01 April 2023
Urban microclimates tend toward extremes, particularly heat and drought. Plants that are tough enough to survive heat and water stress without resorting to dormancy (and loss of ornamental value) are highly prized by urban specifiers.
Hailing from the American Midwest, the striking sub-shrub Amorpha canescens is so well-suited to harsh city life that it seems inexplicable that it is not more widely encountered.
Performing as a herbaceous perennial where winters are colder, it grows to approximately 90cm, with pinnate foliage and individual purple flowers with orange anthers. What makes it distinctive and attractive is the way the late-summer flowers are held advantageously in groups of spikelets at the top of plants like the fingers of upturned hands. The foliage colours are attractive in autumn.
The attractive grey tinge to its stems and pinnate leaves comes from the covering of fine hair that limits water loss by slowing transpiration and reflecting sunlight. Just as important, its tough roots penetrate deeply into the soil, gaining the nickname ‘devil’s shoestrings’ from American pioneers, who cursed as they attempted to plough native prairie into arable condition.
By contrast, modern horticulturists are more likely to view these long roots as a blessing since they help the plant to survive—and even thrive—in schemes designed for minimal irrigation.
Though it performs best in fertile soils, as a legume, its roots offer the additional advantage that (like almost all pea relatives) they possess nodules containing symbiotic bacteria that fix nitrogen from the air, lessening the plant’s need for additional fertiliser input.
As an additional bonus, Amorpha canescens is highly tolerant of road salt. All of this adds up to a genuinely distinctive plant—even distinguished—in appearance, but which offers a great deal more in addition to its ornamental qualities.
By Martin Deasy, a tutor on the RHS Mhort. He also runs his own landscape design business.