Sally Petitt invites us on a tour of the tropical rainforest that’s right on our doorstep – and shows us how to recreate a little of it at home
I have always had a love of plants and gardens, but despite developing a familiarity with common garden plants as a youngster, nothing prepared me for my first encounters with tropical plants in the Palm House at Kew Gardens and the Tropical Houses at Cambridge University Botanic Garden.
The plants were alien to me, with enormous leaves and exotic blooms, all growing in warm, humid conditions. The blend of plants and tropical atmosphere created an intimate environment, and I was mesmerised by the lush foliage of the Asian taro (Colocasia esculenta), Colombian black-gold philodendron (Philodendron melanochrysum) and cocoa (Theobroma cacao). Seeing such diverse, lush vegetation was mind-blowing!
Where do tropical plants come from?
Originating from equatorial tropical rainforests of South America, Central America, Central Africa and South-East Asia, tropical plants have evolved to survive – and even thrive – in hot, humid conditions.
Tropical rainforests are made up of distinct plant layers, with the highest formed of slender-stemmed emergent trees that can reach 60m in height and tower above the rainforest. Beneath these sits a dense canopy of shorter trees up to 40m high, which create a rich habitat for wildlife including insects, reptiles, birds and mammals, but also block out light to cast dense shade across the rainforest floor.
An understorey of emerging saplings grows in cooler, shady conditions, and here seedlings may grow to plug the canopy where gaps appear. The lowest layer is dark, formed from decaying roots and rotting plant matter that form a moisture-retentive, humus-rich forest floor.
Together, these layers make up a complex habitat containing an array of highly adapted plants.
What is a tropical plant?
Tropical plants have evolved to suit shady, hot, humid conditions, in which annual rainfall is high and temperatures remain constant, usually between 26 and 29°C.
Plants show a range of adaptations to these conditions. The tallest emergent trees have buttress roots to help stabilise them, and tree trunks have smooth bark to help water flow freely to the roots. Waxy leaf surfaces and drip tips help channel water from leaves to prevent diseases. Woody vines (lianas) climb up in search of light and produce leaves and flowers in the rainforest canopy. Meanwhile, epiphytic plants are those that grow on host plants without taking nutrients from them. They gather moisture and nutrients from the air, rain and debris high in the forest canopy.
Exploring the Botanic Garden
The Tropical Houses at Cambridge University Botanic Garden grow a host of plants to inspire and create wonder.
The one that seems the most captivating to visitors is the jade vine (Strongylodon macrobotrys), an endangered bat-pollinated liana from the rainforests of the Philippines, which can reach 20m in height and produces pendent clusters of brilliant-turquoise, claw-like flowers along a central stem (racemes).
But other plants are equally beguiling. The titan arum (Amorphophallus titanum) is native to the rainforests of Sumatra. Unfortunately for us it is shy to bloom, but the flower has a ruffle-like collar, an erect thick spike of tiny flowers (spadix), grows up to 3m tall and emits a foul aroma to attract pollinating beetles. Perhaps not a horticultural beauty in everyone’s eyes, but certainly a botanical curiosity.
The calabash nutmeg (Monodora myristica) is a tree bearing speckled, lantern-like flowers up in the canopy of our Tropical Houses. Carludovica palmata (Panama hat palm) produces
large, pleated, fan-like leaves and is used to produce Panama hats. Members of the ginger family are at home in the tropical conditions here, and include the beehive ginger (Zingiber spectabile) and the torch ginger (Etlingera elatior). Several Dutchman’s pipes scramble through hosts such as the Darth Vader plant (Aristolochia salvadorensis) and the Aristolochia arborea.
Epiphytic plants can also be found here. Orchids in various sizes and colours from rainforests all across the globe nestle among the limbs and the foliage of host plants, and Spanish moss (Tillandsia usneoides) from the Americas hangs curtain-like from tree branches.
These are just a few of my favourites to whet the appetite, but there is always something to capture the imagination in these verdant houses.
Bringing the tropics home
Chances are, if you grow houseplants, you might already be growing a tropical plant. Asian moth orchids (Phalaenopsis species) adorn many kitchen windowsills, often with little horticultural attention, though lots more tropical orchids are available. Other popular tropical houseplants include the herringbone plant (Maranta leuconeura), the money tree (Pachira aquatica) and the fiddle-leaf fig (Ficus lyrata).
While we are fortunate at the Botanic Garden to be able to dedicate space solely to the cultivation of tropical plants, most of us don’t have that luxury in our homes. But there is still scope to grow elephant’s ears (Alocasia species and cultivars), parlour palm (Chamaedorea elegans) and many other tropical plants if you can provide the correct conditions. All will thrive in indirect light, a minimum temperature of 15°C, a humid environment provided through regular misting, well-drained compost and watering when the compost surface dries out. Many of these will also relish spending a summer in your garden, providing they aren’t in bright sunlight, which may scorch them.
If you’re encouraged to start growing tropical plants at home, now is a good time to dip your toe in, with garden centres and online retailers offering a plethora of plants to choose from. What better way to create a horticultural talking point in your home or garden than by adding a touch of the exotic such as a hardy banana (Musa basjoo) or a staghorn fern (Platycerium bifurcatum) to your plant collection? Many of these will grow outdoors in pots or in the garden over summer, but will need the protection of a greenhouse or conservatory in winter, or wrapping with fleece or hessian if left outside. Alternatively, try adding pineapple lilies (Eucomis pole-evansii) or arum lilies (Zantedeschia aethiopica) to a warm, sunny corner to bring a hint of tropical climes to your summer garden.
If you’re really not inclined to test your green fingers, a visit to the Botanic Garden is sure to provide a taste of the tropics.
Gardening jobs for August
1. Take a moment to relax and enjoy your garden.
2. Harvest crops such as lettuce, carrots, beetroot, beans, sweetcorn and onions.
3. Continue deadheading flowers in your borders and pots to encourage more flowers.
4. Take softwood cuttings of plants such as lavender, pelargoniums and penstemons.
5. Pinch the growing tips out of tomato plants in the garden to encourage fruit to develop and ripen.
6. Keep pots well watered.
Sally Petitt is head of horticulture at Cambridge University Botanic Garden