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Origin and Habitat: Impatiens tuberosa is native to NW Madagascar (Antsiranana province). It is known only from 2-5 localities.
Altitude range: Above 500 metres.
Habitat and ecology: This species grows in seasonally dry tropical lowland forests on limestone rocks, where temperatures get to 31°C. Seasonally dry habitats are usually avoided by Impanties with the exception of a few specialists with particular morphological adaptations such as the unusual, water storing tubers (caudexes) of I. tuberosa and Impatiens cinnabarina, which enables the plants to survive in time of drought. In the wild, the plants normally drop their succulent branches during the dry winters, and emerge from dormancy in spring. This species is threatened by deforestation. In the Mountain of the Français, North of the island, it is easy to notice the rarefaction of I. tuberosa in the places where the vegetation cover has become insufficient.
- Impatiens tuberosa H.Perrier
Description: In Madagascar there are numerous non-succulent species of Impatiens, but only Impatiens tuberosa can be considered a succulent plant. This is a glabrous perennials that develops in time an attractive, large, basal caudex which produces several annual shoots which drop off after fruiting. This beautiful species is quite different from the common bedding Impatiens. Its orchid-shaped blossoms are a vivid shade of magenta, and have yellow and red markings inside the white throat. The blooms are about 2.5 cm long and appear at the tops of the branches pretty much non-stop throughout the year.
Stems: Basally thickened into an irregularly shaped depressed-globose woody tuber with smooth brownish to greenish-brown bark. Annual shoots are numerous, deciduous, softly juicy-succulent, erect to 60 cm long, but usually shorter and branched. The shape of the caudex is variable, with each plant taking on a different shape. The tubers appear the first year from seed and the growth above the tuber will die off that year leaving the tuber to overwinter. It produces new growth the following year. The caudex gradually increases in size each year, eventually growing up to 30 cm wide.
Leaves: Spirally arranged, dark green or olive green with reddish undersides when grown in bright light, thin-textured, ovate-lanceolate, to 12 cm long, 5.5 cm wide, margin slightly crenate. Petiole 1-2 cm long.
Flowers: Solitary from upper leaf-axils about 2.5 cm long and 2 cm wide. Peduncle 3-5 cm long, thin. Petals dark pink to pinkish-violet, lower lip with yellow markings, throat white with red lines.
Bibliography: Major references and further lectures
1) Urs Eggli “Illustrated Handbook of Succulent Plants: Dicotyledons” Springer Science & Business Media, 2002
2) Werner Rauh “Succulent and xerophytic plants of Madagascar”, Volume 1 Strawberry Press, 1995
3) Klaus Kubitzki “Flowering Plants. Dicotyledons: Celastrales, Oxalidales, Rosales, Cornales, Ericales” Springer Science & Business Media, 11 November 2013
4) “Impatiens tuberosa.” in: Strange Wonderful Things -Rare and exotic plants & seeds web <http://www.strangewonderfulthings.com/266.htm>
5) “Impatiens tuberosa.” in: Paul Shirley Succulents weblog for succulent friends web: <https://paulshirleysucculents.wordpress.com/2012/06/23/impatiens-tuberosa/>
6) Jean-André Audissou “Madagascar: which future?” Cactus-Aventures International N° 76
7) Attenborough D. “Madagascar, a natural History”, Preston-Mafham 1991
8) Guillaumet J-L., Koechlin J. & Morat P. “Flore et végétation de Madagascar”, Gantner 1997
9) Martin Cheek and Eberhard Fischer “A Tuberous and Epiphytic New Species of Impatiens (Balsaminaceae) from Southwest Cameroon” Kew BulletinVol. 54, No. 2 (1999), pp. 471-475
Impatiens tuberosa Photo by: © Plantemania
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Cultivation and Propagation: Impatiens tuberosa is rare in cultivation and seldom seen for sale. Despite the fact that it grows in tropical conditions, it can take quite a bit of dryness and survive happily on a minimum of water. In the winter it will lose all its leaves and will start growing again in early spring. It is a good grower needing a lot of water; seeds it-self and can become a greenhouse weed. In the right conditions, it is an easy and long-lived plant. In cultivation, the plant often stays evergreen if watered regularly and kept warm.
Substratum: It requires very fast draining soil. The plants like a rich soil mix containg 2 parts coarse sand, 2 parts pumice rock, and 1 part potting soil or coir fiber. Mix in a small amount of dolomite lime, to simulate the plant's limestone habitat, but aren't usually too picky as to soil type.. The tuber normally grows above the ground in the wild, so do not be tempted to plant it under the soil or it will rot off.
Exposure: It can take more sun than typical Impatiens, but give it some protection from strong afternoon sun. Bright shade works well too. Shade is sometimes provided in hot climates.
Water requirements: It is fairly drought tolerant, but it looks best when given regular water and fertilizer, just like other Impatiens. Over about 50% humidity is preferred.
Fertilization: They are fertilized on a monthly basis with a balanced fertilizer during active growing phase.
Temperature requirements: It is a tropical species that does best above 15°C. It seems to be happiest between about 21-32°C. It can often tolerate warmer conditions than that if nights are cool (below18°C). However, in the winter, keeping it cool (about 10°C) gives the plant a needed rest.
Pests and diseases: The only problem with pests is likely to be red-spider mite and keeping the air moist will help to combat that.
Propagation: By seeds that need warm (23°C) and moist humiferous substrate, germination in 4-5 weeks. If you want to catch the seed, make sure to put something around the seed pod or they will spring all over the place. The seed can be kept up to 3 months before sowing, but can lose viability quickly after that. Flowering from seed can take 3 to 4 years. Propagation by cuttings – although difficult – are also possible, but then the tuber does not come back and the vegetative part will only last a year, unlike cuttings taken from Impatiens flanaganae which do produce a tuber. Therefore, the flowers of tuberosa need to be pollinated to get seed in order to produce a plant with a tuber. The flowers are produced in abundance, close to the top of the stem in full view. They are normally pollinated by bees, but can easily be pollinated by hand.
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