Tropical Nursery blog
The Tropical Nursery at Kew provides a home to the permanent tropical and temperate collections. It comprises 21 different climate zones and holds an estimated 35,000 plants.
The nursery is split into four teams that each have a specialism; orchids, moist tropical, cacti and succulents and temperate and island flora. The Tropical Nursery is normally off limits to the public so this blog provides a peek into the fascinating world of the largest nursery at Kew, and the work of the staff there.
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
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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!
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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 -
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The genus Sarracenia comprises a number of carnivorous species that grow in the marshes and bogs of North America. Some are under threat from habitat destruction as many of the species have a very limited range.
Here at Kew, we have a relatively good representation (241 accessions, giving us a total of around 800 plants) of the genus, you can see them on display in the Rock Garden and the Princess of Wales Conservatory. In recent years they have become increasingly popular as they have been used for their striking pitchers in the RHS Chelsea Flower Show and have had coverage on the BBC Gardeners World show.
As far as cultivation at Kew is concerned, they have herbaceous rhizomes, they love to sit in water during the spring and summer when in growth, and they really like insects! I’ve seen some of our larger specimens that we keep outside in water filled tanks almost full to the brim with partially digested bodies.
Kew Diploma student Ben Houston working on an accession of Sarracenia minor (Image: RBG Kew)
Potting is undertaken once every 2-3 years in March/April as they really like to be constricted in their pots. Sarracenias are one of the only collections that we still use peat for. We have tried our usual coir based potting mix but these plants really reacted badly to living in that medium. Currently we are doing trials with other peat free mixes and we are committed to finding an alternative for them. Our mix is 60% perlite and 40% peat; we are trialling a range of alternatives, from pure Seramis (expanded clay particles) to finely chipped bark.
First, the previous years brown pitchers are removed and the plant is taken out of the pot. The rhizomes are cleaned of old leaves, the spent compost is shaken off and the plant is checked for pest. If the rhizome is long and unbranched, the tip is removed to encourage it to send out side buds and become a clump. We use square pots for this collection; you can get more into the tanks this way and they provide a stable base on a windy day when they are outside. The clean rhizomes are potted into clean pots, with the rhizome at the back of the pot, facing into the center, so that as it grows the branching rhizome will fill the pot. We try to put the plants into as small a pot as possible, and we firm the compost really well. This way they will become constricted quickly and they will have a solid base from which to produce large pitchers. The plants always sulk for a season after potting, so we expect to see the fruits of our labour next summer!
The finished product, ready to go outside for the summer (Image: RBG Kew)
This is our fifth day of potting and we should be finished tomorrow, the collection is looking healthy and clean, and the tanks are all given a good clean as they are emptied. It always brings a smile of satisfaction to the faces of the team when we finish a batch and the plants look fresh and happy in their new homes.
- Nick -
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Nymphaea ‘Arc-en-ciel’ is an unusual day-blooming waterlily. Its multi-coloured leaves range from mid-green, to pink and dark red, often looking like it has been ‘tie-dyed’. Several waterlilies have blotches, but this is the only one that is variegated in this distinct way.
The colourful, marble- patterned foliage of Nymphaea 'Arc-en-ciel' (Image: Perry Slocum)
'Nymphaea' is Greek for water nymph and 'Arc-en-ciel' is French for rainbow (literally 'Arc in the sky'). This waterlily has broadly ovate to rounded floating leaves which can reach 25 cm in length and 12 cm across. It has a distinct cleft at the base of each leaf. Nymphaea ‘Arc-en-ciel’ produces white to pale pink star-shaped flowers, with yellow stamens. This unscented, hardy cultivar is considered to be relatively slow growing; an excellent plant choice for a small, sheltered pond or even for an indoor pool.
History of Nymphaea 'Arc-en-ciel'
Nymphaea 'Arc-en-ciel' (Image: Kit Knotts)
Little is known about this cultivar (particularly in terms of its parentage). Indeed, it was thought to have disappeared from cultivation. That changed when waterlily enthusiast Philip Swindells came across one that had no label while visiting Denver Botanic Gardens. He correctly identified it as Nymphaea ‘Arc-en-ciel’ as he had seen it a decade earlier in Europe before it disappeared.
Nymphaea ‘Arc-en-ciel’ has added significance as this waterlily, like many others, is inextricably linked with Joseph Bory Latour-Marliac (1830–1911). Marliac was one of the first to hybridise waterlilies, opening his French nursery in 1875. He introduced many new colours and forms. Sadly, Marliac’s ‘secrets’ of hybridising died with him, but he developed about 70 first-class Nymphaea hybrids, many of which are still popular, such as Nymphaea ‘Marliacea Rosea’, N. ‘Marliacea Chromatella’ and N. ‘Marliacea Carnea’.
In 1889, Marliac presented his collection to the World Fair of Paris; the same year that the Eiffel Tower was completed. Installed in the water gardens in front of Le Trocadéro, his hybrid waterlilies caused a sensation and won first prize in their category. They also caught the eye of impressionist painter Claude Monet. The exhibition inspired him to create the water garden in Giverny, as well as around 250 of his trademark oil paintings.
Cultivating Nymphaea 'Arc-en-ciel'
Waterlily tanks in the aquatic zone of the Tropical Nursery (Image: RBG Kew)
N. ‘Arc-en-ciel’ has been growing in the aquatic zone in the Tropical Nursery in tanks that are heated to 21˚C. Above the tanks are supplementary lights which not only increase the light levels in the zone but can also extend the day length. The temperature of the zone ranges from a minimum of 18˚C to a maximum of 22˚C. The evaporation from the tanks keeps the humidity in the zones quite high. The waterlilies are potted into loam. Crocks are used at the base of the pot and then topped with grit, which prevents the loam from escaping when submerged into the water. This cultivar is propagated by dividing the plant.
N. ‘Arc-en-ciel’ has been on display to the public in the small pond on the west side of the Princess of Wales Conservatory.
- Sam -
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About the Tropical Nursery
The main functions of the Nursery are to:
- Form a back-up collection of tender tropical plants used to support science, display and education within the gardens. We supply plants for use in displays in the Main Kew conservatries, for festivals and events organised by the Foundation and the Directorate.
- Supply plants for education purposes to the Schools & Families department.
- Act as main propagation facility for the Princess of Wales Conservatory, Palm House and Temperate House as well as supporting the exchange of plants between Kew and other institutions and collections.
- Provide direct education in nursery techniques and the cultivation of tropical plants for the Kew Diploma course, Apprentice and Trainee programme, Internship and work experience programmes and visiting staff from other UK and overseas institutions.
- Support conservation by working with the UKOTs team undertaking propagation and cultivation protocols on targeted endangered species.
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