Although cerei are not the most popular cacti for restricted by cultivation in glasshouses, almost all collections have at least one representative of the beautiful genus Espostoa. Most species are easy to cultivate, and the hairy stems of many species make them an attractive addition to any collection, even in a small glasshouse. They are slower growing in pots than many cerei - a factor which also makes them good show plants, and members of this genus are frequently seen winning the cereus class. For exhibitors, those species with stems covered with white hair seem to particularly catch the judge's eye, and the hair can help to hide minor blemishes on the stem which could otherwise he seen by the judge, and which would not he viewed favourably.
In keeping with the fashionable trend towards the broader concept of genera, Espostoa has been expanded in recent years to include species previously included in three genera erected by Backeberg: Pseudoespostoa, Thrixanthocereus and Vatricania.
The flowering of espostoas is rarely seen in northern European collections, although I have seen a few mature specimens in large glasshouses in Holland and Belgium. Espostoa flowers are usually produced from a lateral cephalium, or at least from modified areoles on the upper parts of the stem. In some species the cephalium sits in a deep groove which extends down from the growing tip of the stem and increases in length as the plant grows. There can, occasionally, be two such cephalia on opposite sides of the same stem. It has been reported that the cephalia always grow on the side of the stem with a particular orientation, but I have observed wild populations with cephalia facing in various directions on the same plant.
I was encouraged to write this article after making a trip to Peru, where I saw many of the species in their natural habitat. Their range in the wild extends from central Peru northwards into southern Ecuador (see map), and one species, Espostoa (Vatricania) guentheri occurs in Bolivia. The plants are spectacular, often covering a hillside, or forming a cactus forest with other species, and the white, hairy species are particularly splendid. They are the only cephalium-bearing cereoid plants in Peru, inhabiting the coastal valleys and the warmer inland valleys to the east of the main Andean cordillera.
The genus Espostoa was erected by Britton and Rose in volume II of their book, The Cactaceae, in 1920. It was named for Nicolas E. Esposto, a botanist connected with the National School of Agriculture in Lima. They designated Cactus lanatus Humboldt, Bonpland & Kunth (described in 1823) as the type; it was the only species in their new genus. Their description and illustrations, however, make it clear that they included the plant we now know as E.melanostele within their concept of E.lanata. They did not realize that Cephalocercus melanostele Vaupel (described in 1913) belonged to their genus Espostoa, and instead made it the type of another new genus Binghamia, together with a second species, Binghamia acrantha (now Haageocereus acranthus). I imagine that they came to this conclusion because they thought Vaupel's description referred to a plant we would now call a Haageocereus, rather than E.melanostele which grows with it.
The easterly occurring espostoas are generally more cold sensitive in cultivation, particularly so the species which live at low altitude in the deep valleys, where the rivers enter the forest zone. Here one finds the remarkable and little known Espostoa calva Ritter which is hairless; before it grows its typical Espostoa cephalium it is difficult to recognize as a member of this genus (see Fig.4).
Like so many genera, the number of species and varietal names erected in Espostoa is far greater than a conservative view of the plants would support. In consulting the literature to try to put names to the plants I had seen in Peru, I was firstly surprised to find how little had been published in English and secondly, how many different names had been applied by subsequent authors to what appeared to me to be the same plant from the same place! My attempt at rationalizing this situation is set out below, but I am left with the feeling that there is much which remains unknown about the relationships of Espostoa species.
The terrain and the few roads in these parts of Peru mean that we can only sample populations in the few places which are accessible, and are unable to determine the extent to which these populations link up. For instance, in the river Maranon gorge and its tributaries, it appears that if the altitude is right then you will find an Espostoa, a fact which makes me suspect that they colonize just about all the river valleys in the area. It could be that there is just one species exhibiting variability great enough to cover all the published forms from the currently accessible locations in the area.
The flowers of Espostoa are nocturnal, usually white, and are borne over the length of the cephalium. E.(Thrixanthocereus) senilis has purple-pink nocturnal flowers; I wonder why this species has evolved flowers of this colour? The berry-like fruits remain largely buried in the wool of the cephalium, eventually splitting to reveal the black seeds. These berries are reported to be sweet and are eaten by the local people.
In cultivation in northern Europe, espostoas enjoy a full sun location with plenty of root room and generous watering in the summer. The species from the warmer, low altitude habitats are sensitive to cold and should be kept at a minimum of 100C. Such species include E.mirabilis, E.blossfeldiorum, E.senilis, and E.calva. Young plants of E.melanostele and E.lanata are particularly attractive, their 'cotton wool' covering making them look like shaving brushes. These two species are also amongst the slowest growing, making them particularly suitable for showing. A well grown E.melanostele will be about 25cm tall after 10 years.
