Eryngium planum, the blue eryngo or flat sea holly, is a striking addition to the front of any herbaceous border, gravel garden, dry open meadow, between ornamental grasses or even in a container. It is a hardy, perennial thistle that forms a clump of rounded, dark green leaves that are spiny and deeply lobed. These are held upright on tall, open-branched stems up to 50 cm long. The stems are silvery blue and bear numerous small blue conical flower heads in late summer and early autumn. In favourable conditions it can reach a height of 50 to 100 cm after two seasons. Eryngium planum is native to central and south-eastern Europe and central Asia.

Grow in dry, well-drained, poor or unimproved soil in full sun, but avoid wet places in winter. Water after planting and do not water again until after one or two rains. The plant can be cut back to ground level in early spring or when the flower heads and stems lose their colour and fade to a grey straw colour. You can leave the stems over the winter, they add interest to any border or meadow and are a good source of winter food for birds, but it is often best to remove them as soon as the new spring shoots appear, at the same time removing any surrounding vegetation that might block out any light, Eryngium needs light and sun and doesn't like competition for it.