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PLANT OF THE MONTH

Along with other articles, columns and club updates, each monthly issue of the Henry Shaw Cactus Digest includes an article or two on members' favorite cactus and succulent species. The articles typically include photos and facts on the plants' natural origins and distribution, growing conditions, common and scientific names, care and cultivation tips, and helpful hints for encouraging flower production.
 
Follow the links below this month's offering(s) to enjoy previous Plants of the Month or click to read a sample of the HSCSS Digest in PDF format.

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March 2019 -- Ariocarpus retusus

By Pat Mahon
 
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 Of course, this species has belonged to a fair share of very diverse, outdated genera that are more defunct than a 2002 Daewoo.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!

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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
 

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March 2019 -- Didiereaceae

By Bob Williams
 
Madagascar separated from Africa millions of years ago. During this time, the plants and animals adapted to the new conditions. There was no human activity on the island until 1200 B.C. This allowed the flora and fauna to evolve in isolation. Over 90 percent of Madagascar's plants and animals are found only on the island. This distinctive ecology has led some to refer to Madagascar as the "eighth continent," and the island has been classified by Conservation International as a biodiversity hotspot.
 
The majority of the island gets plenty of rain via the trade winds from the Indian Ocean. The rainy season lasts from November through April. The eastern part of the island is tropical with a central highland that is cooler, but still wet. This geographical highlight is of the most interest to us in the weird succulent world.
 
This highland area can reach an elevation of over 4,000 feet. Its geology creates a "rain shadow" in the southern and southwest sections of the island. This nearly 17,000-square-mile area, commonly referred to as the "Spiny Forest," is on many of our "bucket lists."
 
In order to understand the plants that grow in this region, we need to understand the geology and the ecosystem. Even though it is dry, this region can get 12 to 20 inches of rain during the rainy season. This sounds pretty good when you compare it to rainfall levels in Namibia and the southwestern regions of southern Africa. However, this area of Madagascar can be subject to periods of prolonged drought that last years.
 
The geology is also a factor. There are two major rock types in this region. In the south and southeast areas, red sand is the predominate soil. In the higher elevations in the southwest, a fast-draining limestone is found. There is little organic material to hold moisture. When rains do occur, the water drains quickly. The highly porous nature of the soil and bedrock provides little opportunity for vegetation to absorb moisture.
 
In order to survive, plants here have large tap roots that seek out ground water. The plants are woody, and when the rains do come, their trunks swell. The plants have a single goal: Store water and don't lose it. Some of the more majestic plants found in this area are baobab trees.
 
This brings us to the family Didiereaceae. This is a small family of just four genera and 11 species of flowering plants. The genera are Alluaudia, Alluaudiopsis, Decaryia and Didierea. The most significant fact is that these plants are only found in the Spiny Forest and nowhere else in the world outside collections.
 
Didiereaceae plants closely resemble some forms of cacti, but are not related. Unlike cacti, Didiereaceae species produce small, deciduous leaves, which are protected by mean-looking thorns and spines that grow directly out of the plants' many branches. The leaves can have a waxy covering that helps prevent water loss. During the dry season, these leaves dry up and fall off.
 
These are not small plants that live low to the ground. They can grow to heights of 20 feet or more. Growth can be relatively rapid when rains occur. Some people let them grow with a wild look, while many train them as bonsai plants.
 
The plant I have in my collection is Didierea madagascariensis. It is nicknamed the "octopus tree" due to the long, thin leaves that completely cover the spines of the plant. These plants are from the part of the Spiny Forest where the red sands are found. My plant is still young, only 10 inches tall. In the wild, this plant can be found in the 12- to 20-foot range.
 
During the growing season, D. madagascariensis can be watered regularly with a good soaking. Allow the soil to dry before another watering. In the fall, watering should be cut way back. I have found some sites that say not to water at all during the dormant period.
 
It goes without saying that a fast-draining mix is preferred. Mine is in the same mix I use for most of my plants, but you may be able to use a coarse sand mix for this plant.
 
D. madagascariensis can be propagated by cuttings. These plants are not rare and can be found online and at our show and sale.
 
Sources:
World Wildlife Fund -- https://www.worldwildlife.org/ecoregions/at1311
Wikipedia -- https://en.wikipedia.org/wiki/Madagascar_spiny_forests, https://en.wikipedia.org/wiki/Madagascar
Llifle Encyclopedia of Succulents -- http://www.llifle.com/Encyclopedia/SUCCULENTS/Family/Didiereaceae/11208/Didierea_madagascariensis
 

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