Origin and Habitat: Pediocactus knowltoniiSN|14412]]SN|14412]] is a rare, endemic cactus that is presently known to occur on a single hill in a very limited area in the northwestern part of San Juan County and Rio Arriba counties and Colorado's Archuleta County, New Mexico on the Colorado border (extent of occurrence and area of occupancy approximately 0.1 km2). There may be only 2 Knowlton's miniature cactus populations, the largest of which occurs in a 6 ha area south of La Boca, Colorado and the other occurs in nearby Reese Canyon. Type locality: 1 mile south of La Boca, Colorado.
Altitude range: ± 1800-2300 metres above sea level.
Habitat: Gravel pavements and hill sides on red-brown clay soils in pinyon-juniper (Pinus edulis-Juniperus scopulorum) woodlands and in mixed sagebrush (Artemisia spp.) shrublands often in sites where other vegetation is often lacking. The climateis characterized by little precipitation, extreme summer heat, low humidity, and very cold winters. Annual precipitation is approximately 300 mm, and most comes in the summer and winter months.
Ecology: It has a unique growth pattern that facilitates avoidance of desert droughts and high temperatures. Generally there are 2 growth periods in a year. In the early spring (April-May), plants grow, bloom, and set seed. From June to September, aboveground growth stops, and plants shrink into the ground. Summer dormancy is considered "very definite". Cultivated plants receiving supplemental water also experience summer dormancy. Root growth continues when aboveground plant parts recede into the ground. After late summer rains, it cactus grows again. "Offsets", or new stems, are often produced at this time. Often flower buds are produced in the fall; buds remain unopened through the winter. Seed dispersal: The limited distribution of and the dry fruits produced by Knowlton's miniature cactus suggest that wildlife species are unlikely dispersal agents. Dispersal is probably a result of water and wind movement that results in scattered high density colonies. It has suffered from drastic declines in the past and even though, for several decades the population increased, however more recently there has been unexplained declines and extreme fluctuations in the number of individuals, which are presumably related to droughts. The species is protected, but still it is affected by illegal collection. The current trend of the population suggests further declines in the next years. The main threats to this species are illegal collection, urban development and droughts.

• Pediocactus knowltonii L.D.Benson
• Pediocactus bradyi var. knowltonii (L.D.Benson) Backeb.
• Pediocactus simpsonii var. knowltonii (L.D.Benson) Halda

Description: Morphology (Identifying Characteristics): Very small plant (one of the world's smallest cactus) usually solitary or a few in a cluster (Clusters of up to 30 cylindrical stems are reported) barely protruding above ground level.
Stems: Globular to short cylindrical, 3.5-5,5 cm tall, 2.5-3 cm in diameter.
Tubercles: Not grooved.
Areoles: Circular or slightly elongate, At first woolly but become smooth with age.
Central spines: This species lacks central spines.
Radial spines: Smooth, relatively hard, pubescent, mostly 18-26 per areole, somewhat pectinate, radiating from areole and appressed to the plant reddish tan, pink, or white, 1-1.5 mm and darkest at tip.
Flowers: Funnel-shaped 1-3.5 cm long by 1-2.5 cm in diameter, fragrant, borne from previous years areoles near apex of plant, pink, , tepals white to magenta, usually pink, stigmata yellowish or pale green, ovary with few tiny papery scales.
Blooming season: Early in spring (April or May). Flowers are produced when plants reach 3 or 4 years of age and last only 2 to 3 days and are open for approximately 3 hours in mid-morning and early afternoon. Flower buds are set in the fall after a period of summer dormancy and remain unopened through the winter.
Fruit: Flat, barrel-shaped, fleshy, not juicy, on top and approximately 4 mm long. Most plants produce 2 or 3 fruits. Fruits are green-tannish, drying, dehiscent by circumscissile split at top and one or more vertical splits. Fruits are produced from late May to early June and dehisce in mid or late June and on average contain 10 to 12 seeds.
Seeds: 1.5 mm long and 1 to 1.2 mm wide teardrop-shaped with low nearly flat tubercles, hilum sublateral and with a whitish strophiole.
Similar Species: Could be confused with some species of Escobaria, but it differs in having no groove on tubercles, ovary and fruit with scales, fruit dehiscent, and seeds larger and not pitted. Closely related to Pediocactus simpsoniiSN|19699]]SN|19699]], but it is separable by recurved appressed radial spines with no central spines; some juveniles of P. simpsonii lack central spines and are nearly identical.

Bibliography: Major references and further lectures
1) Butterworth, C. & Porter, J.M. 2013. Pediocactus knowltonii. The IUCN Red List of Threatened Species. Version 2015.1. <www.iucnredlist.org>. Downloaded on 10 June 2015.
2) Edward Anderson “The Cactus family” Timber Press, Incorporated, 2001
3) James Cullen, Sabina G. Knees, H. Suzanne Cubey "The European Garden Flora Flowering Plants: A Manual for the Identification of Plants Cultivated in Europe, Both Out-of-Doors and Under Glass" Cambridge University Press, 11/Aug/2011
4) David R Hunt; Nigel P Taylor; Graham Charles; International Cactaceae Systematics Group. "The New Cactus Lexicon" dh books, 2006
5) Lyman David Benson “The Cacti of the United States and Canada” Stanford University Press, 1982
6) Fritz Hochstätter: “The Genera Pediocactus, Navajoa, Toumeya.” Selbstverlag, 2007

Cultivation and Propagation: They are relatively difficult to grow from their own roots, and are cultivated only by expert cactus enthusiasts. Nevertheless, plants on their own roots are very hardy.
They don’t tolerate living in humid and heated green houses. Place them in a ventilated position; they like "wind". Avoid direct sunlight exposure in summer. Water moderately, only from early March to May. The rest of the year keep them in dormancy.
They are exceptionally slow growers. For this reason they are frequently grafted on strong hardy cactus stocks like Opuntia and Echinocereus.
The flowers buds stay dormant on the plant apex all winter, they will then open in spring when the temperatures rise. To avoid any damage to the buds, the grafted plants that stay in heated greenhouses should not be let to completely dry out. Some light watering in winter is useful for keeping the flowers hydrated and healthy.
Propagation: Seeds: Temperature for optimum germination: night minimum approx 17 day maximum to near 40 C at any time of year with proper temperatures and daylength (ca.13-14 hours); it is possible to extend day-length with artificial lights. Anyway the seeds germinate slowly with difficulty and a low rate of success (But germiation is comparatively easy if compared with other Pediocactus and Sclerocactus species and often new seedling cluster spontaneously around the plant every year), scarification and stratification help, alternate freeze and thaw both wet and dry, but don’t keep wet, alternate wet and dry with changing temperature.
Germination can take several years so keep pot and try again next year. Seedlings do not do well either, and some die each year, for this reason plants are commonly grafted on hardy stocks like Opuntia compressa in this case they are easy to grow and no special skill is required.