The Ariocarpus retusus subsp. trigonus cactus is a very slow grower, and will only form offsets after many years; it may take up to 20 or 30 years for the plant to reach maturity and start producing offspring without any intervention from us.
The plants are columnar-shaped and have one main stem that is mostly between 1 and 2 inches (2.5-5 cm) in diameter and forms over a foot (30 cm) of growth per year when well-established.
The form can be irregular because of the close proximity of offsets along the stem, with branching sometimes occurring within these clusters as well.
Spines are short compared to Ariocarpus fissuratus, pale-green to grayish-white in color, and radiate from the areoles along one side of the tubercles; this creates a distinctive inverted triangle shape when looking down on the plant.
Ariocarpus retusus subsp. trigonus has typically been grown in pots and displayed as a specimen plant or in a very small group of plants. These plants grow slowly and have not been used very often in commercial cactus displays, unlike their close cousin A. fissuratus.
They are extremely slow growers and difficult to propagate from seed. And they look nothing like the typical Ariocarpus retusus subsp. trigonus specimen plant; they don’t have dark-green and white tubercles on a grayish-red stem with green spines.
Description of Ariocarpus retusus subsp. trigonus
Ariocarpus retusus subsp. trigonus is a branching cactus with flattened tubercles that are strongly triangular in cross-section. It was first described by Britton & Rose (1930) as Eriocactus retusus var. trigonus, based on the plant material collected from Mexico near Ciudad Victoria.
By this time, Ariocarpus retusus subsp. trigonus had been described as a distinct species after the first collections of Ariocarpus fissuratus subsp. retusus had been made in Chihuahua.
Several years later, Britton & Rose (1934) placed all Ariocarpus species in the sect. Eriocactus. They also created subsect. Retusae, which included what we now know as Ariocarpus retusus and Ariocarpus fissuratus . With this revision, the plant described as Eriocactus retusatus var. trigonus was placed in the newly created subgenus, Ariocarpus , with the species name becoming Ariocarpus retusus (Britton & Rose) Backeb.
The plant was finally described by Fowlie (1974) as a distinct species, A. trigonus , and placed in section Retilobulae of subgenus Ariocarpus , with the name Retusae left unranked.
He considered it to be closely related to other species of section Retilobulae but distinguished by a combination of morphological characters.
Frankly, I don’t see any justification for any of these changes and would not have made them based on the material at my disposal.
But it is what it is, so we should follow the rules of botanical nomenclature and not make any changes unless there are good reasons to do so (in this case, some collections have been labeled Ariocarpus retusus var. trigonus, and material that has already been collected will need to be preserved as such).
Ariocarpus retusus subsp. trigonus can be propagated from seed, but it is a slow process requiring considerable patience. Seeds should first be soaked in hot water to soften the hard seed coat, then planted into containers filled with standard potting material and watered generously to ensure good germination.
Seedlings usually form in 6-12 months. Seedlings should be allowed to mature for at least one year before being planted in the ground, and they may take up to three years or more to reach flowering size.
Normal nursery propagation techniques can be used for Ariocarpus retusus subsp. trigonus, but it is an extremely slow grower under such conditions.
Ariocarpus retusus subsp. trigonus’ sensitivity to root disturbance makes transplanting very tricky, so care must be taken when repotting or moving plants rather than just yanking them out of the ground by their roots as one would do with most other cacti species without damage.
Propagation by cuttings or offsets is also quite difficult, with Ariocarpus retusus subsp. trigonus taking up to two years to produce roots from cuttings and often failing to grow at all from such procedures.
While it can be propagated in this manner, it is much easier and faster to propagate by seed than by these other methods which are of only moderate success.
Propagation by grafting is the most widely used method for Ariocarpus retusus subsp. trigonus. Grafting can be done as stick grafts, air layers, or even combined with other methods of propagation to increase the odds of success.
For stick grafts, scions should be cut in the early summer; cuttings should be taken in June or July. It is important to leave a portion of the fat taproot on the cutting, and care must be taken not to damage it.
The scions are then allowed to dry for a few days at room temperature before being inserted into pre-bored holes in host stock (the grafted plant). The scions should be inserted so that at least 1/2 to 1 inch of the fat taproot is buried in the ground.
General care information
Ariocarpus retusus subsp. trigonus is a winter grower though light levels should be lowered in winter months to simulate autumn conditions, this plant can also tolerate very low light conditions and has been observed growing under artificial illumination with no supplemental lighting (grow lights) for 4 hours a day.
This species needs as much light as possible without burning the plant and will tolerate full sun for short periods of time with few negative effects.
Ariocarpus retusus subsp. trigonus requires porous soil with excellent drainage. This species will benefit from the addition of small, clean river rocks to the soil mix to reduce the likelihood of root rot due to excess compacted material or standing water.
In well-draining soil, this species will grow tall and narrow. To encourage wider growth, include a small amount of sphagnum moss or perlite in the soil mix. Ariocarpus retusus subsp. trigonus can be successfully cultivated in regular potting soils in pots or plant tanks when appropriate drainage holes are included.
Ariocarpus retusus subsp. trigonus requires good drainage and should be watered with care to prevent overwatering and root rot. This species dislikes standing water and will likely rot in containers that collect run-off water or are left sitting in a saucer of any type after watering.
However, Ariocarpus retusus subsp. trigonus also does poorly in container gardens where water remains stagnant, therefore great care must be taken when watering this species to ensure that drainage and aeration are adequate.
Temperature and humidity
Temperature: In Winter, temperatures can drop no lower than 70°F (21°C) if the plant is under artificial light and if the plant has been allowed to become root-bound. If temperatures drop below this threshold, decrease watering frequency.
This species will tolerate overnight lows of 50°F (10°C) for short periods of time and can also endure extended cold weather due to its high tolerance for drought once roots aerate properly.
Ariocarpus retusus subsp. trigonus does not require high humidity, though it will tolerate more than other species in the genus to varying degrees of success. Species that require higher humidity are recommended for terrarium culture with their own unique watering requirements and temperature preferences.
In summer, temperatures can reach as high as 95°F (35°C).
Fertilizer and soil
Ariocarpus retusus subsp. trigonus is a fast-growing species that benefit from regular fertilization. A high-nitrogen, well-balanced fertilizer such as 20-20-20 or similar should be used at the rate of one teaspoon per gallon (5ml per L) every watering or weekly.
The addition of small amounts of clean river rocks (1-2 mm) to the soil can greatly reduce the likelihood of root rot due to excess compacted materials that would inhibit proper drainage. The addition of sphagnum moss, in small quantities, will also aid in improving soil aeration and drainage.
Pests and diseases
Although Ariocarpus retusus subsp. trigonus is generally considered pest and disease-free, the rare reports of mealybugs or spider mites can be controlled with a systemic insecticide containing imidacloprid mixed into the soil at the time of transplant.
Aphids are effectively controlled by spraying with water from the base of the plant.