– Ariocarpus retusus (2019)

By Pat Mahon (March 2019)

Who can deny that the genus Ariocarpus is one of the most atypical cacti in the family? Often mistaken as fake, a succulent or a stubby agave, these plants are truly a pinnacle in people’s collections.

Ariopcarpus can range from the size of a quarter to filling out a 10-inch pot. Their versatility and pest resistance make them an exceptional addition to cultivation, but many are put off by their seemingly nonexistent growth. Priced From $20 to over $100, this could be a cactus that survives winter and flourishes in spring and summer for years to come.

Ariocarpus retusus ssp. retusus (Scheidweiler) is a magnificent geophytic cactus hailing from a wide distribution in multiple states of Mexico. The species overall does not seem to face much threat in situ. The amount of plants and their ease of germination seem to allow the cactus to be prolific in many environments.

The specific epithet retusus derives from the Latin “retunsae” or “blunt,” which may allude to the seemingly rounded tubercles of the type species. According to the original species description, the first Ariocarpus retusus was found near the purple rocks of San Luis Potosi at 6,500 to 7,000 feet elevation. Of course, this species has belonged to a fair share of very diverse, outdated genera that are more defunct than a 2002 Daewoo.

With wide distribution in the array of environments and elevations of the Chihuahuan Desert comes a range of forms and varieties. This species is highly subdivided by phenotypic variations.

One of the forms most commonly encountered in cultivation is A. retusus ssp. trigonus, which is found in lower elevations. It is visibly identified by longer, narrower tubercles and pink-white to yellowish flowers. Another large contender is A. retusus ssp. furfurascens, which differs morphologically with equilaterally triangular tubercles.

Along with Ariocarpus retusus ssp. retusus, these three commonly cultivated subspecies have very similar culture requirements. Even within the same subspecies or same progeny, plants can vary in the amount of wool, as well as tubercle size and shape. It is hard to find two plants exactly alike!

Not too surprisingly, these cactus geophytes have adapted to take a beating in nature. Having a large taproot and succulent tubercles gives this cactus a huge advantage in longevity and water storage. The ability to shrink and recess into the ground during droughts, reducing surface area exposure to the sun, is a remarkable demonstration of long-term commitment to surviving.

In cultivation, it is obvious that overwatering can prove quickly fatal to these extremophiles. Always consider a fast-draining substrate with minimal moisture retention and little to no organic matter. This species grows among limestone, so it seems to favor some calcium in the substrate. Plant in a deep pot to allow the taproot to grow comfortably and survive.

A little bit of water goes a long way with ariocarpus. In winter, withhold water altogether, unless tubercles are so shriveled they have lost dimension. When watering in spring and summer, do so very lightly. Choose a fertilizer that is very weak and has calcium in it, and perhaps apply only once or twice during the growing season. In pot cultivation, try to water very small amounts around the root zone only.

A. retusus is an extremophile, so it can take plenty of sun. DO NOT purchase a plant grown indoors or in a greenhouse and immediately expose it to full sun. Slowly introduce the cactus to more and more light. Avoid having any ariocarpus in the rain, as it can lead to a quick death. A. retusus ssp. retusus seems to bloom in fall to early winter. Whenever the days become shorter, the growing season is coming to a halt.

Blooming and pollination can occur very quickly, so do enjoy the once-a-year show! Seed pods usually swell and mature within the safety of the tubercles and wool, and expel without warning six months to a year after blooming.

These cacti do like it on the warmer side, but are unusually hardy to below freezing for a short amount of time. They do not seem to respond negatively to humidity, but I personally pull them into lower light during the extremes of summer. My reason is to reduce stress in order to reduce the chances of fungal pathogens coming to fruition. This species seems to be very pest-resistant, but be on the lookout for mealy bugs in the more tender areas of the tubercles close to the main stem.

A. retusus is a species that is popular in collections as a strong plant that can survive most winters. Although Ariocarpus retusus ssp. retusus is the fastest-growing and largest example of the genus, it seems like time stands still with them. And whenever you feel these plants need water, get a watering can and dump it out on the floor, because this genus can survive with minimal watering!

Sources:

Llifle Encyclopedia of Cacti – http://www.llifle.com/Encyclopedia/CACTI/Family/Cactaceae/2079/Ariocarpus_retusus
Ariocarpus, Living Rocks of Mexico – https://www.living-rocks.com/retusus.htm
CactiGuide.com CactiForum – https://www.cactiguide.com/forum/viewtopic.php?t=754