Cultivation notes on Melocactus

About the genus

The original description of the genus included only four species. From then, there was a gradual increase in the number of known taxa. About 35 species are currently ‘accepted’ and new forms are still being found.

Their distribution is wide-ranging, from Rio de Janeiro in Brazil and Arequipa in southern Peru, northwards to the Caribbean and Jalisco in western Mexico. The greatest concentration of species is in Bahia, Brazil where at least 15 species are found.

The initial growth is not capable of flowering, but when melocacti reach a certain stage the spines and areoles increase at the crown of the plant and a dense pad of specialised spines develops, increasing in width and length to form the terminal cephalium from which flowers appear.

Their size ranges from about 10cm in diameter and height (M. matanzanus) to the largest, over 50cm in girth, (M. zehntneri and M. levitestatus), excluding the cephalium which can be over a metre in length. Melocactus flowers are small or very small, red, pink or magenta in colour.

Most species flower continuously throughout the growing season. Reproduction is solely by seed. Melocacti are solitary and do not branch or form clumps, unless damaged. Almost all are self-fertile and will produce a great many seeds from the red, pink or white fruits.

How to grow them

A standard, very gritty compost comprising no more than 30% John Innes No. 2 or 3 with 70% or more coarse grit will be sufficient. It is also possible to grow the plants successfully in pure pumice or crushed lava but feeding will have to be more frequent.

Melocacti require good light for optimum growth and flowering but scorching has to be avoided. A minimum of 17ºC should be maintained although some species will survive at temperatures down to 10ºC.

Watering can take place throughout the year as roots must not be allowed to dry out completely. How often and how much they should be watered depends on the nature of the growing medium. If the soil is very well drained there is no limit to how much water can be applied but it has to be done regularly. Conversely, if the soil is poorly drained watering must be less frequent and great care is needed to judge just when water can be reapplied. The plants should be fed every other watering with a good cactus fertiliser.

Repotting of mature Melocactus plants should not be done too often as the sensitive, fibrous roots can easily be lost or damaged. Almost all species of the genus are easy to grow from seed. These are usually produced in great quantity by the adult plants and, providing the seeds are fresh, almost 100% germination can be expected.

The most common pests of succulent plants are mealy bugs, probably the only ones that frequently affect Melocactus. Root mealy bug is rare. Scale causes tissue damage to the plants but rarely kills them. Good cultivation practice, physical removal and, as a last resort, chemical treatment should keep the plants pest free. Perhaps surprisingly, melocacti make very good houseplants, if kept in a warm room with plenty of light.

Text and photos: George Thomson

No part of this article may be reproduced without permission. Copyright BCSS and the Author 2021

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