Origin and Habitat: Cleistocactus baumanniiSN|4742]]SN|4742]] subs. horstii occurs at the border of the Pantanal swamps near Porto Murtinho (in the lowlands of Rio Amoguiy), south-west of the federal state of Mato Grosso do Sul, Brazil.
Altitude range. 100–200 metres above sea level.
Habitat and ecology: This species grows in the boundary region between the Pantanal and the eastern reaches of Chaco. Pantanal is a huge marshy area extending from western Mato Grosso to Bolivia. Large areas are flooded every year, with dry areas only located above the grasslands and cerrados. Cleistocactus baumanniiSN|4742]]SN|4742]] subs. horstii grows together with the palm Copernicia in dry forests. It creeps among other vegetation mostly at the edges of the hummocks (floodplain) between the vegetation of the wooded islands that stands just above the high water level of the rainy season. The plants, that are characterized by pendent growth, occasionally hang down from the tops termite hills. The average annual temperature is between 24 and 25 ° C, the average annual rainfall is slightly more than 1000 mm. Gravely sites and crystalline rocks are the preferred place for the growth of cacti.

• Cleistocactus baumannii subs. horstii (P.J.Braun) N.P.Taylor
• Cleistocactus horstii P.J.Braun

• Cleistocactus baumannii (Lem.) Lem.
• Aporocactus baumannii Lem.
• Cereus baumannii Lem.
• Cereus flavispinus (Colla) Haw. ex Steud.
• Cactus flavispinus Colla
• Cereus flavispinus var. hexagonus Salm-Dyck
• Cereus subtortuosus C.F.Först.
• Cereus tweediei Hook.
• Cleistocactus aureispinus Frič non D.R.Hunt
• Cleistocactus baumannii var. colubrinus (Otto ex C.F.Först.) Riccobono
• Aporocactus colubrinus (Otto ex Salm-Dyck) Lem.
• Cereus baumannii var. colubrinus K.Schum.
• Cereus colubrinus Otto ex Förster
• Cleistocactus colubrinus (Otto ex Förster) Lem.
• Cleistocactus baumannii var. flavispinus (Salm-Dyck) Riccobono
• Cereus baumannii var. flavispinus K.Schum.
• Cereus colubrinus var. flavispinus Salm-Dyck
• Cleistocactus colubrinus var. flavispinus (Salm-Dyck) Borg
• Cleistocactus flavispinus (K.Schum.) Backeb.
• Cleistocactus baumannii var. paraguariensis (F.Ritter) P.J.Braun & Esteves
• Cleistocactus paraguariensis F.Ritter
• Cleistocactus bruneispinus Backeb.
• Cleistocactus jugatiflorus Backeb.

• Cleistocactus baumannii subs. anguinus (Gürke) P.J.Braun & Esteves
• Cereus anguinus Gürke
• Cleistocactus anguinus (Gürke) Britton & Rose

• Cleistocactus baumannii subs. chacoanus (F.Ritter) P.J.Braun & Esteves
• Cleistocactus chacoanus F.Ritter
• Cleistocactus margaritanus hort.
• Bolivicereus margaritanus F.Ritter
• Cleistocactus margaritanus f. cristatus hort.
• Bolivicereus margaritanus f. cristatus hort.

• Cleistocactus baumannii subs. croceiflorus (F.Ritter) P.J.Braun & Esteves
• Cleistocactus croceiflorus F.Ritter

• Cleistocactus baumannii subs. horstii (P.J.Braun) N.P.Taylor
• Cleistocactus horstii P.J.Braun

• Cleistocactus baumannii subs. santacruzensis (Backeb.) Mottram
• Cleistocactus chacoanus var. santacruzensis F.Ritter
• Cleistocactus santacruzensis Backeb.

