Espostoa senilis

Espostoa senilis grows as an upright, cylinder-shaped cactus whose surface is divided into many vertical ridges called ribs. Along these ribs sit small cushiony spots called areoles, and it is from the areoles that the plant's long white hairs emerge. Those hairs are actually modified spines, so what looks like soft fleece hides shorter, stiffer spines pressed close to the green stem. Young stems often start with light brown spines that fade toward white as they age, and seedlings put out especially long, bristly hairs near the base. A single plant may stay solitary for years, then begin branching from the base to form a small group of hair-covered columns. With age, it develops a cephalium, a dense woolly patch of hair and bristles along one side of the stem where the flowers are produced.

This cactus comes from the dry valleys of the Marañón River in Peru, where it grows on rocky mountain slopes. Days there are hot and sunny, while nights can turn cold - and the heavy white hair helps in both directions: it scatters strong sunlight away from the stem and traps a thin layer of still air that slows heat loss after dark. Like other cacti, it uses a water-saving process called CAM photosynthesis, opening the tiny pores in its skin at night rather than during the hot day, so it loses less moisture. The ribbed stem works like an accordion, swelling as it takes up water and shrinking during dry spells. In the garden, these traits make it a good fit for a fast-draining gravel bed or container in frost-free areas, where it needs very little water once settled in. It grows slowly and dislikes cold, wet soil, which is the most common cause of rot.

One feature makes Espostoa senilis stand out among its relatives. Most cacti in this group open white, night-blooming flowers, but this species produces purple-pink blooms instead, an unusual color that some botanists think may draw hummingbirds rather than the bats and moths that visit its white-flowered cousins. The flowers open at night from the cephalium, and the small, berry-like fruits that follow stay tucked in the wool until they split to release black seeds. Gardeners may still see this plant sold under its older name, Thrixanthocereus senilis, from a time when it was placed in a separate genus before being moved into Espostoa. One gentle caution: the fluffy coat looks soft but conceals short, needle-like spines, so handle the plant with folded newspaper or thick gloves and keep it clear of walkways where people brush past.