Passion flowers are beautiful plants and it is almost unbelievable with their exotic look that many of the climbers can be grown outside in the UK.
It is a huge genus, including 576 species, some of which are shrubs and herbaceous plants, but in the UK it is the climbers, both tender and hardy, that are used ornamentally and for fruit.
You might believe it is their beauty that leads to the name, but in fact it is a reference to Christ on the cross - with the filaments resembling a crown of thorns, the three stigmas the nails, and the five anthers his wounds. In non-Christian cultures, the flowers have other meanings: a likeness to clocks in Israel and Japan and a symbol of Krishna in India.
The most popular species in the UK is Passiflora caerulea Award of Garden Merit (AGM) because of its hardiness combined with its elegant blue flowers. If given a sheltered position, it can tolerate temperatures as low as -15 degsC. In colder areas it will die back in the winter but reshoot in the spring.
Most of the hardier Passiflora have caerulea as a parent and there are some excellent varieties, including 'Eden' with sweetly-scented purple flowers and the white-flowered 'Constance Eliott' and 'White Lightning'.
There are also excellent varieties for the conservatory or greenhouse. P. quadrangularis AGM is one for the heated conservatory. Also known as the 'Giant Granadilla' because of its edible fruit, it has incredible fragrant flowers with red petals and white-and-violet banded filaments that curl at the ends. It requires a minimum temperature of 10 degsC. Another stunner is P. antioquiensis AGM, which only needs the protection of a frost-free conservatory or greenhouse. It produces rich, rosy-red flowers from June to October.
In a warm summer, the common P. caerulea will produce fruit, but while not poisonous, they are not tasty. P. edulis and P. quadrangularis do produce nice fruit if you have somewhere warm inside to grow it.
Passion flowers like a well-drained, not particularly fertile soil. If given a rich soil they tend to produce foliage at the expense of flowers. Add sharp sand when planting to improve drainage if necessary. They do best when their roots are restricted so do well in large planters and in the border - a barrier helps them to thrive. They prefer a sunny spot but have a tendency to get scorched if caught in the glaring summer sun. The ideal situation is a south-facing, sheltered spot close to a wall or fence.
Give them a wire or trellis support. They are self-clinging tendril climbers but can benefit from some training if you like a neat plant. Follow the RHS's guidelines and train it into a fan and carry out pruning just after flowering by shortening flowered tips and sideshoots to a couple of buds from the main fan framework. However, other passion-flower growers prefer the plant to scramble more naturally, and just prune tips in the spring after growth has restarted.
What the specialists say
- Kevin Bosustow, manager, Cross Common Nursery, Cornwall
"Passion flowers are one of our favourite plants in the nursery and we grow more than 100 varieties. The beautiful formation of the flowers and the different colours and look of the plants always make our customers stop and marvel.
"The most popular are the hardier 'White Mirror', 'Lavender Lady', 'Purple Haze' and 'Clear Sky'. We also stock several new varieties bred by Myles Irvine, with 'Betty Myles Young' and 'Justine Lyons' being the most popular.
"Our favourite Passiflora are a couple of the slightly more tender varieties. 'Empress Eugenie' is a beautiful hybrid with spectacular large pink, white and blue flowers. It is heavily scented and free-flowering, and can be grown outside in the summer and in a cold greenhouse over winter with a bit of frost protection.
"The spectacular 'Giant Granadilla' is also popular among customers with a heated conservatory. This vine produces red, violet and white flowers that are highly scented and sweet, edible fruit. It's a real show-stopper.
"Passiflora are best grown in a sunny position in a free-draining soil. Use a high-potash feed and, if indoors, grow away from the glass because they may suffer leaf scorch in hot weather. Keep the roots dry in winter to avoid rot."
- Steve Dance, office manager, Burncoose Nurseries, Cornwall
"My favourite has to be P. caerulea. This plant is surprising hardy to -5 degsC. It flowers all summer long and is followed by orange fruit. As long as it is planted in a sunny position, away from strong winds, with well-drained soil, it is an easy plant to grow.
"The other favourite is P. antioquiensis, although it's not hardy and will have to be grown in a frost-free conservatory/greenhouse."
