Monstrous deliciousness and devilish fruit - Kew's edible aroids
Not many people have heard of edible aroids yet they are among the oldest cultivated crops in the world. Tropical Nursery horticulturalist Louisa Hall introduces some of these exotic species.
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 -