Ariocarpus - Cactus Club

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Plant of the Month > Species A to B


by Bruce Brethauer

    As a rule, in writing columns for this Journal, I have tried to focus on plants which are reasonably easy to grow in cultivation; but in my humble opinion, the various species of Ariocarpus do not qualify as easy plants. Many of the members of the Central Ohio Cactus Society (myself included) admit that they have killed a plant or two from this genus. But the various species are amongst the most unusual and attractive of all cacti, with low stems (in some species, growing nearly flush with the surface of the ground, with odd triangular tubercles barely protruding above the ground’s surface). All species are compact, growing to (at most) several inches in height, and to about 10 across or perhaps slightly larger in the largest species (most of the species tend to remain much smaller). While most plants tend to remain solitary, mature plants may eventually produce a few offsets, and a few exceptional clones may even produce compact clusters of a dozen or so stems. The tubercles vary considerably between the species, and for that matter, often show considerable variation within a species: these may be flat, convex or concave, smooth, or intricately rough, long and narrow or short and squat depending upon the species, variety or clone. Except for their earliest months of growth (and in some plants of A. agavoides), no mature plants will produce spines. Some plants of Ariocarpus retusus and A. scapharostrus, may not even produce areoles, which is usually considered one of the fundamental diagnostic traits of the Cactus Family. But let there be no confusion, these ARE cacti, despite their many distinctly un-cactus-like traits: I think that the management of Aridlands Nurseries phrased it best when they declared the various species to be "honorary succulents" (succulents in this case meaning any succulent plant outside the cactus family).

   Very old, and well grown plants are hardly to be forgotten, as these are unique and very attractive plants; possibly best described as living cubist sculptures. Oftentimes, the greater portion of these plants grow below the surface of the soil, with large tuberous roots which may be carrot-like, beet-like or turnip-like depending upon the species. The apex of the plant is usually covered with a thick matt of wool, presumably to protect new growth from scorching light, desiccation, and other environmental factors. The stems and tubercles may be dark green, or a grayish green color, and are usually exceptionally tough, feeling almost horn-like. Because of their very low growth habit, and the near grey coloration of the stems of most species, these plants are well camouflaged in habitat, and can be nearly impossible to locate when plants are not in flower. Flowers range in color from bright magenta (which is typical in most plants) to pink, white and even yellow in some plants which had formerly been identified as Ariocarpus trigonus - a name which appears to have been recently been merged with A. retusus, as a variety.

   Because these plants produce such very large tuberous roots, all species of this genus tend to be very sensitive to excess water, and may have a tendency to rot in cultivation unless provided a potting medium with very sharp drainage. Plants also need very bright light to produce good growth, and also seem to prefer warm temperatures (from the upper 80's to mid 90's and possibly warmer) during their growing season, so plants are best grown in a bright greenhouse, or should be moved outdoors onto benches in direct sunlight (after plants have been sufficiently acclimated to the increased light) during the warm months from late spring through early autumn. Provided with excellent drainage, these plants will tolerate frequent thunderstorms (although it may be wise to erect screens over these plants to protect them from hail). Exposed to summer sun, high summer temperatures and rain, my plants have rewarded me with growth, and the occasional flower. Plants perpetually maintained inside the home, will probably suffer from a lack of light and temperatures which are too cool to produce any significant growth. These plants, if they produce any growth at all will grow considerably slower than plants moved outdoors or grown in a greenhouse.

   The various species of Ariocarpus have a well deserved reputation of being slow growers. Several years ago, I ordered a few plants from Mesa Garden, and these included data on when they had been planted - one was a 16 year-old plant which only measured slightly more than about ½ inch across. But this is an exceptional case: Steven Brack (of Mesa Garden) has a reputation for growing his plants very hard (with infrequent watering, exposure to extreme temperatures, and given little fertilization to duplicate some of the extremes which these cacti may experience in habitat), and therefore, many of his plants grow very slowly. Other growers, who give their plants more frequent watering, and fertilization report faster growth rates; some have even claimed that they can grow some of the slowest growing species to flowering size in as little as 18 months. In my plants, the growth rate falls somewhere in-between these two extremes. Even so, it is often difficult to discern any difference in the appearance of my plants from one year to the next. In past years, most of the plants which found their way into collections had been collected from the wild, and as a result, many species and varieties have been virtually eliminated from their habitat. Collecting plants from the wild is now illegal, although there is still an illegal trade in some of the rarest species. Plants grown from seed in nurseries have only become available in the last two or three decades or so. Growers are advised to be certain that their plants are from nursery grown stock - while these may be somewhat smaller than field collected plants, these are more likely to survive in cultivation, as most field collected plants are usually damaged in collection, and seldom recover from their translocation. Plants in the wild are rare enough as it is: any additional pressures on the few wild populations could very well result in the extinction of these few wild plants.

