The beautful blue passionflower makes a spectacular addition to any garden and local butterflies (at least their larvre) will thank you!
Blue passionflower is a twining vine that can grow to 30 ft (9.1 m). The shiny leaves are usually palmately lobed with five parts, but they can have as few as three lobes or as many as nine. They are evergreen in tropical climates, but deciduous where winters are cool. The white and purple-blue flowers which appear in summer may be as large as 4 in (10.2 cm) across. They are followed by egg-size deep orange fruits from late summer through fall. Of the many cultivars of P. caerulea, the most widely known is 'Constance Elliott', which has fragrant white flowers and bright orange fruits. 'Regnellii' has exceptionally long corona filaments. 'Grandiflora' produces 8 in (20.3 cm) flowers. 'Chinensis' has pale blue flowers. Numerous crosses have been made between P. caerulea and P. racemosa, P. alata, and P. 'Amethyst'.
Blue passionflower is native to southern Brazil and Argentina.
Blue passionflower likes loose sandy or gravelly soils and does best planted in old brick rubble that retains heat during cold winter weather. Too much manure or compost will result in lush vegetative growth and poor flowering. This species will flower in a small pot, but it prefers plenty of root space and will do better in a roomy container. In Zone 8-9, the ideal location is against a warm south-facing old brick wall where an overhang prevents excessive drenching by heavy rains. Go light on fertilizer and water deeply, but infrequently; passionflowers should be encouraged to reach deep into the earth for water. When motivated to do so, they are capable of developing amazing root systems to sustain them through droughts and freezes. Passionflowers love high humidity, but they are subject to fungal diseases if they don't get good air circulation in the greenhouse. Blue passionflower does better overwintered in a cool greenhouse where it can go semi-dormant as opposed to in a hothouse where it will be tempted to put too much energy into weak off-season growth. In either case, it is important to keep the soil on the dry side in the winter. Blue passionflower may be wound around a hoop support to keep it within bounds so that it may be grown as a houseplant in a sunny south-facing window. Passifloras flower on new growth, so they may be pruned early in the growing season. It is best to cut some stems back nearly to the base, rather than just trim the tips. The terminal buds may be pinched out to encourage branching. Always keep some green foliage on the plant to keep the sap rising and encourage rapid regrowth. The roots may be weakened and become subject to fungal infection if too much top growth is removed at once. Don't try to train a passionflower to be too neat and compact; branches allowed to hang loose and droop a bit will be the ones most inclined to flower. Passionflowers are subject to a wide array of pests and diseases, but most of them have minimal impact on well grown plants. Butterfly larvae are the exception; caterpillars readily devour the foliage of healthy mature plants.
Light: Passionflowers like full sun and will scramble over trees and shrubs to get it. Moisture: Good drainage is essential.
Hardiness: USDA Zones 8 - 11. Blue passionflowers will regrow from deep roots after even severe freezes. They have been known to survive temperatures as low as 5ºF (-15ºC) when the ground was frozen over two feet deep! It is nevertheless important to keep the soil as warm as possible, especially in the winter greenhouse. Gardeners in zones 6 and 7 can grow the maypop passionflower (Passiflora incarnata).
Propagation: The best way to start passionflowers from seed is just to plant them fresh, pulp and all. It doesn't even matter if they get a bit moldy looking. They can be grown from seeds that have been dried and saved for a year or more, but germination of dried seed is slow and erratic. Seedlings from such sowings may not appear until the next year. To accelerate germination of dry seed, you can either soak them overnight in warm water or, better yet, mix them with the pulp and juice of a fresh passionfruit (any species will do) and let them marinate for 24 hours before planting the whole mess. Seedlings don't like to be crowded; they should be transplanted into their own pots as soon as possible. Be careful not to yield to the temptation to prop up the delicate stems by burying them more deeply than they originally grew; they will straighten up as they grow. Blue passionflower can easily be rooted from cuttings taken in the early spring or during the rainy season in tropical climates. Tip cuttings may be used if you can find nice fat sturdy ones from near the base of the plant; long skinny shoots that are reaching out for new territory will be more inclined to wilt and die than to take root. Nodal cuttings also work well, but you have to be careful to keep track of which end is up so you don't plant them upside-down. Rooting hormones, misters and soil heating cables will facilitate the rooting process.
Blue passionflower is typically grown in tropical gardens or greenhouses for the exotic beauty of its flowers. This species is widely cultivated for its value as a hardy parent plant in Passiflora breeding programs. The fruits are also edible. They aren't very tasty raw, but they have a vaguely blackberry-like flavor and can be substituted in blackberry pie recipes.
Passionflowers and butterflies come together as a package deal. Passiflora species are the exclusive hosts for numerous species of Heliconian butterflies. This group includes zebra longwings and fritillaries, but it is the large, solid orange julia butterfly that is most fond of the blue passionflower. If you are going to grow passionflowers, and not turn into a pesticide-wielding fiend, you simply have to develop a philosophical attitude about butterflies and experiment to find strategies for sharing the foliage with them. If you are clever (and lucky!), you can figure out how to pick off just enough caterpillars to have flowers, fruit, and butterflies too.