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Plant of the Month

August 2015

Dorstenia gigas

Dorstenia gigas is the pachycaul that most lovers of caudiciform plants dream of owning. Its statuesque form, reptilian skin and space-age flowers have even been known to earn it the grudging respect of the odd diehard cactus enthusiast – not that they would care to admit to this in public.

Dorstenia is a large genus of tropical shrubs and herbs, related to the fig, that has evolved its own special flower aggregate (hypanthodium) simulating a large single bloom adorned with tentacle-like bracts. Dorstenia gigas hails from the island of Socotra, some 240 km from the Horn of Africa. Known as the Galapagos of the Indian Ocean, Socotra is listed as a World Heritage site by UNESCO for its rich and distinct flora and fauna.

As with other succulents from Socotra, Dorstenia gigas likes heat and benefits from plenty of extra drainage: around 30% additional aggregate in the potting mix is a good rule of thumb. When in growth they will take plenty of water and the odd feed with a low nitrogen fertilizer, but it is wise to aid drainage with a clay pot to help with the vagaries of the British summer. It likes bright light, but the foliage can burn so some light shading is a good idea on the hottest days of summer.

Dorstenia gigas has a justified reputation for being touchy to keep through the winter. It does not do well at low temperatures with 10°C being the minimum during winter dormancy and 15°C being a safer option. I know of one enterprising grower who wraps the trunk of his plant in a soil warming mat every winter, which seems to do the trick. During dormancy, at the minimum temperature, Dorstenia gigas will shed all its leaves and will require several weeks to re-root at the onset of the growing season. Plants kept at higher winter temperatures will keep some leaves and cautious light watering can help retain some live roots.

The start of the growing season is heralded by the appearance of bizarre flower forms, at which point it is time to get your soft paintbrush out in the hope of setting seed. Dorstenia gigas is very capricious in my greenhouse when it comes to setting seed: some years I have more than I know what to do with and others nothing at all. I suspect this has something to do with the frequency of attention with the brush and when the day job gets in the way, the chance of setting seed plummets. When ripened, Dorstenia gigas seeds explode from their seed capsules so I cover the flowers with cling film. That way the precious seeds are safely collected whatever time of day or night they are ejected and the clear film enables me to see them. Seeds are slow, taking several months to germinate, but a joyful sight when they finally emerge.

Gillian Evison

Fig 1 Dorstenia gigas

Fig 2 Dorstenia gigas flower

Fig 3 Dorstenia seed capsules (D. ellenbeckiana) ready to burst

Fig. 1: Neochilenia floccosa KK1173

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