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The genus Melocactus comprises just under 40 recognised species. The range extends from southern Mexico, through the Caribbean islands to central South America. It is a group which is loved or hated, embraced or rejected by growers. The requirement for warm, light and airy conditions, and the consequent cost of heating, puts the genus out of reach of many. However, it is not well-known that they make excellent houseplants, if kept in a warm room near a south-facing window where they can have daily attention.

1117 Fig.1 curvispinus koolwijkianus

Fig. 1 Melocactus curvispinus subsp. koolwijkianus in habitat, Arikok National Park, Aruba.

M. curvispinus is a taxon which includes many forms sunk as synonyms or relegated to subspecies. Names such as loboguerreroi, maxonii and oaxacensis still appear on plant labels. However, subspecies caesius, koolwijkianus and dawsonii are still recognised. The curvispinus group is characterised by relatively large flowers and fruit compared with those of other species. An interesting feature of curvispinus is that, with the exception of subsp. caesius (arguably a distinct species), the cephalium rarely grows to more than a few centimetres high. This compares with the cephalium of some species such as macracanthos and intortus whose cephalia can grow to more than a metre in length.

1117 Fig.2 cephalia

Fig. 2 Melocactus curvispinus subsp. curvispinus (left) and M. ernestii (right) in cultivation. Both these plants are about thirty years old.

Like many cacti which can be tricky to cultivate if not given ideal conditions, melocacti are best grown in a high mineral compost comprising up to 70% grit. The top third of the pot should be pure grit but the soil should never be allowed to dry out completely, even in the winter. Although about 17º Celsius is a recommended minimum, much depends on the fluctuation between day and night. A friend in Arizona allows his melocacti to experience temperatures as low as 7ºC at night without a problem, but the following day temperatures always exceed 30ºC. A word of warning; most plants of the genus for sale are cultivated in the Canary Islands. These often fail to establish and, if they do, they are short-lived. If possible, buy plants grown in Europe, or raise them yourself from seed which can be much easier to obtain.

1117 Fig.3 caesius flower and fruit

Fig. 3 The large flowers and fruit of Melocactus curvispinus ssp. caesius.

We do not yet fully understand the relationships between Melocactus forms. It is a very young genus and it may be that we will find that it comprises a few ‘superspecies’, one of which could be curvispinus – but that’s another story.

George Thompson

No part of this article or the accompanying pictures may be reproduced without permission. Copyright BCSS & the Author 2017