Tropical Nursery blog

The Great Glass Houses and Tropical Nursery at Kew are home to some world-famous edible plants. You might see familiar fruits such as bananas and papayas, or find your daily caffeine boost in its natural, unprocessed state – as dark coffee beans or the leaves of the tea plant, Camellia sinensis. But not many people know that Kew is also home to the oldest cultivated crop in the world. It is a member of the Araceae family, otherwise known as aroids - a diverse group whose edible species include one that gives ice-cream its vanilla flavour (despite being named after a cheese), and the intriguingly named 'fruit of the devil'.

The oldest aroid

The oldest cultivated crop in the family is Colocasia esculenta, commonly known as taro.

Taro (Colocasia esculenta) in Kew's Princess of Wales Conservatory

It originated in tropical Asia and has been cultivated for more than 10,000 years. Today, taro is naturalised throughout the humid tropics and subtropics, and around 400 million people include it in their diet. With the crop from one square kilometre capable of feeding 5,000 people for a year, its long-term popularity comes as no surprise. The tubers are a good source of carbohydrate (13-29% of the cooked weight depending on cultivar) and are richer in protein than most other major starch crops. The leaves of certain cultivars are eaten like spinach and are an excellent source of vitamin A, vitamin C and minerals. The tubers can be ground into a flour to make bread and other bakery products, or sliced and fried to make crisps. Like most aroids, though, the presence of toxins means that all parts must be cooked to render them edible, and in general they are boiled or roasted. Taro’s natural habitat is tropical humid forest and in the tropical nursery at Kew it is grown in humus-rich compost at around 22°C and 70% humidity. As it often grows along stream and pond edges, taro does tend to be rather thirsty.

The second most important edible aroid is the genus Xanthosoma, commonly known as tannia or cocoyam, and again both the tubers and leaves are eaten.

Tannia (Xanthosoma sagittifolium). Note the purple stems

Widely cultivated in tropical America since pre-Columbian times, Xanthosoma spread to Africa and Asia and is still in the course of migration. X. caracu is now commonly grown in Florida in response to the growing population of immigrants from the Caribbean. The main cultivated species is X. sagittifolium, which can be seen in the Palm House showing its striking purple stems and purple veins beneath large, light green leaves.

A giant aroid

Where Colocasia might reach one metre and Xanthosoma two metres in height, Alocasia macrorrhizos, commonly known as giant taro, can grow up to four metres tall and have leaves one metre long.

Giant taro (Alocasia macrorrhizos)

This giant aroid can be seen in the Princess of Wales Conservatory. Its corms are coarse and acrid and generally it is the thick, starch-filled stems that are eaten. In Malaysia it is used in ‘curry santan’; it is first soaked in water with betel nut chips or slaked lime to remove the acridity, and then boiled in coconut cream with spices and dried prawns. When correctly prepared and cooked the stem also yields white flour.

Other giant aroids can be found in the Amorphophallus genus. A. paeoniifolius - common name the elephant yam - has tubers that can reach 25 kg in weight. It is cultivated in tropical Asia and India and is an important standby if the rice crop fails. Yielding slightly smaller tubers is A. konjac, the konjac plant, which weighs in at a mere 10 kg. This species has been cultivated in China for 2,000 years and is used to make a variety of food products. It is used not only as a root vegetable, but also to make 'konnyaku' (a gelatinous ingredient in many Japanese foods), flour (from which noodles and cakes are prepared) and jellies. The tubers are also the commercial source of mannose, a substance used in diabetic foods. These impressive plants appear treelike but are in fact composed of just one leaf. They are also seasonally dormant and so may be hard to spot in the public glasshouses but, should you visit on a tropical nursery open day, an array of Amorphophallus may be seen in zone 8.

The swamp aroid

There is even an edible aroid that grows where nothing else will – in stagnant, brackish swamps. Cyrtosperma merkusii, the swamp taro, is cultivated in Southeast Asia and the Pacific. It needs the longest growing time of all the edible aroid root crops, on average requiring 3-6 years before harvesting, but goes unchallenged by quicker growing crops as they won’t tolerate swampy conditions. On coral atolls in the Pacific it was successfully grown by being lowered in compost-filled baskets into pits, often more than one metre deep, dug through the coral limestone or sand to the water table. As easier, imported foods have become available in the region many pits are now unused.

The Swiss cheese plant (Monstera deliciosa)

Monstera deliciosa, named the Swiss cheese plant because of the holes in its leaves, is actually used to flavour ice cream. It is cultivated in Mexico and Central America and the flowers and fruits only appear once the plant has reached maturity - at around 22 metres in height with leaves up to 90 cm long and 75 cm wide. The fruits are protected by an irritant outer layer, but when the berries are ripe the protective cover breaks away. They have the scent and taste of pineapple and banana combined.

