What are Erysimums?


What is an erysimum? We are on firm ground in stating that the genus erysimum (wallflowers) is part of the family brassicaceae or cruciferae (mustards). So far, simple enough but, after this, it gets a bit more complicated. What is cheiranthus and is it different from erysimum? And what about the bedding wallflowers beloved of British gardeners? Are they erysimums or cheiranthus? Some plants are sold in garden centres as perennial wallflowers. How do these fit in to the scheme of things?  
If we go back to the early years of the Journal of the Royal Horticultural Society (RHS Journal), we find references to erysimums, cheiranthus and wallflowers. In the cumulative index to the RHS Journal from 1838-1935 there are 26 references to erysimums, 64 references to cheiranthus and 51 references to wallflowers. 
In the main, the references to erysimum relate to perennial species - arkansanum, asperum, kotschyanum, linifolium and ochroleucum, for example, rather than the perennial cultivars we see in garden centres today (such as  ‘Bowles’ Mauve’ or ‘Apricot Twist’). This is not surprising, as we are talking about the era of the Victorian plant hunters, when many of these species were being described and classified for the first time. 
And, in the main, the references to cheiranthus relate to biennial species and the cultivars bred from them. Most importantly for horticulture is Cheiranthus cheiri - the English Wallflower, from which most of our bedding wallflowers derive (although it is not English but, rather, native to the eastern mediterranean). There are also many references to Cheiranthus allionii and Cheiranthus x marshallii. These are the same plant – the Siberian Wallflower. It did not come from Siberia, but is a cross between two species E. perofskianum and E. decumbens, bred by John Marshall in 1846, rather than a naturally occurring species in its own right. It is also important in horticulture, being the forerunner of a number of varieties of bedding wallflower.
So, the broad distinction between what were called erysimums and what were called cheiranthus  is clear – erysimums referring generally to perennial species, cheiranthus generally to biennial species, crosses and cultivars. The term ‘Wallflower’ was used mainly to refer to bedding wallflowers, with many familiar varieties dating back a century or more, although also to some alpine and sub-shrubby perennials (E. pulchellum – the Rock Wallflower, for example, or E. scoparium – the Teide Wallflower).
In [date] the genus cheiranthus was integrated into erysimum so that, strictly speaking, all types of wallflower are now erysimums. However, this National Collection only covers those considered to be perennials.