflora Facebook Group we are often asked to ID which selection of P. edulis people have. There is rarely a correct answer. To explain, if you buy a decorative hybrid (meaning grown for its flowers not the fruit) like Passiflora ‘Snow Queen’ as an example, all the ‘Snow Queen’ plants are propagated from cuttings of the original plant. Now if someone names a selection P. edulis ‘Red Rover’ as it is particularly tasty, then in theory all ‘Red Rover’ plants should be grown from cuttings of that plant. In practice however people sell seed from P. ‘Red Rover’ fruit. Whoever buys it will probably label it as P. ‘Red Rover’ and possibly sell or share some of their plants or seeds as ‘Red Rover’, when they aren’t. So unless you can buy from the original source you won’t necessarily be disappointed by the fruit, but you won’t quite have what you think you have.
Hybrids between the two P. edulis species of course have characteristics of each, but generally flower during one long season. A noted USA hybrid is Passiflora edulis ‘Frederick’ by Patrick Jesse John Worley. It is only partially self fertile though and may need hand pollination to set fruit. In UK fruit of a Passiflora edulis selection, P. ‘Ester’, which originated in South Africa in 1995 is excellent and widely available.
* There is considerable confusion re the correct names for the above.
John MacDougal, taxonomist, has kindly provided a summary:-
So, the “correct names” (according to the official Rules of nomenclature) is the easy part, but which of those names to use (in other words, a different kind of correctness) is the problem.
There are officially correct names that must be used whether one considers the two Passifloras to be minor variations or completely different species. These names are all synonyms of each other and all are correct at the rank (level) indicated.
The problem comes in deciding at what rank or level you think they are different – are they cultivars or species? Different people think different things, so there is no “correct” answer, just opinions, some based on evidence. Everyone knows that recognizing them as simply “FORMS” is probably wrong (not based on accepted evidence), but for now it is the practical thing to do. Say “form” and then wink, and roll your eyes.
The question of “what is a species?” or “how do we define species?” comes into play, and then what is the evidence in this case of Passiflora edulis? The scientists are still investigating it, new evidence continues to develop, and I personally have a slightly different opinion than some others, so for now I personally just use “forma flavicarpa” in the interest of promoting understanding, while leaving it open for future revision.
In 2008, Luis Bernacci published an expert paper and summary of the evidence about this, and suggested that it should be considered and cited as a cultivar, “P. edulis ‘Flavicarpa’”. (See Luís Carlos Bernacci et al. 2008. Passiflora edulis Sims: The correct taxonomic way to cite the yellow passion fruit (and of other colors).Rev. Bras. Frutic., Jaboticabal – SP, v. 30, n. 2, p. 566-576, Junho 2008.) Read here. His conclusion is based on much evidence but it has not been widely followed yet.
I personally find the article difficult to read (even after six complete times). I think that I do understand and can agree with his decision to call it a cultivar ‘Flavicarpa’ under his criteria, but I also can envision that using a different definition of species (there are several that scientists use), it still would be reasonable to conclude that it be recognized at the variety or even species level. Bernacci seems too narrowly focused only on the color of the fruit. But no one has laid out the species/variety-level argument in modern times using modern genetic or morphological evidence.
And then we would have to say “P. verrucifera” or “var. verrucifera” and not only is that is very confusing to everyone (think of all the books and articles already written), but because this crop has been hybridized and cultivated so much, the reality of “what is a species here” is muddled and difficult to define, and becoming harder by the year. Then this all starts to become something to only argue about, not something that really matters. The use of cultivar names looks like a good idea, a very good solution for the future in this case. Think about beans or tomatoes or cherries… species or cultivars? Names are mostly for communication, so practically speaking I think for now we are stuck with the name flavicarpa, either as a form or cultivar.
The story is not completely over, however, and I look forward to additional scientific studies that might shed light on the origin of the species and its relatives.
John MacDougal July 2020