The 'fruit of the devil'

And the 'fruit of the devil'? This is Montrichardia arborescens, cultivated in South America for its starchy tubers, and acquired its nickname due to its irresistible fruiting spadices which produce large infructescences, each containing about 80 edible yellow fruits. At Kew we're growing a juvenile specimen in our Tropical Nursery.

Kew's Montrichardia arborescens in the Tropical Nursery. Note the tree-like stems which give the plant its Latin name. We're hoping this specimen will fruit later this year. (Image: Simon Port, RBG Kew)

Aroids have long been a food staple to many populations and even though lifestyles across the world may be changing there’s still plenty of potential for their use. As just this small selection has shown, aroids can provide high yielding nutritious crops, substances for specialist diets and gels for the food industry. With further research and development other beneficial applications may well be discovered. Perhaps one day we Brits may even find the 'fruit of the devil' turning up in our local greengrocers! 

- Louisa -

Related links

• World Checklist of Selected Plant Families
• Aroid pictures
• The International Aroid Society   

That time of year is upon us again... it’s Kew’s Tropical Extravaganza! This year’s festival is inspired by the forces of nature – earth, fire, air and water - and I was thrilled to be given the opportunity to help transform the Princess of Wales Conservatory into an explosion of colour in celebration of the elements.

Orchids really are the stars of the show and this year’s selection won’t disappoint. Bright yellow Oncidiums and vibrant Phalaenopsis decorate the pillars, stunning blue Vandas form a tropical arch and elegant Dendrobiums festoon the giant hanging baskets.

Volunteers behind-the-scenes working on the plants for the displays

Decorating the pillars

My first task of the week, and one of the largest, was to help decorate the pillars. Behind the scenes, an army of volunteers prepared each pot for mounting. They drilled two holes in the back of the pot, through which a wire hook was inserted, and then they attached a wad of moss to the front of the pot with a rubber band to camouflage the container in the display. After watering, the pots were passed to those of us perched at the top of a ladder where we could begin to fix them to the pillar. Each pillar was already wrapped in coir and covered with a strong plastic mesh to which we attached the pots using a cable tie through the mesh and the wire hook of the pot. The cable tie was tightened to hold the pot in position and we worked down the pillar attaching each individual plant in much the same way as a patchwork quilt is assembled.

The team dress the pillars from ladders and reach the top of the tallest pillar from the cherry picker

In at the deep end

The large ‘twister’ in the lily pond was designed to give visitors a sense of the movement of air and wind, the element used by many plants to disperse their seeds. Blood red Begonias and Guzmanias, yellow Anthuriums and orange Kalanchoes were mixed with silver foliage plants such as Tillandsia usneoides to represent air and cool wind. I donned a pair of (leaky) waders, filled a floating wheelbarrow with plants and compost and lowered myself into the pond. This was a fun job! I love planting up any kind of display but doing it waist deep in water just made it cooler. The plants are only going to be in this display for the short term (the duration of the festival) and won’t have much time to grow and fill out a large space, so we planted them unusually close together. The result – a display that looks full and appears well established. Stunning!

Working on the ‘Twister’ display in the lily pond

Every year I marvel at the finished product and I’m really happy that this year I got to participate. It was busy, physical, hot, challenging - and one of the most memorable and fun weeks I have spent in this job. I can’t wait to take my friends and family around the festival to point out all our hard work! I’m sure they will appreciate it, just as I did when I could only guess at the imagination that went in to it.

Enjoy...

- Kirsty -
 

Find out more... 

• Visit Kew’s Tropical Extravaganza
• Find out more about the plants featuring in the displays

The titan arum is not the sort of plant you can be neutral about. It is curious, weird, rather obscene, revolting and even terrifying. Scientifically known as Amorphophallus titanum, it is by no means easy to tell that it belongs to the Araceae family, which includes the ‘Swiss Cheese Plant’ (Monstera deliciosa) with its unmistakeable foliage, the virginal ‘White Arum’ or ‘Calla Lilly’ (Zantedeschia aethiopica), the ‘Flamingo Flower’ (Anthurium andraeanum), and the ‘European Arum’ or ‘Lords-and-Ladies’ (Arum maculatum and A. italicum).

The flower of Amorphophallus titanum is indeed very odd. It consists of a single bract (modified leaf) called a spathe, which provides cover for a central flower-bearing protuberance, the spadix. This floral peculiarity is technically an inflorescence, not a flower; the real flowers being tiny and protected at the base of the spathe. The shape of this inflorescence has inescapable sexual connotations beyond the fact that it is the plant’s reproductive apparatus.

