The passifloras are known in the Temperate Zone as flowering plants, but the species commonly grown in the tropics are cultivated principally for their edible fruits. The most important one is the purple granadilla, P. edulis, known in Australia, where its culture is extensive, as passion-fruit.
The plant is a strong-growing, somewhat woody climber, with deeply three-lobed, serrate leaves. The flower, which is white and purple, is attractive but not so handsome as that of some other members of the genus. The fruit is oval, 2 to 3 inches long, deep purple in color when fully ripe. Within the brittle outer shell are numerous small seeds, each surrounded by yellowish, aromatic, juicy pulp, the flavor of which is rather acid.
From its native home in Brazil the purple granadilla has been carried to all parts of the world. It attains its greatest importance as an economic plant in Australia, but it is grown also in Ceylon, the Mediterranean region, in the southern United States, and elsewhere. The fruit is used for flavoring sherbets, for confectionery, for icing cakes, for "trifles," - a dish composed of sponge-cake, fruits, cream, and white of egg, - and for other table purposes. The pulp is also eaten directly from the fruit, after adding a little sugar, or it may be used to prepare a refreshing drink by beating it up in a glass of ice-water and adding a pinch of bicarbonate of soda.
The term passion-fruit, which is often applied to this species, confuses it with other members of the same genus, many of which are known by the same common name. In order to distinguish between these different species, it is well to adopt a different name for each. P. edulis is called lilikoi in Hawaii.
In California this fruit is easily grown, but it has not yet reached a position of importance in the markets; indeed, it is rarely seen in them, - a condition which contrasts strikingly with its prominence in Australia. It withstands light frosts, but when young is injured by temperatures more than one or two degrees below the freezing-point. While it bears abundantly in California, plants grown in Florida have in some instances failed to produce fruits. The reason for this is not definitely known, but it may be due to defective pollination. The pollination of this and other edible-fruited passifloras deserves investigation, for it is probable that the secret of many failures in their cultivation lies in this detail. Paul Knuth, in his "Handbook of Flower Pollination," states that the passifloras are protandrous (the anthers shedding their pollen before the stigmas are in condition to receive it) and adapted to cross-pollination by humble-bees and humming-birds. In describing the pollination of P. coerulea he says: "In the first stage of anthesis, a large insect (such as a humble-bee) when sucking the nectar, receives pollen on its back from the downwardly dehiscing anthers. In the second stage the styles have curved downwards to such an extent that the now receptive stigmas are lower than the empty anthers. It follows that older flowers are fertilized by pollen from younger ones."
The passifloras are easily propagated by seeds or cuttings, the latter method being preferable in most cases. Seeds should be removed from the fruit, dried in a shady place, and planted in flats of light soil. They do not germinate quickly, but the young plants are easily raised, and may be set out in the open ground when six months to a year old. Cuttings should be taken from fairly well-matured shoots, and should be about 6 inches in length. They are easily rooted in sand, no bottom-heat being required. Cuttings of the purple granadilla will often fruit in pots at the age of two years.
Directions for the commercial cultivation of this fruit, based on American experience, cannot be given, since no commercial plantings, with the exception of a few small ones on an experimental scale, have yet been made in this country. The following extracts are taken from an article by W. J. Allen in the Agricultural Gazette of New South Wales for November 2,1912 :
"Although this fruit is not grown so extensively as it should be throughout the many districts on the coast where it will do well, it nevertheless plays quite an important part in some of the young citrus orchards in the County of Cumberland, on the Penang Mountain, and around the Gosford district, where it is frequently planted among the trees. As it begins to bear very early, growers are enabled to make considerably more from this crop than pays for the working of the orchard until the young trees begin to produce crops of fruit, which they invariably do after the third or fourth year.
"Generally speaking, the vines are most productive before having attained to four or five years of age. After that period they begin to lose vigor and gradually die out, or cease to be very profitable, and are in consequence removed.
"The passion-vine is found to thrive well on many classes of soil, - some so poor that one is led to wonder how anything could profitably be grown on it. On the light sandstone and poorer coastal country there is no other fruit which will give the same return as this, and with proper working and heavy manuring, it is wonderful the amount of fruit that can be taken from an acre of such vines. The area planted is comparatively small, and, in consequence, the fruit usually commands very high prices. As an addition to a fruit salad there is no flavor that can surpass it, and when eaten with cream it rivals the most delicious of strawberries. If this fruit were known in Great Britain and America, I venture to say that there would be an unlimited demand for it, if once we were successful in landing it in those countries in large quantities.
"In selecting a site for the planting of a vineyard, one of the important points to keep in view is to avoid a district or situation where frosts are at all severe or of frequent occurrence in the winter. There is one thing which this vine will not stand, and that is severe frosts; and the Easter, winter, and spring crops are those which are in most demand. During the summer time there is a superabundance of other fruits, and hence the consumption of the passion-fruit is not so great; from Easter until Christmas time there is a splendid market for all well grown fruit. It is during part of this time that we have our coldest weather, and a severe frost or two would destroy the whole crop, and in all probability kill the vine back to the root.