A description of the species follows. I have rationalized the list by including some names under previously described species. In some cases, these synonyms are recognizable forms which may have horticultural value and so be worth retaining as names in your collection. I have not listed all the old synonyms for each species, but these can be found by consulting volume 4 of Ritter's Kakteen in Südamerika.
Following its discovery by Blossfeld, this distinctive plant was tentatively described by Werdermann in 1937 as a Cephalocereus but almost immediately Backeberg erected the new genus Thrixanthocereus to accommodate it because of its hairy flower tubes. In 1959 Buxhaum explained his reasons for transferring it to Espostoa and made the combination in Krainz's Die Kakteen.
It is usually solitary, growing to about 3 metres tall. The cephalium is not in a deep groove as in other espostoas, but is composed of a slightly depressed zone of bristles and hair on one side of the stem. The young plants have a characteristic growth of long, fine bristles at the base of the stem, a feature also seen in E.mirabilis. The flowers are nocturnal and white, sometimes appearing simultaneously in large numbers, and remaining open for a time the following morning.
All the reported habitats are in the Maranon river system, and the type locality is Huancabamba. As I mentioned before, the reported populations are where roads give access to the valleys, so it is more than likely that this species grows in suitable places throughout the whole river system between the reported localities. I saw it near Chamaya, and also further south at Balsas, while at Puente Crisnejas I saw the closely related Thrixanthocereus cullmanianus. This form was described by Ritter in 1961. It is not as large as E.blossfeldiorum, so will flower at a smaller size in cultivation. Ritter also described Thrixanthocereus longispinus from even further south at El Chagual - this is a plant that I have never seen, but which appears to be another form of this species. It was interesting to note that wherever I saw this plant in habitat, it grew only on shallow ground over rocks, for instance on rocky outcrops, where it grew in large numbers. There were none to be found in the surrounding areas where other cacti were growing.
All these forms make fine specimens in the glasshouse. If you can keep them above 100C and give them good light and plenty of water in the summer, then they will grow surprisingly quickly and flowering is certainly possible, particularly with T.cullmannianus.
E.calva makes a tree up to 9m high, the tallest in the genus. In places it makes dense stands on the steep rocky slopes of the valley sides. At lower altitudes it shares the hillsides with other cacti including a species of Cleistocactus (Clistanthocereus) and Browningia altissima.
This is a rare plant in cultivation. Young individuals look like Weberbauerocereus and would certainly look out of place in an Espostoa class on the show bench. I first saw top cuts of this at DeHerdt's nursery in Belgium in the early 1970s. It was described as "species Utcabamba" and a few seedlings came into cultivation. Seed of what I believe is the same plant was distributed as Thrixanthocereus jelinkyanus nom. nud. KK282. The seedlings certainly look very similar to those of the DeHerdt plant.
This species looks like E.melanostele, branching from the base and growing up to 2m high. It differs from E.melanostele in not having small, shiny black seeds - instead they are larger and matt black like those of E.lanata. The type locality is at San Francisco on the road from Santa Isabel to Pasaje in southern Ecuador. In cultivation young plants look just like E.melanostele, although as yet they are rarely seen in collections. Perhaps because this part of Ecuador is not often visited by cactus enthusiasts, I have not yet seen seed offered commercially.
This plant was found by Prof. Dr. C. Troll in 1927 and described as Cephalocereus Güntheri by Kupper in Monatsschrift der DKG in 1931. Backeberg erected the genus Vatricania in 1950 for this one species on account of the form of its flower and cephalium, but Buxhaum later included it in Espostoa. It is found in the valley of the Rio Grande, near El Oro, Chuquisaca, making it the only Espostoa to grow in Bolivia.
The attractive golden-spined stern grows in habitat to about 2m high and will branch in age. The characteristic cephalium, which is a zone of long, fine spines, first occurs superficially on one side of the stem but eventually grows to completely encircle the top of the plant. This can he achieved in a glasshouse but takes many years since the plant tends to he slow growing in cultivation in northern Europe. Its warm habitat means that a minimum of 10oC should be maintained to avoid marking of the stem. There is a large, mature specimen growing outside at Grigsby Cactus Gardens at Vista, California; it is over 3m tall and multi-stemmed (see Fig.5).
It is rarely seen in cultivation, but I photographed a plant in the DeHerdt collection early in the 1970s which had retained its thin stems, and my seedlings are also similarly slender.
There have been many names erected for forms of this widespread species, some old, but many in recent times. Some of the species I have listed separately here could probably be included in E.lanata, but certainly some synonyms are E.laticornua Rauh. & Backbg.,E.procera Rauh & Backbg., E.ritteri Buin., E.sericata Backbg., and E.superba Ritt.