Inflorescence of the titan arum

How we grow them at Kew

The titan arum is a tuberous species that characteristically has dormant periods when leaves are not present, and these normally correspond to the dry season (or winter) in the habitats where it grows. At Kew, we grow them in huge containers that hold tubers which can reach up to 90 kg. The leaf is a marvel in itself and at first glance resembles a sapling tree. It is patterned with irregular pale green blotches, which at a distance render it indistinguishable from the moss and lichen encrusted on the trunks of surrounding trees. At the apex it divides into three, and then again into several more divisions, making an umbrella of leaflets about 7 m in diameter.

 

Titan arum in the Tropical Nursery at Kew

In the Tropical Nursery at Kew, Amorphophallus titanum is not very difficult to grow, provided a number of conditions are met. These are roughly: a rich soil and trace elements, the use of additional fertiliser, shading against direct sunlight, a minimum temperature of 22⁰C during the day and 19⁰C during the night, a well defined resting period, and strict control of pests.

The plant must be kept in a well drained soil. If the soil becomes too dense (because of excess water or too much loam) the roots will rot. The health of a tuber is finally measured by its increase in size, and a tuber that is doing well may triple its weight in a single season.
 

Charles Shelton (on the left) and Marcelo Sellaro weighing the titan arum tuber
 

In cultivation at Kew, A. titanum shows a strong tendency to take dormant periods, usually twice during the growth cycle. When a leaf dies down after a regular growing season, the tuber will invariably take a rest period. Following this another leaf may emerge. When the plant is mature enough to flower, the resting period is usually considerably shorter and may take no longer than a month, or sometimes even less.

Rare flowerings of the titan arum

Pollination of different clones in cultivation rarely occurs. In 2009 two specimens flowered at Kew. This allowed cross-pollination using both fresh and frozen pollen grains, which were put on the stigmas on the first day of flowering. When the female flowers are receptive, the stigmas are very sticky and ready to receive pollen. The yellow rind of the appendix emits an overpowering, nauseating stench that gives the plant its Indonesian name of ‘bunga bangkai’, meaning “corpse flower”. The smell is perhaps the most powerful and disgusting odour produced by any plant. It was described by Sir Joseph Hooker (1891) as a mixture of rotting fish and burnt sugar, which turns your stomach over and makes your eyes run.

The stalk formed about 100 fresh berries, which turned into orange reddish fruits when ripe. After removal of the pulp the seeds were sown in a mixture of ½ part coir, ½ part sand, and watered daily. Seedlings were established after one month. Any seeds stored dry lose viability very quickly because the seed coat is rather thin and does not protect the embryo from desiccation (drying out).

Any success in propagating A. titanum, by any means, is a triumph of hope over adversity. For almost a century it was considered near impossible, but now the corner has been turned. Not only have viable seeds been produced in cultivation, but young plantlets have also been raised vegetatively by leaf cuttings.

If the features outlined in this text have captured your imagination, I hope that seeing a specimen growing in the Princess of Wales Conservatory will persuade you to explore the world of Aroids further, and serve as a general introduction to this impressive family of plants.

- Marcelo -
 

Find out more...

• More from Kew about the titan arum (Amorphophallus titanum)
• International Aroid Society
• Fairchild virtual herbarium
• Recommended reading – ‘Aroids: Plants of the Arum Family’ by Deni Brown

Recently a request came through our facebook page for some information on the cultivation of bromeliads, so I ventured forth with our resident expert, Marcelo Sellaro to look at this unusual exotic family and it’s cultivation at Kew.

Behind the scenes in Kew's Tropical Nursery

Marcelo has been growing bromeliads since he was a young boy growing up in the city of Sao Paulo, at the age of 18 he took the opportunity to study at Sau Paulo University and majored in agronomy. As he studied he became fascinated with a subject that became his first horticultural love, bromeliads!

After university, Marcelo decided to broaden his knowledge and he came to Kew to study cultivation techniques as an intern. That was 11 years ago, he never left. Kew has a burgeoning collection of epiphytic bromeliads, which is housed in one of the larger zones on the west side of the Tropical Nursery complex. Most of the house is occupied by large A-frames on which are hanging hundreds of bark pieces, each one a home to an accession of this diverse family. The majority come from the South Atlantic rainforest, an area under serious threat due to deforestation for slash and burn agricuture and logging concessions. Our collections come from a dedicated network of botanical gardens and amateur enthusiasts, such as Elton Leme, a man who has devoted more than 20 years to the rescue and rehabilitation of many unique species from this area. Kew serves as an ex-situ back up for the family collections.