   The species of Ariocarpus can be quite a challenge to grow well, especially without the benefit of a greenhouse or Florida-room, But the challenge of growing these plants, and the serendipitous reward of finding a flower bud on a plant after a year or so in your care may perhaps make up for their contrary nature.

   I cannot pretend to be an expert with these plants: I have managed to maintain several for a number of years, and several plants have managed to produce a flower or two each year for the last few years, but on the other hand, I have also killed several plants over the years, and since I have treated all of my plants more or less identically, I can only surmise that the plants which continue to grow and flower under my care have done so in spite of my treatment, rather than because of it. I provide my plants with a potting medium with a bit of extra grit worked into it to provide a particularly free-draining medium. Bonsai soil will probably make a good potting medium, but I usually mix my own soil, blending equal volumes of a calcined clay product (Turface All Sport), coarse Perlite, coir (coconut fiber), and a good quality potting soil or topsoil. This is a good basic mix, but other growers may choose other ingredients and proportions for their plants. I only fertilize my plants from spring through early fall, using a water soluble fertilizer. I prefer Dyna-Gro products for my plants because these often provide nutrients in proportions more suited to cacti and other succulents, with lower proportions of nitrogen, and higher percentages of potassium and phosphorous. One of the other pluses of the Dyna Gro fertilizers is that these usually also include a complete compliment of the "trace elements", minerals which plants require in minute quantities. Unfortunately the Dyna Gro products are largely designed for use on plants grown in hydroponic systems, and are not widely available in many nurseries, so if these are unavailable, other fertilizers (such as Miracle Gro or Peters fertilizers for tomatoes and other vegetables) may also be suitable. I usually fertilize at about 1/4 the recommended application rate, and fertilize every 4 to 6 weeks during the growing season. I withhold fertilizers altogether through the cooler months. I also will keep my plants quite dry from late autumn through early spring, usually only watering them about every 6 weeks or so. Some growers withhold water altogether through the winter months, but I find this to be a little extreme. In mid to late spring, I move my plants outdoors, gradually acclimating them to the increased light by first placing them in partial shade for a few days, and gradually moving them into increasingly sunny sites until they are in full sun. Plants will produce their greatest growth during the summer months, and (if the fates are with you) will produce their flowers in fall. I have had my greatest success with the smaller species, however, I have also seen blooms on my plants of Ariocarpus retusus.

The plants which I have had the greatest success are:

   Ariocarpus agavoides: a miniature species growing to about 3 inches across, but usually remaining much smaller. This species produces very long and narrow tubercles, these growing to about 1½ in length and to about ¼ inch wide. Plants usually only produce a relatively few tubercles, and in my plants, these are practically deciduous, lasting perhaps 2 years before drying, and being replaced with newer growth. Flowers are usually large for the size of this plant, opening to as much as 1½ inches. Flowers are deep pink to magenta in color, and petals tend to be acute, terminating in a point. This species is also unique in that some plants may occasionally produce an odd spine on its areoles. These spines are quite short, and generally grow appressed against the tubercle.

   Ariocarpus fissuratus: plants usually grow to no more than 6 inches in diameter (and are usually smaller). Plants are usually very low, with broad triangular tubercles with very rough upper surfaces. Flowers are pale to deep pink, opening to about 1.5 inches across. This is the only species known to occur in the United States, with several populations identified from part of the lower Rio Grande Valley in Texas; otherwise, all other populations of this species and the remaining species of this genus are endemic to Mexico.

  Ariocarpus kotschoubeyanus is another very small species, growing up to about 3 inches in diameter, but are usually much smaller, reaching flowering size at slightly more than ½ inch in diameter. Plants are flat-topped, growing flush with the surface of the ground. Tubercles are small, triangular, and bear felted groves along their midlines. Flowers open to just over 1 inch across, and may range from nearly white or shell pink to bright magenta.

   Ariocarpus retusus. This is perhaps the largest growing species, growing to as much as 10 inches in diameter. Its stems are gray-green , and tubercles are more or less smooth, although these are quite variable in shape and size. Flowers vary in color from white to pale pink, or in variety trigonus, may produce bright yellow flowers.

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