Erysimum is a genus of the brassicaceae family. Most are evergreen biennials and short-lived perennials. Their geographic range covers central and southern Europe and the Canary Islands, Mediterranean Africa, the middle east and through Asia to western China. It also includes North America. This suggests that early erysimums or their precursors may have occupied a great swathe of northern Eurasia before the separation of the two continents some 100 million years ago.
Within this geographic range, erysimums have colonised many different sorts of ecosystem, again suggesting they are a fairly ‘old' group of plants. Some, such as E. ammophilum are adapted to live in coastal sand dunes. Many live in scrubby garrigue environments. Others are alpine or scree plants, such as E. nivale, which lives high up in the Rocky Mountains of Colorado. Some inhabit quite specialised niches - for example, E. scoparium lives on the volcanic soils on the slopes of Mt. Tiede in Tenerife.
We can recognise a number of different, typical erysimum habits - related to where in the geographic range they live and which ecosystems they occupy. Most of the Mediterranean species, and the cultivars derived from them are properly herbs or sub-shrubs, which develop woody, branching stems with the new growth and flower-bearing racemes towards the ends. These are very variable in size - a good specimen of E. mutabile can reach 2m across and 1m high, whereas some of the compact cultivars such as E. ‘Butterscotch' grow only to about 20cm high and 30cm across.
The European and Asian alpine species and their cultivars generally have a spreading, mat-forming habit. Some - such as the species E. kotschyanum and the cultivar E. ‘Parkwood Gold' have tiny leaves and the leaf clusters are densely packed together, forming a ground-hugging carpet. Others, such as E. helveticum, have a much looser habit and grow slightly taller (up to 15cm). These mat-forming alpines all bear their flowers on upright stems above the foliage.
Many of the North American species have a tight rosette of very elongated, almost grass-like leaves and the flower stem emerges from the centre. In alpine species such as E. amoenum and E. nivale the flower stem is very short, but in some lowland species, such as E. asperum, the flower stem shoots up to 60cm or 70cm. Its common name of the ‘Prairie Rocket' may be derived from this growth habit.