Oreocereus is a genus of cacti in the family Cactaceae (order Caryophyllales), comprising a small group of high-altitude columnar cacti found exclusively in the Andes mountains of South America. The genus name translates to "mountain cereus", reflecting the distinctive montane habitat that sets these plants apart from most other cacti.
Plants in this genus are shrubby and branch sparsely from the base, typically reaching 2 to 3 metres in height. True trunks are rarely formed. The erect, cylindrical stems are ribbed and notched or warty between the ribs. A defining characteristic of the genus is the dense covering of long white woolly hairs on the areoles, alongside stout spines. This conspicuous fuzz — which consists of modified spines — gives several species their popular common name "old-man cactus", a name shared with the unrelated Cephalocereus senilis and Espostoa lanata. The name "old man of the mountain" is also occasionally applied to some species.
Flowers are tubular to funnel-shaped, roughly radially symmetrical, and range in colour from orange and red to violet. They emerge near the shoot tip or occasionally from a terminal cephalium, and open during the day. The flower tube is straight to slightly curved and sometimes laterally compressed; the areoles on both the pericarpel and tube are hairy. Fruits are spherical to oblong, containing broadly oval black seeds.
Oreocereus celsianus is among the most widely recognised species and is frequently encountered in botanical collections. The genus contains approximately five to nine accepted species depending on the taxonomic treatment applied.
Etymology
The name Oreocereus means "mountain cereus", combining the Greek prefix oreo- (ὀρεο-), meaning mountain, with the Neo-Latin cereus, meaning wax or torch. It reflects the genus's exclusive occurrence at high elevations in the Andes and its columnar, torch-like growth form.
Distribution
Oreocereus is known only from high altitudes of the Andes in South America, where species occur at elevations where few other cacti can survive. This makes the genus unusual among cacti in its tolerance for cold, high-montane conditions.
Cultivation
Species of Oreocereus are cultivated as ornamental cacti in collections worldwide, prized for their distinctive woolly white areoles and colourful flowers. They require excellent drainage and tolerate more cold than most cacti due to their high-altitude origins; however, they generally need frost protection in temperate gardens and bright light to maintain their characteristic woolly appearance.