- David Anderson, plant manager, Thompsons Garden Centre, Welling
"I have found that Passiflora can be a bit tricky to merchandise and keep looking their best. This is partly due to their rampant habit and their need for sunlight, while avoiding scorching. To eradicate this, we have a fenced area with cream-painted panels in partial shade. This we find reflects a lot of light and obviously increases temperature due to the heat absorbency of the wood.
"From a merchandising point of view, the cream background also reflects the vibrant tropical flowers and enhances their unique makeup. We space the plants out to allow for their growth habit. The last thing customers want is to pick up a plant that is entangled with another.
"Regular pruning is required to keep them in check and to remove flowers that have gone over, thus increasing their flower yield and attractiveness to the customer. Coloured point of sale and care information is ideally placed to give the customer confidence in their purchase."
- Chris Day, plant publicity manager, Buckingham Garden Centre & Nurseries, Buckingham
"Sales of passion flowers have been quite stagnant over the past couple of years due to the harsh December in 2010. We know that many of our customers lost their well-established plants and are reluctant to replant them. That said, sales of P. caerulea and 'Constance Eliott' have been buoyant, whereas 'White Lightning', 'Lavender Lady' and rubra have been slow or nonexistent.
"Customers who lose their plants have usually put them in the wrong location, either in heavy clay or not in a southor west-facing position."
Species and varieties
- P. 'Amethyst' AGM (H1) is a vigorous variety with deeply lobed leaves and large, pale-purple flowers and deeply trilobed leaves. The filaments of the flower are dark purple and the tepals gradually become reflexed. Orange fruit. Height: 4m.
- P. 'Betty Myles Young' is a large and spectacular near-evergreen climber bred by Myles Irvine in 2005. It has large soft-blue flowers with darker-blue filaments produced freely from March to December. Said to tolerate prolonged spells of at least -8 degsC. Height: 3.5m. Spread: 6-9m.
- P. 'Eden' = 'Hil Pas Eden' is a cross between P. caerulea and 'Amethyst' AGM (H1). Large, sweetly-scented purple flowers and deep-green foliage. Fully hardy.
- P. 'Marijke' is a striking tender passion flower, with pink petals and blue-and-white banded filaments. Best kept above 8 degsC.
- P. 'Purple Haze' is a caerulea hybrid with fragrant, pale to darkpurple flowers from summer to autumn. Given a sheltered spot in full sun it will fruit. Frost-hardy.
- P. 'White Lightning' is a highly floriferous, white-flowered Passiflora. A cross between P. caerulea and 'Constance Eliott', it has ivory-white sepals and petals with light-violet filaments darkening to purple at the base, and green and black stigmas. Large oval orange/yellow fruit.
- P. caerulea AGM (H3) is the most popular species, being hardy and producing beautiful creamy white flowers with purple-blue-zoned coronas. Sometimes followed by orange fruit in autumn. The foliage is lobed and glossy green.
- P. caerulea 'Clear Sky' (PBR) is a form with larger flowers than the species. They have bright-blue centres. Vigorous and hardy to -10 degsC.
- P. caerulea 'Constance Eliott' has unusual fragrant, large, ivory-white flowers with white or pale-blue spiky filaments from July to September, followed by orange-yellow fruit. The foliage is deeply lobed, dark green and glossy. Height: 10m. Spread: 2m.
- P. edulis produces arguably the best edible fruit. It has white flowers with purple-zoned white coronas, followed by purple or yellow fruit. Can be grown outside, although best protected inside. Top growth is likely to be killed off in harsh winters.
- P. x exoniensis AGM (H1) has attractive red flowers, produced from mid summer. It has large, lobed leaves. Hardy down to around -1 degsC.
- P. quadrangularis AGM (H1) is a tender Passiflora with a really exotic flower. It is sweetly scented and has red petals with white-and-violet banded filaments that curl at the ends. It requires a minimum of 10 degsC.
- P. x violacea 'Victoria' is one of the oldest known cultivars, with deep-pink flowers with dark-purple and white centres in summer and darkgreen evergreen leaves. Only hardy down to -5 degsC so best grown indoors.