by Bruce Brethauer
I first became interested in growing members of the Pterocactus genus when I was making experiments to test the cold hardiness of many different cactus species. While these are remarkably cold hardy plants - to about 0 degrees Fahrenheit and colder in several species - I eventually had to give up on them as garden plants here in Ohio - they were not quite cold and moisture tolerant enough. Nevertheless, I have continued to grow these plants, because their odd combinations of unusual growth habit, strange appearance, relative ease of cultivation, and distinctive flowers have endeared these plants to me. The species of Pterocactus are native to the western provinces of Argentina - primarily within Patagonia, and typically inhabit dry, rocky grasslands at high elevations. They often grow in areas exposed to temperature extremes, with temperatures soaring by day in summer, and plunging to nearly freezing and sub freezing temperatures at night and in winter. They often grow in exposed situations where they may be buffeted by high winds, and very high solar radiation. These are all rugged plants adapted to a very rugged environment. And while some of their other cactus cousins in similar regions - the Tephrocactus, Austrocylindropuntia etc. have often developed heavily spined stems as defenses against the elements and grazing animals, the pterocacti typically produce comparatively small, bristly spines, and have adopted a strategy of growing underground to survive in such a challenging environment.
Some of the traits of this family are distinct, and are regarded as primitive by some taxonomists. This species has terminal flowers, - that is, the flowers are produced at the ends of the stems, and no additional growth can be produced at the ends of these stems once they have flowered, and the ovaries of the flowers are actually imbedded within the stems. Traits which distinguish plants of this genus from other genera of their "opuntiod" cousins.
Pterocactus decipiens produce odd tapering cylindrical stems measuring (on my plants) to about 1/2 inch in diameter, and to a length up to 10 inches (these may eventually grow longer still in cultivated plants). The size of the stems produced on plants in cultivation may be atypical, and may reflect excessive growth due to the comparatively moderate conditions under cultivation: habitat photographs typically show plants with very short stems - usually to no more than a few inches in length. The stems initially grow upright, but will eventually become scrambling as they either grow too large for the plant to support their weight, or when these stems becomes flaccid in response to periods of drought or other stresses. The stems of plants in habitat tend to be deciduous, and are typically shed in response to extreme cold, drought or other environmental stresses. The stems of this species are of an unusual purplish to brownish green coloration, and lack tubercles entirely.
Spines are very short, and bristle-like: Radial spines lie almost flat against the stems: some areoles produce a single very short bristle-like central spine. The spines are so few and short, that this cactus seems virtually spineless, a trait which may endear it to those growers who are not especially fond of the spinier cacti.
Plants produce immense tuberous roots, these are usually long and tapering, in most plants, growing to about 1 inch across and to about 2 inches or so in length, but Britton and Rose illustrate a plant in their The Cactaceae with a very large beet-like root, which grew to over 3 inches in diameter and to about 5 inches in length. Some growers choose to raise the roots of this species in cultivation to show them off. In times of extreme drought, and possibly in times of exceptional cold, the aerial stems may become deciduous, dehiscing from the roots to conserve moisture. When conditions become more favorable, the roots will grow new stems to replace the old ones. This is very likely an adaptation to a very harsh environment: in habitat, plants are probably subjected to extreme daily fluctuations in temperature; below the soil, the roots are insulated from these daily temperature extremes. In habitat, plants are also likely to experience frequent frosts, and are occasionally subject to hard freezes.
The flowers of this species are rather small (from about 1 to perhaps 2 inches tall and across), but are attractive with inner petals which are of a pale lemon yellow, and outer petals of a more brownish to coppery bronze/yellow color. The stigma lobes are of a vivid vermillion, adding a striking highlight to the flowers of this species. The flowers are borne at the ends of the stems, with the fruits being imbedded within the stems, a trait which distinguishes Pterocactus from other cactus genera.
This species is fairly easy to grow in cultivation provided that it is given a very porous potting medium, that it receives very intense illumination during its growing season, and is kept cool and dry through the winter months. Due to its large tuberous roots, this species can be fairly susceptible to rot if plants are subjected to perpetually moist conditions - particularly when temperatures are cooler. For this reason, it is best to provide this plant with a very porous potting mix which does not retain excessive moisture. I incorporate a significant percentage of pea gravel to my potting mix, and give my plants a 2 to 3 inch top dressing of pea gravel to keep the aerial stems dry. Also, it is best to grow this plant outdoors during the warmer months to benefit from direct sunlight: this species typically grows at higher elevations where the solar radiation is intense, and with a significantly higher percentage of ultraviolet radiation - once a plant has become properly acclimatized to direct exposure to sunlight, they will thrive when provided with maximum exposure to sun and seem to be immune to scorching. In winter, it may be best to maintain very bright illumination to maintain compact growth. Stems can grow significantly during the summer months - especially when provided with regular watering, and frequent fertilization - but remember that plants in habitat tend to produce much of their growth underground, producing ever larger tubers. In the wild, the aerial stems are typically deciduous, and are shed in response to extreme stress, be it drought, extreme heat, cold, or any combination of the above. Some of the plants which I tested in my unheated greenhouse, to determine their tolerance to our cold winter temperatures, lost all of their aerial stems, but some of these retained their underground tubers and stems, and re-grew the lost stems once temperatures returned to warmer levels. Also remember that because this plant can be exposed to very cool temperatures during its growing season in habitat, cooler temperatures alone will not induce or maintain dormancy in cultivated plants, so keep these plants as cool and dry as possible in winter - most of my plants survived temperatures of 0 degrees Fahrenheit in my unheated greenhouse - but temperatures between 40 and 50 degrees would probably be ideal - also, keep this species particularly dry in winter - while it would probably survive without any watering through the winter months, I tend to give my plants a bit of water about once a month at this time. I really believe that such a cold and dry dormancy is essential for this species to produce flowers, but my plants have only flowered on a few occasions, and admittedly, I have pushed my plants to the very limits of their cold temperature tolerances - perhaps plants which are grown at more moderate temperatures would be more likely to flower in cultivation. It is also my impression is that this species does not require much fertilization, - I would recommend making several applications of a water soluble, low nitrogen fertilizer through the summer months, at about 1/4 the recommended application rate, and to withhold fertilizer altogether throughout the remainder of the year.
Unfortunately, plants of Pterocactus tuberosus are not frequently encountered in cactus collections, and are equally difficult to find in the trade. Aside from a general lack of demand for plants of this species, there is no reason why plants should be so hard to locate in the trade: I have found most species to be of easy culture, and can be easily propagated from cuttings and seed (on those rare occasions when seed are available). Where these plants are happy, growth is rapid, and plants will readily flower in the spring, following a very cool and dry winter dormancy.
I find this plant to be especially interesting, its flaccid, nearly spineless stems are a welcome contrast to the more viciously spined denizens of my hardy cactus collection. This species is also more compact, growing only to a few inches tall, and spreading (so far) to less than 12 inches, making plants suitable for smaller spaces. This is not the prettiest cactus plant, but its unusual, purplish brown, and virtually spineless stems give it a very distinctive appearance. And its flowers, with their brilliant vermillion stigma lobes are quite attractive. If you ever manage to find this plant for sale, give it a try - it will be one of the most distinctive plants in your cactus collection.