Exotic plants appeal to the eye with their sculptural forms and vibrant colours. But how do you protect them from the worst of the British climate? Val Bourne has some tips
They may serve as a nostalgic reminder of a sun-soaked Mediterranean idyll or a spell in the spice island of Grenada. Or they may bring back memories of the rugged South African cape. Whatever the reason, more and more British gardeners are being seduced by exotic plants. Now that our winters are so much warmer, we are as likely to buy an olive tree as an apple. Managing these exotic plants in British gardens can be a challenge, though, as they enjoy a range of differing conditions. Here's how to do it.
Adding a Mediterranean touch
Areas of the world with a Mediterranean climate enjoy dry, sunny summers and warm, wet winters. The plants have their growth spurt in frost-free winters and in summer they flower and fruit. The leaves of most Mediterranean flora tend to be soft silver or sage green and many have aromatic, oily foliage, which helps the plant to survive in strong sunlight. Others have very tough, leathery leaves. We already grow hardy Mediterranean plants such as lavender, sage and thyme. But now we are also trying to grow tender plants like the olive. These long-lived small trees are capable of recreating the Mediterranean landscape at a glance with their dusky, leathery foliage and sparse canopy.
But olives are not fully hardy. They can stand the odd cold night, but they need winter temperatures to stay above freezing consistently and they also need excellent drainage. You may be able to create your own suitable microclimate if you have a sunny, frost-free site close to buildings. But you will probably still have to keep your olive trees in containers with a free-draining compost.
In winter, make sure the pot is standing on feet to allow any surplus water to drain away and, given a very sheltered site, you should be all right. But most of us will have to move our trees into a dry position, either up against a sheltered wall or inside a frost-free greenhouse (see below for more seasonal tips). You can only get away with growing an olive tree in the ground if you live in a very sheltered position. The largest specimen olive, for instance, grows at the Chelsea Physic Garden, close to the River Thames. But the site of this apothecary's garden (founded in 1673) was specifically chosen for its warm microclimate. Even here, olives only ripen in exceptional summers.
If your garden is too exposed and cold for you, let alone an olive, there is an alternative: a hardy shrub with a similar look. It's a shrubby umbellifer called Bupleurum fruticosum and the leathery leaves support lime-green umbels of tiny flowers in August.
Rugged and macho
There are other exotic-looking plants that are much tougher than the gentle olive and they include the handsome Chusan palm (Trachycarpus fortunei) from China. It is hardy to -17ºC and the erect fan-shaped leaves split into dozens of sections as they rise up from the fibrous brown trunk. This "Venetian blind" palm needs its own space to shine, as do all bold plants with attitude. But its corrugated leaves brown in strong winds, so a sheltered position is vital—otherwise your plant will lose its leaves, look miserable and then die.
The best time to plant a Chusan palm, or any other frost-tender plant, is early summer and it's a good idea to feed the plant at the same time. Add well-rotted manure to the hole, or sprinkle pelleted chicken manure round the base. Surrounding the plant with a gravel mulch will aid drainage in winter and conserve moisture in summer.
There is a less hardy Mediterranean palm (Chamaerops humilis) that will take temperatures as low as -10ºC, but only for short periods. This more delicate-looking palm has finely divided, silver-green leaflets that split at the tips, while the trunk is pineapple-like in texture. You often see Chamaerops humilis thriving in seaside resorts where cold weather is a rarity. It tolerates windy conditions well and is generally easier.
Perhaps the most tempting plant on offer is the soft tree fern (Dicksonia antarctica) found naturally in a belt from Tasmania to Queensland. This fantastic fern has large, exuberant fronds, radiating from a stout brown trunk in Jurassic Park style. Garden centres often sell them as "logs" with no leaves at all, but they should have a certificate to say they have been collected with permission. Dicksonia antarctica is hardy to -10ºC (for short periods only), but needs fertile soil and shelter from strong winds. It also demands a position away from full sun, doing best where the air tends to be humid. An overhead canopy of tree branches definitely helps.
The damper, western half of Britain is probably more ideal for tree ferns and there is at least one garden in Hereford where they thrive en masse. Some gardeners cleverly compensate for dry air by setting up a mist system to keep the leaves and trunk moist: this seems to work well and adds a steamy atmosphere, too. If you do get it right, this majestic fern will grow quickly. You can wrap it up with fleece in late autumn or pack straw into the crown of the trunk.
A hardy backbone
The best way of using tender plants like these is to support them with a hardy backbone of plants that share the same exotic look. Phormiums come from New Zealand and are very variable in size and leaf colour. But they all have upright sword-shaped leaves and occasional beak-like flower heads. They make dramatic foliage plants in winter with black, striped or green leaves. "Dazzler" is well named, for this metre-high phormium has bronze leaves striped with red, orange and pink; "Platt's Black" has dark purple foliage.
Phormiums need a sunny position to keep their leaves vibrant, but they also need good drainage and a sheltered position. Give them a good tidy up every spring to keep them looking pristine. You could also use hardy ferns and larger euphorbias.
If you've decided to go exotic you may well need a shelter belt. Some bamboos do it well and the canes can look fantastic in winter light when the sun slants through them. Many keep their foliage in winter too and these leaves cast their own magic-lantern pattern on the ground as the sun moves around to the accompaniment of a gentle, grassy rustle. The tall and upright Pseudosasa japonica has thumb-thick, pale-green canes which are regularly banded with pale-straw-coloured sheaths. These papery sheaths are cast off throughout the year and should be left to decompose on the soil surface to provide slow-release nourishment. Old canes should be thinned out every year. They are easy to spot, they look duller than the others. P. japonica can attain a height of 4m and spread to 3m in width within 10 years—so it's not for the faint hearted, or the gardener with little space.
One of the most popular trends uses strong colour with lavish foliage and these plants demand warmth and water from late spring until late summer, as they thrive in tropical climates where daily rainfall is high. The banana, an upright exotic with paddle-shaped leaves, is one of the key plants. The hardiest is the Japanese banana (Musa basjoo) and these seem to survive in the well drained ground in sheltered locations as they are hardy to -6ºC. Mulching with a thick layer of pale gravel helps, as it throws the heat back up from the ground and acts as a storage radiator too. You can also protect your plants with chopped bracken or another dry material kept in place by a circle of sticks.
There are more exotic bananas with deep red leaves (like Ensete ventricosum "Montbeliardii") but you will need to lift and place these in a heated greenhouse during winter. They can be bedded back out in late spring or early summer, after fear of frost has passed, and it's possible to keep them in their pots and sink them into the ground. Cannas, those highly colourful plants with large leaves, will need treating in the same way. All will need warmth and water in summer to perform well.
Mix in some equally colourful, frost-hardy plants such as dahlia "David Howard" and the red ginger lily (Hedychium coccineum "Tara"). These can stay in place in well drained soil throughout the year. Or add some hardy perennials in sunny colours. These could include crocosmias, heleniums and kniphofias in suitably tropical tones.
So, whether you want a splash of tropical colour, architectural plant sculpture, or a Mediterranean hillside, there's a world of hardy plants on offer. Be adventurous and push back the frontiers.
Exotic gardens to visit
Henstead Exotic Garden Suffolk: www.hensteadexoticgarden.co.uk
The Exotic Garden Norwich: www.exoticgarden.com
Cotswold Wildlife Park Burford, Oxfordshire: www.cotswoldwildlifepark.co.uk
Hardy Bamboos: Taming the Dragon by Paul Whittaker (Timber)
Encyclopedia of Exotic Plants for Temperate Climates by Will Giles (Timber)