This is one of the largest espostoas, said to grow up to 7m high, with numerous branches. These branches usually appear from the upper part of the main trunk, giving it a tree-like appearance. Near to Bagria I saw the densest cactus forest I have ever seen anywhere. It consisted of Browningia altissima, Armatocereus rauhii and Espostoa lanata -- the Browningia and Espostoa having a similarly branched, tree-like appearance.
In cultivation this is one of the most frequently grown species. It needs plenty of root space so that it can grow properly and if planted in free root run conditions it can grow several inches a year and make an impressive specimen, particularly if it forms branches. It usually has to be over 2m tall before a cephalium is formed and I have seen such specimens in European glasshouses. Seed is freely available and is easily grown.
I saw it at its type locality at Puente Crisnejas where the road crosses the Crisnejas river, quite near to where it joins the Marañon, along which so many espostoas can be found. Plants I saw on the hills further north, above Balsas, looked to me to be a form of E.lanata, but Ritter states that the plants growing there are E.lanianuligera.
At Puente Crisnejas I was puzzled by the extensive damage done to the stems by human activity, but I have since read in Ritter's book that the local people cut down the stems to harvest the wool from the cephalia. This activity is also reported by Madsen in connection with E.lanata in Ecuador, and he says that it results in few plants reaching their full, potential size.
E.melanostele grows to 2m high, branching freely from the base to form a cluster of stems. It is found in the river valleys of western Peru which drain into the Pacific Ocean, from about 800m to over 2,000m altitude. It has a wide distribution range from the Pisco valley in the south to the valley of the Rio Sa&ntild;a in the north.
This is one of the most popular species in cultivation, the seedlings being particularly attractive with their covering of long white hair, and as I mentioned earlier, they are often seen on the show bench. Seed is always available and raising plants from seed is easy in an open soil and good light. I cannot recall seeing this species flowering in a northern European glasshouse, but it earns its place without flowers.
Balsas is a hot place -- which gives a clue to this plant's intolerance of low temperatures in cultivation. Seed is often available and the resulting seedlings hardly look like espostoas at all but, interestingly, they do grow very long spines at the base of the stem just like Espostoa (Thrixanthocereus) blossfeldiorum with which it shares the lower part of its habitat. I saw a 2 metre tall plant with a cephalium nearly 1 metre long in a collection in Holland many years ago. The owner told me that it flowered almost every day of the year, and indeed it was in flower when I saw it, even though it was a dull, wet day.
Interestingly, this plant was possibly mentioned by Britton and Rose in their description of the genus Espostoa, where they speculate that a member of the new genus might occur at Chagual, having seen a photograph taken there.
In cultivation E.nana is indistinguishable from E.melanostele, except perhaps that it is slower growing. Its extremely dense covering of hair makes this a very attractive plant for our collections, but I am unaware of any plants flowering in northern European glasshouses.
Espostoa ruficeps was first offered as seed in the Winter catalogue of 1957, although Ritter admitted to some confusion in later editions of the catalogue with his E.lanianuligera which he first treated as a variety of E.ruficeps. Plants of this species, which is probably a form of E.lanata are rare in cultivation. The central spines are described as red or sometimes yellow, so it would probably be an attractive form to grow.
From even further to the south, Ritter described another species, E.huanucoensis from near to Huanuco, which is in the next river system to the east. Here it grows in extensive stands on the hillsides, as pictured in his book (1981). This is another relative of E.lanata but since I have not seen it, I cannot make any further observations.
This outstanding plant was described in 1961 by Ritter in Kakteen und andere Sukkulenten as Thrixanthocereus senilis. It was the first new Species to be found since the erection of the genus in 1937 by Backeberg for Thrixanthocerus blossfeldiorum and it remains the only other distinct species.
The stems, which grow up to 4m high, are branched and densely covered in white spines, making this one of the finest species for cultivation. The cephalium can be formed in cultivation when the plant is about 1m high, from which the unusual deep pink nocturnal flowers are produced. It is slower growing than E.blossfeldiorum but flowering specimens are often seen in European glasshouses. I have had a few flowers on one of my plants which is about 1m high.
This species is reported from only a few localities in Ancash and La Libertad. I saw it near Rahoapampa, growing on steep slopes among quite dense low vegetation following an unusually wet rainy season (see Fig.11). Quite a number of the largest stems were dying back from the tips, but new vigorous branches were growing from lower down. Only a few plants had cephalia but (in April) I found no flowers or fruits. It is unfortunate that seed is only occasionally offered for sale but I recommend you it grow it if you get the chance.