Eryngium planum is a plant of spare continental steppes, carrying into the garden a certain European severity that sits beautifully alongside softer perennials. Its flower heads — each a small globe of steel blue surrounded by a ruff of spiky bracts — have the quality of a botanical illustration made three-dimensional, precise and almost implausibly decorative. The species name planum, meaning flat, refers to its unarmed leaves, which distinguishes it from spikier relatives; the plant’s texture comes from its flowers alone. In full sun, the blue is almost startling. In partial shade, it fades toward grey-green and the stems grow lax.

Plant it in the driest, poorest-draining spot in the border and it will reward the neglect. Rich soil and moisture produce leggy plants that fall open; gritty, lean ground keeps them upright and vivid from June through September. Fresh-cut stems have an unfortunate barnyard fragrance, but this passes once dried, and dried stems hold their color and form for months. Sea holly moves slowly through self-seeding but rarely becomes a nuisance. Butterflies work the flowers steadily through summer.