Marcelo Sellaro looking after bromeliads in the Tropical Nursery

Almost exclusively from the New World, the general public best know this family for one of our favorite tropical fruits; 'the pineapple'. It has a diverse range of genera, from the 'air plants' in the genus Tillandsia to the carnivorous bromeliad that lives atop the Tepuis of northern Venezuela, Brocchinia reducta. Many species of bromeliads live in trees, high up in the canopy, so Marcelo mounts his specimens onto Cork (Quercus suber) bark, in much the same way as we mount orchids (see previous blog, ‘Mounting Orchids onto Bark’). Propagation of individuals is undertaken by removal of ‘pups’ (offshoots) once they are in their second year. The base of the pups are dried a little to prevent rot and then rooted in a coir based compost with added bark. After a year of growing happily in a pot, the plant can then be established on a bark. From seed, Marcelo broadcast sows onto a coir/sand mix, and germinates them in a closed cabinet which gives a very hot humid environment.

Watering of bromeliads is easy in a moist greenhouse with high humidity. Marcelo simply mists the moss around the base of the plants daily during the summer, and keeps the distinctive tanks that form at the base of the leaves topped up with water. Even in winter the plants are watered daily to counteract the drying effects of the heating pipes.

I asked Marcelo what he recommends for home cultivation, he said "try Vriesea splendens hybrids ('flaming sword plant') and Guzmania lingulata ('crimson star'), as these are tougher and can contend with the difficulties of lower humidity and light of a household environment. These plants can be bought in pots and the soil should be watered sparingly, but they really appreciate the misting of their leaves and don't forget to keep those tanks watered! The majority of bromeliads are plants with sympodial growth, so, once the plant has flowered and the bracts start to fade, cut it down to the ground to allow the developing pups space to develop."

Parts of Kew's bromeliad collection is on display in the Princess of Wales Conservatory and the Palm House for the public’s delight. Marcelo has been instrumental in setting up these displays and is proud to share little tastes of the Brazilian wilderness!

-Nick-

Related links

• Kew's Neotropics Key for bromeliads
• Bromeliad Society International
• The Bromeliad Encyclopedia 

Each year in the UK, around 2.5 million cubic metres of peat are sold to amateur and commercial gardeners. In Great Britain, over 94% of the 69,700ha of peat bogs have been damaged or destroyed.

Since 1997, Kew has been committed to finding alternatives to growing plants in peat based composts. There are many in the trade nowadays but for large scale horticulture, one of the best mediums is coir. This is made from shredded coconut fibre and is a versatile product, as well as coming from a sustainable source. The only major drawback is the miles it has to cover to get here! We use coir as the basis for our potting mix in the nursery, named ‘Kew Mix 3’. It was developed in the late nineties and consists of coir (45%), Silvafibre (commercially available well rotted leaf-mould) (45%), loam (equal parts sand, silt and clay)(10%), Kieserite (Trace) and a slow release fertiliser (15.9.11).
 

A comparison between raw coir fibre and Kew’s ‘Mix3’

Many people in the industry have said they have difficulties switching from peat to coir and that they prefer to use peat. Believe me, when I first started using it I had the same problem! But, to use coir successfully, you need to change the way you pot plants and the way you water. Once you adjust your way of doing things, coir is actually a remarkably good substitute and in some cases it even outperforms peat.

When potting with coir, we use the soft-potting method. Coir tends to lose its air field porosity when firmed. This means that it will hold too much water and not enough air. Eventually this will cause the roots of the plants to rot and die. The pot is tapped gently as we go, and at the end it is tapped onto a hard surface to settle it in. No firming-in is needed. We tend to pot the plant quite high in the container. Once it is watered in, this should cause the compost to sink enough to give us a good watering gap.
 

Horticultural apprentice Jude Goddard teases the roots in preparation for potting up

When watering, you should always remember that coir drains differently to peat. Peat drains evenly, so if it looks dry on the top, chances are it will be dry throughout and require watering. Coir doesn’t do this. Even if it looks dry on the top, it could be very moist a third of the way to the bottom. As with other mediums, overwatering can cause the compost to ‘sour’ and the roots to die. So we pick up the pots and feel the weight of the compost. After a while, this becomes like second nature, you soon get the hang of how heavy a moist pot is in comparison to a dry one. I think after nearly ten years of using coir I could water a nursery blindfolded, as long as I can pick up the pot and feel the weight. I might trip on the hose though, so I probably won’t be trying this anytime soon!

- Nick -