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Origin and Habitat: Pina County , southeast of Phoenix (around Kelvin and Sacaton), Arizona (Southwestern U.S.A., Northern America)
Altitude range: 500-1000 metres above sea level.
Habitat and ecology. Sonoran desert scrub, edges of grasslands, rocky flats and slopes, rolling hills. O x keivinensis from the lower population are in un-farmed flats between Sacaton Flats and Blackwater, where Opuntia fulgida may found as well. Because the plant reproduces by rooting of joints and fruits, this microspecies can be very abundant locally.
Description: Opuntia x kelvinensis is a natural intergeneric hybrid between Opuntia fulgida and Opuntia spinosior. It is trunked but intermediate in growth form, joint, size, and ease of detachtability. Spine length is intermediate too, but the spines are grey to pink. Flowers, variably coloured, are diurnal and many tepaled. The fruit is intermediate in colour, tubercle size, and depth of apical cavity. To complicate matters, there are two clones In Pima country: one somewhat more like one parent, the second more like the other. It is considered a clonal microspecies, that is to say it is a plant population that has a definite and narrow geographic range and that reproduces for the most part, if not exclusively, by vegetative propagation. Usually have no seeds in the fruits (rarely one or two seeds) and very little viable pollen. Because typical plants of Opuntia fulgida are diploid, but in part triploid (2n= 22 or 33) and typical Opuntia spinosior are diploid (2n = 22 ), when hybrids are formed between these two species, some peculiar genetic combinations result. For example, in a single population plants may be either diploids or triploids, but for the major part Opuntia x kelvinensis are triploid, with 33 chromosomes (that is to say they have 3 sets of chromosomes instead of the ordinary 2). The very high percentage of sterile seed produced by triploid O.× kelvinensis is almost certainly a consequence of unequal segregation of chromosomes in pollen mother cells during anaphase I. The ability of Opuntia x kelvinensis to reproduce vegetatively is attributable to its O. fulgida parentage.
Habit: Shrubby or tree-like 1.5-2 m 1-2 m tall, with a commonly branched trunk. The crown may be open to compact and usually bears whorled branches.
Stem segments: Often easily dislodged, if terminal, green or purple, 5-13 long, 1.8-3 cm in diameter.Tubercles prominent, broadly oval, 0.7-1.2(-1.5) cm long.
Areoles: Triangular to rhombic, 4.5-6 long, 3-5 mm broad, with yellow wool, ageing grey to black.
Spines: 0-8(-13) per areole, sometimes with 1-3 bristle-like spines along areole margins at most areoles, usually slightly interlacing with spines of adjacent areoles, pale-yellowish to tan, sometimes with pinkish hue, to red-brown and partially grey coated, aging brown-grey to grey. lowermost spines often angular-flattened, reflexed, the longest to 30 mm long. Uppermost spines straight, erect-divergent, the longest to 18 mm long. Sheaths uniformly whitish or tipped yellow to gold, slightly baggy.
Glochids: Inconspicuous, longer in the upper part of the areole, yellow, to 1 mm long.
Flowers: Variably coloured, diurnal and many tepaled. Inner perianth segment rose to magenta, spatulate, 18-25(-30) mm long, with a short, sharply pointed tip. Filaments deep rose. Anthers yellow; style white, tinged red-purple distally; stigma lobes white with hint of green.
Blooming season: Flowering spring (Apr-Jun).
Fruits: Usually sterile, sometimes forming short chains, yellow-green to yellow, sometimes purplish, 25-45 long, and 15-22 mm in diameter, fleshy, tuberculate, spineless; tubercles progressively longer toward fruit apex; umbilicus 4-7 mm deep; areoles 32-44.
Seeds: Pale yellow, rounded to angular or squarish in outline, flattened to warped, 3.5-4 mm long, 3-4 mm wide.
Bibliography: Major references and further lectures
1) Arthur C. Gibson, Park S. Nobel “The Cactus Primer” Harvard University Press, 1990
2) Marc A Baker, Jon P Rebman, Bruce D Parfitt, Donald J Pinkava, and Allan D Zimmerman “Chromosome Numbers in Some Cacti of Western North America-VIII” in Haseltonia, 15:117-134. 2009.
3) Baker MA, Pinkava DJ. “Cytological and morphometric analyses of a triploid apomict, Opuntia × kelvinensis (subgenus Cylindropuntia, Cactaceae).” Brittonia 39: 387–401 1987
4) “Cylindropuntia x kelvinensis” in Flora of North America @ efloras.org FNA Vol. 4 Page 103, 105, 107, 108, 112. retrieved 13 March 2016 from <http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=242415147>
6) Amadeo M. Rea “At the Desert's Green Edge: An Ethnobotany of the Gila River Pima” University of Arizona Press, 1997
Cultivation and Propagation: Cylindropuntia x kelvinensis is a a much decorative hardy cactus occasionally grown for horticultural purposes. It is a summer grower species that offers no cultivation difficulties. Its cylindrical densely spiny joints provide a striking accent for an otherwise dormant cactus garden.
Soil: Use a very a particularly draining substratum, as it is sensitive to rottenness when in presence of humidity and low temperatures and let the soil dry out between waterings, since it's natural habitat is in sandy or gravelly, well draining soils.
Repotting: 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.
Water: In summer, during the vegetative period, it must be regularly watered, but allowing the substratum to completely dry up before irrigating again (but do not overwater ); in winter, it’s to be kept dry. Preferable not to water on overcast days, humid days or cold winter days.
Hardiness: It is a quite frost resistant cactus, hardy to - 12° C (or 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. USDA Zone 7-10. It can handle extremely high temperatures in summer.
Exposure: Outside full sun or afternoon shade, inside needs bright light, and some direct sun.
Use: It is suitable for “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.
Warning: It is armed with treacherous spines that are extremely sharp (This is one of the most dangerous of all cactus). Handle it with extreme caution, and keep it away from gangways and areas frequented by children and animals. Spines must be meticulously removed with tweezers.
Propagation: Stem division. The stem segments root easily and grow rapidly when placed in loose, well-draining soil.
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