Echinocactus grusonii has the somewhat dubious distinction of being more common in cultivation than in habitat. Plants are raised in their thousands for the retail trade and large specimens are frequently seen in botanical gardens where the climate is suitable.
In habitat it grows in the Rio Moctezuma valley in the states of Querétaro and Hidalgo, east central Mexico. The story of the Zimapán dam, which caused a portion of the valley to be flooded, from which at least some of the plants were rescued from a watery death, is well-known. More recently a new population has been found in the state of Zacatecas. In habitat they tend to grow on medium to steep slopes often on volcanic rocks. They often lean to the south or south west so that the spines can best protect the body of the plant from the sun, and it is said to be possible for travellers to use the plants as giant compasses.
A mature E. grusonii may reach over one metre in height. While plants can remain solitary for many years, they may well offset once they reach maturity forming magnificent clumps. A juvenile looks quite different to the mature plant with a much more tuberculate appearance. There are a number of forms in cultivation including one with white spines, and another which is nearly spineless.
E. grusonii can start to flower once they reach around 40-50cm in diameter. The flowers are comparatively small in relation to the size of the plant, and appear in a ring around its apex. It is difficult to flower in the UK however, generally needing warm and sunny conditions.
Joel Lodé, in his Taxonomy of the Cactaceae suggests that E. grusonii may have originated as a hybrid between Echinocactus and Ferocactus, and has placed it in a new monotypic genus, Kroenleinia (named for Marcel Kroenlein, Director of the Jardin Exotique of Monaco between 1969–1993). It is such a well-known and popular plant however, that I suspect it will continue to be known as an Echinocactus for some time to come.
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