Astrophytum myriostigma, "Bishops Cap" - Cactus Club

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Astrophytum myriostigma, "Bishops Cap"

Plant of the Month > Species A to B

Photographs by Bruce Brethauer

   Members of the genus Astrophytum have been long time favorites of mine. The very first cactus that I remember growing - back when I was 5 or 6 years old, was a seedling of Astrophytum oranatum which I selected from an assortment of succulents at a local nursery. Even as a child, I was attracted to the remarkable geometric shapes of these plants, and the distinctive flecks of white flocking on their stems. I'm sad to report that this first plant is now a very distant memory, but my love affair for this genus continues to this day, having grown and propagated many plants over the years.

   While the genus includes some very attractive and curious plants, I believe that the "Bishop's Cap" cactus is one of the most iconic of the group; it easily recognized by its distinctive geometric, spineless stems, and its characteristic covering of tiny, scale-like clusters of fine hairs (trichomes). It is frequently offered in selections of cacti and other succulents at nurseries and home and garden stores (it is readily available, so it is not just for connoisseurs), and its common name is so perfectly descriptive that many people who are only marginally familiar with cacti can ask for it by name. With a few minor considerations, this is an easy plant to grow, it is compact, and will not soon outgrow its allotted space, it is spineless and non-toxic, so it is a suitable plant to grow in households with inquisitive pets and young children, and its delightful summer display of lemon-yellow flowers is the icing on the cake - What's not to like?

   The species is native to northern and central Mexico, growing at high elevations ( above 6500 feet ) in the Chihuahuan Desert. Stems are typically globular to cylindrical, growing to about 7 inches in diameter and to about 10 inches tall in most plants. The flowers are produced in summer and are typically of a uniform, lemon-yellow color, sometimes with an apricot blush. Flowers typically measure to just under 3 inches tall and wide, but in some plants, these can be somewhat smaller.

   For years, I was under the mistaken belief that the specific epithet of this plant, myriostigma, was  reference to the many stigmas of the flowers of this plant - but this always seemed a bit odd; while this plant does have a number of stigma lobes on its flowers, there are no more of these on this plant than on most other species of this family - it seemed an odd feature upon which to distinguish this species. I finally came to the realization that the name is better translated "thousands of spots" and is in reference to the many thousands of individual trichomes which dot the surface of typical plants.

   This is a highly variable species in the wild, with many named varieties (although in recent classifications, many of the old varieties have fallen by the wayside). Plants vary in a number of attributes; in their degree of flocking - with densely felted plants at one extreme, to plants completely lacking any flocking in variety nudum; in their size, from the typical low squat to barely columnar stems of most varieties, to tall, very columnar forms with stems growing to nearly 3 feet tall in varieties columnare and tulense; and in the number of ribs, from typical plants with 5 or more ribs, but in variety quadricostatum, there are only 4 ribs, and in the cultivar 'tricostatum' there are only 3 ribs. The number of ribs is not a stable feature; plants typically add additional ribs as the plant matures, increasing to as many as 8 or more in the typical plants. Plants also vary in flower size, and the the sharpness of the ribs, which can vary from some plants with very sharp ribs to plants with very rounded ribs. Selective breeding and hybridization has also produced a number of outstanding cultivars, for example, Astrophytum myriostigma 'Onzuka' has exceptionally thick, patterned, flocking on its stems. In all, there are probably more than a score of named varieties, cultivars and hybrids of this species on the market.

   Overall, these are fairly easy and tolerant plants, and should grow well given my basic guidelines for growing cacti and other succulents. However, this is a desert species, which is adapted to arid conditions, and very intense solar radiation. The flocking on the stems is believed to be an adaptation to help protect the plant from intense light and temperature extremes, so that the heavier the degree of flocking, the more light the plant will require in cultivation. Grown in anything short of a brightly lit western or southern window, and these plants will eventually fail as a houseplant. Whenever possible, it is always best to grow these outdoors through the warm months of spring, summer and fall to benefit from full exposure to sun and summer heat - given this treatment, it will tolerate a 6 month period in a more dimly lit interior setting, provided that it is kept dormant by keeping the plant cooler and dry through the winter months. Many growers do not water this plant at all in the winter -I generally give my plant a light watering about once a month in winter to prevent root loss: but be warned, extended periods of cold and damp will rot this plant, so it is always better to err on the side of caution. Given this regimen of over-wintering in a cool dry setting (preferably under the brightest lighting conditions available) and outdoor exposure to full sun and summer heat, and my plants have reliably flowered every year once they have achieved flowering size. Sadly, the individual flowers are short lived, each opening for only about 2 days, but plants will produce a succession of flowers, so that the flowering season may last the greater part of mid to late summer. Another consideration in growing this plant is that infestations of mealybugs and scale insect can very closely mimic the flocking on this plants stems; it is important to regularly inspect these plants for these insect pests, and to isolate and treat any plants as soon as problems are detected.

   I suppose that it is hard to be completely objective about long time favorites; some authorities caution that this plant may require a bit of extra consideration, and do not recommend it for beginners. This does not reflect my experience with this plant - I pretty much treat it the same as I treat most of my succulents, and given this treatment, my plants of this species have generally thrived. A well grown plant in excellent health is quite a sight - with its marvelous geometric shape, and its distinctive flocking, this species really has no equal. If you have never grown this plant before, give it a try - I'm sure that it will also become one of your favorites as well.

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