Crocus is one of the most popular flowers in the early- spring garden. Among the approximately 90 known species, there are also species that flower in autumn and winter. The native distribution of the genus Crocus extends from central and southern Europe through the Mediterranean region and North Africa into the Middle East and across Central Asia into western China. Species occupy habitats from sea level to subalpine altitudes, through woodlands to meadows.
The solitary flowers vary dramatically in colour, although purples, yellows and whites predominate. The most popular species in cultivation are Crocus chrysanthus, Crocus flavus, Crocus sieberi, Crocus tommasinianus and Crocus vernus, together with hundreds of varieties and hybrids derived from them. In contrast to their flowers, the grass-like leaves, with their central white stripes, are very similar among species.
The first crocuses seen in north-west Europe were grown from corms transported from Constantinople in the 1560s, and sent to the Flemish botanist Carolus Clusius in Leiden.
A corm is a short, swollen, vertical underground stem with lateral buds that can form new plants, and is used by the plant to store energy reserves. Specialised, scale-like leaves, which form the tunic, protect lateral buds. The tunic is one of the important characters for crocus identification and classification. The crocus stem remains underground; only the flowers and leaves emerge above it.
Corms produce two different types of roots. As new shoots emerge above ground, normal fibrous roots are produced at the base of the corm. A second sort of thick, wrinkled root forms as new corms grow from lateral buds. These contractile roots, which pull the corms into the soil, appear to be formed in response to fluctuating soil temperatures and light levels. Contractile roots do not grow when temperatures are uniform, or when light is absent.
Besides their use as ornamentals, the other major use of the genus is as the source of the spice saffron. Saffron comes from the dried, crimson stigmas and styles of Crocus sativus flowers. Saffron has been cultivated and traded for thousands of years in Eurasia, and has the record for being the world's most expensive spice by weight. Approximately 140,000 flowers must be harvested to produce one kilogramme of dried saffron. During the sixteenth century and seventeenth century, saffron was an important crop in parts of southern England, a fact reflected in modern place names such as Saffron Walden in northwestern Essex.
Mathew B 1983. Crocus: A revision of the genus Crocus. Timber Press.
Mathew B et al. 2009. A reassessment of Crocus based on molecular analysis. The Plantsman 8: 50-57.
Petersen G et al. 2008. A phylogeny of the genus Crocus (Iridaceae) based on sequence data from five plastid regions. Taxon 57: 487-499.