A first flower stimulates research       Roland Tebbenham

During June I was pleased to see a bud appear for the first time on a plant I have owned for more than ten years, but to which I had paid scant attention. It was a Denmoza rhodacantha with a collection reference DJF186. I used Ralph Martin’s excellent on-line database http://ralph.cs.cf.ac.uk/Cacti/ and found that it refers to David J Ferguson and he collected at a site called ‘Agua del Toro’ in Mendoza, Argentina. David Ferguson is from Rio Grande Botanical Gardens in New Mexico, USA and he allocates numbers to collection sites, not to individual species; something to be aware of if you acquire a plant with a DJF collection number. In fact DJF186 also covers Pterocactus decipiens and Pterocactus kuntzei. Many other people have collected plants from the same locality including Fred Kattermann [Eriosyce] and Zlatko Janeba [Denmoza, Pyrrhocactus & Trichocereus].

The genus name ‘Denmoza’ is an anagram of the Argentinean province Mendoza, a happy hunting ground for cactophiles, and the specific epithet ‘rhodacantha’ means red spines. The taxonomy of this plant has been complex; it started life in 1821 as Cactus coccinea nom nud and was described by Salm-Dyck in 1834 as Echinocactus rhodacanthus. According to opinions based on the morphology of stems, flowers and seeds it has been assigned to Cereus, Cleistocactus, Echinopsis, Oreocereus and Pilocereus, but Britton & Rose erected the genus Denmoza in 1922 a century after its discovery and this is upheld in the New Cactus Lexicon. It inhabits mountains west of the Andes in the Argentinian provinces of Mendoza and Salta at elevations between 2000 & 2800m.

Above: Roland’s Denmoza rhodacantha flowering for the first time.

I have two small Denmoza rhodacantha plants both growing in 9cm (3½”) pots. One is roughly eight years old with reddish spines and the second is at least twelve years old with yellow spines. The older one bore the bud, which expanded over ten days and opened fully as you can see in the images. The scarlet flowers open during the day, are zygomorphic (asymmetrical), 7-9cm (3>3½”) long and adapted for pollination by humming birds.

Left: Detail of the flower of Denmoza rhodacantha.

When young the plants resemble Ferocacti with their low ribs and strong spination. They are slow growing plants whose appearance changes as they mature, the plants become columnar or more barrel-shaped, develop more ribs and may produce long white bristles. The yellow-spined plants were named Denmoza flavispina, the bristly plants Denmoza erythrocephala and there is a variety named Denmoza rhodacantha var. diamantina, but they all are referred to Denmoza rhodacantha.

So an unassuming seedling made good progress over a decade and developed into a nicely spined flowering gem. It caused me to research the taxon and discover many interesting cactus snippets. I hope you have been engaged and you will seek out a Denmoza or two – they grow slowly and make arresting, sculptural subjects for the discerning cactus grower.

Roland

Above: Detail of spine cluster.

Right: Many, many years of careful culture will produce a Denmoza rhodacantha like the Darbon’s seen at our 2010 Show