Plant of the month: Sea holly

A sea holly with a difference makes a splash in the autumn garden

After the bright sunlight of high summer September brings a softer light as the sun lowers itself in the sky and many plants look at their best set against this gentler light. A plant that looks good during summer and carries on looking great through September is Eryngium yuccifolium.

Eryngiums, or sea hollies as they are often referred to, are a large group of plants originating mainly from Europe and northern America and many of them have spiky leaves and thistle-like flowers. Eryngium yuccifolium doesn’t look like most of its family members and it isn’t instantly recognisable as an Eryngium.
It is a herbaceous perennial that reaches around 1.5m (5ft) with slender, evergreen foliage that is silvery-grey in colour and smooth compared to the usual spiky leaves of other eryngiums. This plant bears straight stems with the leaves clasping themselves around the stem and gradually getting smaller the higher the stem they emerge. During high summer at the tips of the stems egg-shaped flower heads are borne which are silvery-grey in colour and gradually open to produce white or green flowers. These are surrounded by silvery bracts which have small teeth along their margins.
Eryngium yuccifolium is a plant that is grown as much for its foliage as it is for its flowers. It works well in a broad range of schemes as its silvery, vertical form acts as a great contrast to stronger colours such as blues or purples, and works particularly well in late summer surrounded by ornamental grasses like Calamagrostis × acutiflora. At RHS Garden Hyde Hall you can find this eryngium growing in the Dry Garden against great plants such as agapanthus, contrasting well with their deep blue flowers or Hylotelephium ‘Matrona’ (formerly Sedum ‘Matrona’) whose dark purple foliage makes a wonderful foil for the eryngium. It can also be found growing in the Hilltop Garden in a mixed planting near the Modern Rose Garden where it associates well with a wide range of herbaceous perennials such as Diascia personata with their strong, small pink flowers and Echinacea purpurea ‘Magnus’ with their large cone-like magenta pink flowers.
As with all eryngiums this species likes to be grown in well-drained soil and will resent sitting in a wet soil during the winter months - it prefers a light sandy soil and if you have a heavy clay soil you should improve the drainage by adding sharp sand or grit. As with most silver-leaved plants it likes to be grown in an open sunny area and although its evergreen it should be cut back in the spring as it is herbaceous, but once its new foliage emerges it will retain this for the full year until it is cut back the following spring.

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