Growing Flat sea holly in a pot

Eryngium planum is a low-maintenance perennial once established, with minimal water and feeding requirements. Its drought tolerance makes it well-suited to gravel gardens and sunny borders that dry out in summer. Water newly planted specimens during prolonged dry spells in their first season, but established plants rarely need supplementary watering except in extreme drought. Overwatering or planting in moisture-retentive soil is the most common cause of failure, leading to root rot and weak, floppy growth. Feed sparingly. In April or May, apply a light dressing of general-purpose granular fertiliser or a handful of blood, fish, and bone around the base of each plant. Eryngium planum evolved on poor soils and too much nitrogen encourages lush foliage at the expense of flowers, as well as reducing the intensity of the metallic blue colouration. On chalky or sandy soils, you can skip feeding altogether without harm. This plant is fully hardy across zones 4–9 and requires no winter protection in temperate Europe. The basal rosette of leaves remains semi-evergreen in mild winters. Avoid cutting back too early in autumn, as the foliage helps protect the crown from excessive winter wet. Pests are rarely a problem. Slugs and snails generally avoid the spiny foliage. Eryngium planum is occasionally affected by powdery mildew in late summer if air circulation is poor, but this is cosmetic and does not harm the plant. Ensure adequate spacing and avoid overhead watering to minimise risk. Root rot caused by poor drainage is the main disease concern, so a gravel mulch and well-prepared soil are your best preventative measures. Self-sown seedlings may appear; remove them promptly if unwanted, as the taproot becomes difficult to extract once established.