Description: Cleistocactus baumanniiSN|4742]]SN|4742]] subsp. horstii is one of the morphological or local forms of Cleistocactus baumanniiSN|4742]]SN|4742]]. It is a shrubby cactus, branching basally with thin and flexible branches, that scramble on the ground forming tangled clumps to 1 m tall and 2-4 metres in spread, or works its way through shrub branches. The stems are fairly stiff, to 2 m or more long, less than 2,3 cm in diameter, with about 10-13 ribs, 11–15 radial spines, to 5 mm long, yellowish to dark brown, bright tipped and 1 central spine to 2,5 cm long. The flowers are 5–7 cm long, zygomorphic orange-red. The red fruits have a white flesh.
Derivation of specific name: This member of the Cactaceae family was given this name in honour or Leopoldo Horst (1918–1987), German-born Brazilian cactus collector and exporter in Rio Grande do Sul.
Roots: Highly branched.
Stems: At first erect, then arching or creeping, but the tip of the shoot turned upwards (decumbent). Branches light green, 12-23 mm diameter, up to 200 cm in length.
Ribs: 10-13, rounded, flattened, with wavy grooves and often divided in elongated octagonal tubercles, one for each areola. In turgid mature stems these ribs are very flat and barely visible, less than 0.8 mm in height, but in time of drought, the ribs appears wrinkled up to 1.5 mm hight and 2-4 mm wide.
Areoles: 1-1.5 mm long and 1 mm wide, oval, with dark hairs, later becoming lighter, 5-7 mm apart.
Radial spines: 11-15, 2-5 (or more) mm long, variable in thickness and strength, but always very thin, needle-like, brittle, white and grey.
Central spines: Typically 4, arranged cross-wise (placed at right-angles) , thickened at the base; the strongest directed upwards, all light brown to grey-brown, 3 mm long, rarely up to 10 mm in length (12-25 mm long in flowering areoles, the upper spine often up to 30 mm in length).
Flowers: Strongly zygomorphic, double-curved, 7-8.5 cm long, in throat about 1.5 cm wide, in the middle of receptacle 1.3 cm wide, and only 7 mm wide just above pericarpel. Pericarpel almost round, pale brownish-red, 9 mm wide, 7 mm long, grooved, strongly covered with scales; the scales at the base c. 1 mm long, pointed, fleshy, pink, with very thin curly hair 3-5 mm long in the axils. Receptacle tube pink-red, strongly bent upwards at a distance of about 7 mm from the pericarpel, furrowed, with, pink-brownish scales, thickened at the base, and bright red above, about 1.8-3 mm long and 1.8-3 mm wide, with longer white hair in the axils. Perianth segments in 4-5 whorls, the lowest segments sharply curved up, bright red with thin, translucent edges, inner segments wider and more rounded at the top, all the segments with reddish veins and slightly thickened middle rib. The tip of the inner segments is usually bent outwards. Outer segments thinner 5-12 mm long and c.3 mm wide, inner segments gradually enlarging, c. 12 mm long and 6 mm wide. Basal filaments 3.8-4.3 cm long, forming a circle at the base of the tube and fused lengthwise along the receptacle. Upper filaments up to 3 cm long attached at different distances up to about the middle of receptacle. The upper part of the flower tube is strongly grooved due adherent filaments, which are released only at the edge of the flower. All filaments glassy white below, and pink to purple above. Anthers purple, 1-2 mm long and 0.5-1 mm wide, but covered by white pollen grains. All filaments protrude from the flower tube at different levels (up to 1.5 cm). Style 6-7 cm long and 1 mm wide, yellow-white, not fused to the tube; stigmas lobes 5-6 yellow-green, 3-4 mm long, 0.2 mm thick. Flowers are autosterile and remain open for a few days and nights.
Fruit: Spherical, slightly elongated above 17-19 mm in diameter, smooth and shiny, slightly ribbed only at the very top, light pink, darkening above and whitish at the base of the dry flowers remnants. Dry perianth with white tufts of hair. Seeds buried in thick white fruit pulp. Young fruits are dark brown, and then become wine-red and strongly grooved, finally at the end of the maturation process, will take a bluish-purple colour. After that, the fruit becomes turgid, smooth pink and falls off.
Seeds: 1.1-1.5 x 1.1 mm, rusty-brown to black-brown, shiny, ovoid to helmet shaped, obliquely flattened.

Bibliography: Major references and further lectures
1) Pierre Braun. “Cleistocactus horstii P. J. Braun. Eine neue Art aus dem Sumpfgebiet Pantanal - Mato Grosso do Sul – Brasilien.” in KuaS, 16: 2б4-2б9. 1982
2) Urs Eggli, “Etymological Dictionary of Succulent Plant Names” Leonard E. Newton
3) Walther Haage “Kakteen von A bis Z” Neumann Verlag, 1981
4) Cleistocactus horstii in Cacti collection-Holzheu www.Kakteensammlung-Holzheu.de
5) David Hunt, Nigel Taylor “The New Cactus Lexicon” DH Books, 2006
6) Edward F. Anderson “The Cactus Family” Timber Press, 2001
7) Oakley, L. & Pin, A. 2013. Cleistocactus baumannii. The IUCN Red List of Threatened Species. Version 2014.2. <www.iucnredlist.org>. Downloaded on 07 August 2014

Cultivation and Propagation: Cleistocactus baumanniiSN|4742]]SN|4742]] subs. horstii is a a much decorative hardy cactus.
Growth rate: This species grows rapidly enough to need pruning.
Soil: Grow it in a rich and particularly draining substratum, as it is very sensitive to rottenness when in presence of humidity and low temperatures and let the soil dry out between waterings.
Water: In summer, during the vegetative period, it must be regularly watered, but allowing the substratum to completely dry up before irrigating again; in winter, it’s to be kept dry. Preferable not to water on overcast days, humid days or cold winter days.
Hardiness: This is one of hardiest of the woolly columnar cacti from Bolivia that grow at an altitude of up to 1000 meters. It is a frost resistant cactus, hardy to -7°(or even less if very dry). However in cultivation it is better not to expose it to temperatures lower than 0° C, even if in an aerated and protected location, in order to avoid the formation of anti-aesthetic spots on the epidermis. In presence of high atmospheric humidity avoid any frost as it is particularly sensitive to root rot.
Exposure: It need full sun exposures with ample airflow to produce dense hairs, but, as a former mountain dweller, does not care for extremely high temperatures in summer.
Maintenance: Repot in the spring, when their roots become cramped. Generally, they should be repotted every other year in order to provide fresh soil. After repotting, do not water for a week or more.
Use: Excellent as landscape or patio plant. It is suitable for small “desert” gardens, in association with other xerophytes. Where the open air cultivation is not possible due to the climate, it is to be cultivated in pot in order to shelter it in winter.
Propagation: By seeds and by cuttings, provided left drying up well, in summer.