Peniocereus is a genus of tuberous plants in family Cactaceae. Some species were formerly placed in genera Neoevansia or Wilcoxia, which are now submerged into Echinocereus, another cactus genus with some tuberous or stoloniferous members. Two Peniocereus species are found in the USA; others are found from México to South America. They have relatively thin, elongated stems with few ribs and small to tiny spines. They are adapted to sprawl through shrubbery, and are usually very hard to spot when not in flower or fruit because the thin stems mimic twigs and branches.
Flowers have long tubes with many small spines. Many have large, white, scented nocturnal flowers that open after dark and close with morning heat. In a given population most individuals will flower on the same night. Pollination in these species is by hawk or sphinx moths.
Most species have large turnip-shaped tubers which were eaten by Native Americans. Tubers easily reach 50 pounds / 23 kg with age and seedling tubers can reach the size of a golf ball after 2-3 growing seasons.
All species grow well in very loose soil with plenty of summer water. They need little or no water during winter. Few tolerate any frost. Bare tubers of P. greggii have survived over 5 years forgotten in a shed. Seedlings need to stay moist for their first year and cannot tolerate sun. Stem cuttings root readily during warm weather. Many species form tubers from stem cuttings; others do not.
Peniocereus striatus [(Brandegee) Buxbaum] has fusiform tubers similar to Dahlia spp. It is the smallest Peniocereus, with stems just 2-3 mm in diameter and stem spines that will not pierce most adult human skin. Its flowers open in the daytime and last several days. Spiny fruits follow, bright red when ripe. Stem cuttings form tuberous roots and in time cannot be distinguished from seed-grown plants.
Resources
United States Department of Agriculture
http://plants.usda.gov/core/profile?symbol=PEST11
Southwest Environmental Information Network
http://swbiodiversity.org/seinet/taxa/index.php?taxon=1679