A tiny waterlily, forming rosettes 10-20 cm wide and comprising bright green lily pads (laminas) with short petioles. The lily pads can be as little as 1 cm in diameter. The central growing tip sinks in the wet mud, preventing the developing pads from drying out before unfurling.
Nymphaea thermarum in cultivation at Kew
The flowers are white with yellow stamens, and can be self-pollinating. Blooms open early in the morning and close in early afternoon. The flowers are held vertically, a few centimetres above the plant, but once they have finished their flowering cycle the petiole bends so that the fruit is in contact with the damp mud. Once it is mature, the fruit dissolves releasing the seeds. Reproduction is by seed only.
Threats and conservation
The only population of this species has died out as a consequence of over-exploitation of the aquifer that fed the hot spring that kept the plants moist and at a constant temperature.
However, since this species is now easily propagated and cultivated at Kew, and the spring water is still flowing (but sequestered before it reaches the surface), there could be an opportunity to restore the site and reintroduce Nymphaea thermarum to Rwanda.
Back from the brink of extinction
In 2010, Kew’s top propagation ‘code-breaker’, horticulturist Carlos Magdalena, cracked the enigma of growing a rare species of African waterlily – believed to be the smallest waterlily in the world with pads as little as 1 cm in diameter.
Nymphaea thermarum, was discovered in 1987 by German botanist Professor Eberhard Fischer. It was restricted to just one known location in Mashyuza, Rwanda. However, it disappeared from this location about two years ago due to over-exploitation of the hot spring that fed this fragile habitat, and no plant is known to have survived in the wild.
Soon after its discovery, Professor Fischer realised that the species was in jeopardy and he transported a few plants to Bonn Botanic Gardens. At Bonn, horticulturists were very successful at growing these valuable specimens, and they lasted for more than a decade. However, the species proved extremely difficult to propagate.
As a result of a conservation plant exchange between Bonn and Kew, a handful of seeds and pre-germinated seedlings reached Kew in July 2009. The N. thermarum seedlings were initially grown submerged under water like any other waterlily. But, at both botanic gardens, this method was unsatisfactory; seedlings were barely clinging on to life and did not develop to adult stages.
Nymphaea seedlings. All Nymphaea other than N. thermarum are germinated totally under water.
Carlos, who has a track record of bringing the rarest and most difficult plants back from the brink, took on the challenge of propagating N. thermarum. He ran a series of trials involving a range of temperatures, water hardness, pH and depth. Plants grown in harder water at shallower depths seemed to develop further; however no plant reached maturity.
Carlos then decided to investigate ways to alter the concentration of gases in the water, and to gather information on the natural habitat. Returning to the original German description of the species supplied the final clue: 'it grows in damp mud caused by the overflow of a hot spring. Water reaches the surface at 50 °C but the plant colonizes an area where the water has cooled to a temperature of 25 °C'. This meant that, unlike all other known waterlily species, N. thermarum did not grow submerged in the deep waters of lakes, rivers or marshes. The revelation was that this small, extremely rare and unusual species, grows in the damp conditions at the edge of a thermal hot spring – and this was the vital clue needed to crack the code.
With this knowledge Carlos did one final trial. He placed seeds and seedlings in pots of loam within small containers filled with water, thus keeping the water at the same level as the surface of the compost, at a temperature of 25 °C. In this way, the last remaining individuals of the species could be exposed to the higher concentrations of carbon dioxide and oxygen in the air. And to his surprise and joy, soon the plants started to improve and after a few weeks, eight plants began to flourish, growing to maturity with thicker, greener and wider leaves. In November 2009, Kew’s collection of N. thermarum flowered for the first time.