I am neither an expert on Pterocactus nor a botanist. However in my humble opinion, derived from my observations and experiences in growing plants of the species, I would like to pass out a few comments for the benefit of those who want to read about them. I must warn those Buffs and Taxonomists, that I am not re-writing their books, so please do not jump down my throat.
I believe there are two kinds of Pterocactus. The first one has long branches, arms, tentacles call them what you like. These grow in various lengths and thickness from a stem just above ground, supported by a tuber somewhat deeper in the soil. The flowers appear at the end of the arms during the late summer. Plants appear under various names such as tuberosus (fig. 1), kuntzei and decipiens, but they look the same to me, apart from a variation in the length and thickness of the arms and a slight difference in the colour shade of the flowers. Of course there are intermediates such as megliolii and gonjianii with very thick arms. They are not self pollinating and therefore can not reproduce themselves. Seedlings can look different from the parents and therefore, should one need to maintain the clone, one must resort to vegetative propagation.
Fig. 1. Pterocactus tuberosus (Pfeiffer) Britton & Rose
When growing from seed one can wait for four to five years before any growth becomes noticeable and therefore a vegetative propagation is a much easier option. Branches can be removed as they detach easily from the stem at the end of the growing season. I myself prune the plant completely after flowering in the autumn by cutting the branches about one centimetre away from the stem, so avoiding any damage to the epidermis. I then select the most vigorous and turgid bits and lay them flat to heal. One notices in a little while that the cuttings thicken, shorten and start to curl. They must then be planted in loose sandy soil and for the next three to four years not much will happen as they are growing the tuber underneath. One should avoid putting any stones in the soil, if one wants the tubers to be beautiful round balls. When the cuttings start growing new arms, they should be pruned to allow the strengthening of the stem and the enlargement of the tuber.
I grow my plants in plastic tall toms and use a mixture of half John Innes and half coarse sand, fine stone chippings, perlite and fine Cornish grit. I never plant in a dry soil mixture and then I leave the plants to acclimatise. Cuttings and growing plants are planted in moist soil and left to settle for some weeks before watering. The soil is initially prepared, watered and then turned over several times in order to lose the wetness and only when it is loose and moist is it used for planting cuttings into it.
The plants need re-potting every three to four years especially when the pots start distorting due to the pressure from the growing tuber. When eventually the plant fills an eight to ten inch pot it is time to expose the tuber and prepare the plant for the show. Expose two thirds of the tuber and cut back the stem to the required length. One notices that the tuber is a little rough on the outside and the roots are still live on the showing part of the epidermis. There is no need to worry, as within a year the epidermis will harden up and the surface exposed will become clear, shiny and very attractive. The drawback to this is that the growth slows down to about one millimetre every couple of years. Therefore the tuber growth is minimal and if forced by over watering, it will cause the tuber to split and subsequently to rot. People say sometimes that they had their plant for twenty five or thirty years and that during this time it hardly grew. It must therefore be a slow growing plant. I say these plants are not slow growing, because if the tuber is buried, watered, fed and re-potted frequently, then a twenty centimetre tuber can be attained within twelve years.
The second kind of Pterocactus has spherical or oval branches and a shorter thicker stems on a tuber growing just below the surface. Plants appear under various names such as australis (fig. 2), fischeri (fig. 3) and hickenii. The round and oval arms grow in clusters and form beautiful mats above ground. Of course there are intermediates such as reticulatus characterised by its very thick stem and special areoles. In my opinion this group is the better kind by far. They keep within the boundaries of their pots forming beautiful, aesthetic and structural mounds, they flower profusely and need very little yearly maintenance. I also value them very highly on the show table.
Fig. 2 Pterocactus australis (Weber) Backeberg
Fig. 3 Pterocactus fischeri (FK93-238-722) from Buta Ranquil, Neuquen Province, Argentina
(Photograph by Alan James)
Pterocactus require bright light and plenty of direct sunshine, if they are to grow well and flower. They also need watering during the growing season. Left in shaded places, they etiolate i.e. growing long, thin and unsightly arms. One should remember that the tuber produces the arms and the arms, through photosynthesis, feed the tuber back, so helping it to grow in size and vigour. Good growth each year is vital in order to maintain a healthy tuber, which in return will throw up good enough shoots to flower. Flowers are only produced from the new yearly growth. If kept dry in the winter the plants can easily survive very low temperatures. Some eight years ago I had propagated many of them thus occupying precious space in the greenhouse. With no one showing any interest in them I had no choice but to get rid of them. May be this article will rekindle my interest in them and I will start all over again.
I am leaving the taxonomy of the species to the experts. There is plenty of literature on the genus, conveying lots of information on their country of origin (Argentina), habitat, terrain and locations.
Have fun in growing them and they in turn will award you with a cascade of growth and flowers or a mound of yellow blooms.
This article was originally published in Tephrocactus Study Group (the 'TSG Journal'), June 2011, Vol. 17, No. 2, pages 28-30. © TSG and Costas Papathanasiou