Eryngiums are a large genus of annuals, biennials and perennials with spiky leaves and prickly, globe-shaped inflorescences often mistaken for a type of thistle. In fact, they belong to the umbellifer family (Apiaceae).
Each ‘flower’ is actually a compact head of flowers with a characteristic ruff of spiny bracts beneath, supported on stiff branching stems high above a rosette of leathery basal leaves. Some Eryngiums are spectacular and exotic: the evergreen South American species Eryngium yuccifolium, E. agavifolium and E. pandanifolium, named for their subtropical lookalikes, are surprisingly hardy in North-Western Europe, achieving considerable stature in fertile, evenly moist soils when spared winter wet.
In contrast, the Old World (Eurasian and North African) species are hardscrabble street fighters well suited to unforgiving urban conditions. Adapted to relatively poor stony or dune soils, their long tap roots allow them to access and store moisture from deep within the soil profile, and the leathery stem leaves and stiff stems are durable and resilient.
A steely beauty temper this toughness: stems and inflorescence bracts have a distinctive metallic tinge, often strongly blueish (particularly in E. planum and E. amethystinum). Both colour and architectural interest are retained over a long season. Surprisingly, given their forbidding appearance, Eryngium species are an exceptionally rich source of nectar, a promising resource for urban bee populations.
Self-seeding can be a problem with some species, especially the perennials, whose unwanted tap roots are difficult to eradicate once established.
Cultivars vary in their fertility, however, and hybrids such as E. x tripartitum rarely set viable seed. The designer James Hitchmough has exploited both Old- and New-World Eryngiums in several of his ultralow-input ecologically designed communities, notably the extensive planting at the Oxford University Botanic Garden, where E. planum ‘Blaukappe’ is one of the most distinctive and recognisable elements.
By Martin Deasy, a tutor on the RHS Mhort. He also runs his own landscape design business.
This article was first published in the September 2023 edition of FloraCulture International.