Sea hollies slightly resemble globe thistles (Echinops spp.). Both have blue-gray flowers and a thistlelike appearance. Sea hollies have a finer texture, however, with more deeply cut foliage and more spines. The flowers look like silvery or blue globes held atop stems. Most sea hollies bloom summer to autumn. There are more than 200 species of sea hollies to choose from and a handful of cultivars.
- Common name: Sea holly
- Botanical name: Eryngium spp.
- Zones: 3 to 10, depending on species
- Size: To 12 feet tall, depending on species
- From: Areas of Europe, Asia, and North and South America
- Family: Umbelliferae (carrot family)
- Sun: Full sun or partial shade.
- Soil: Moist, but well-drained soil.
- Moisture: Water during times of drought.
- Mulch: Lay a 2- to 3-inch-deep layer of mulch over the soil around the plants if desired. Use winter mulch only after the soil has frozen.
- Pruning: Cut plants back in autumn after freezing temperatures arrive or in spring before plants begin to grow.
- Fertilizer: Fertilizer is generally not necessary in soils that are rich in organic matter. If desired, use a balanced fertilizer in spring.
- Seed: Sow seeds indoors or in the garden in spring. Note: It may take a year or two for some species to reach blooming size.
- Division: Dig and divide clumps in spring.
- Aphids: These small insects often appear in large numbers on new growth. Spray them off daily with a stream of water; they will not attack a plant after being knocked off. Use an insecticidal soap or neem-oil-based spray if infestations are severe.
- Leaf spot: In summer or autumn, the leaves become spotted yellowish or with darker colored spots. Each spot often has concentric rings around it, forming something of a bull's-eye pattern. To deter it, prune the plant to keep good air flow, and avoid wetting the foliage in afternoons and evenings.
- Powdery mildew: This disease tends to appear in mid- to late summer and affected leaves have a grayish powdery covering. The leaves then drop off. To deter the disease, prune the plant to keep good air flow and avoid wetting the foliage in afternoons and evenings.
- Slugs/Snails: Slugs and snails tend to eat at night, chewing up leaves. They leave slick, slimy trails behind the next morning. To deter them, try surrounding plants with a ring of horticultural grade diatomaceous earth or laying down a slug bait. Some people have found success with laying copper strips around plants, but this does not seem to work for everyone. If slugs are not particularly numerous, set out shallow containers of stale beer at ground level. Slugs, attracted to the beer, crawl into it and drown.
- Some species of sea holly are short lived and may only survive a few years, even in the best conditions. Some species may self-seed.
- Some species have tap roots, making them especially difficult to transplant. It's best to plan carefully and plant sea hollies in a permanent spot.
- Eryngium alpinum: Grows to 2 feet tall with heads of metallic blue flowers in late summer and early autumn. Zones 5 to 8.
- Eryngium amethystinum: Grows to 3 feet tall with spiny, blue-green leaves and heads of metallic blue flower in late summer. Zones 3 to 8.
- Eryngium bourgatti: Grows to 2 feet tall with green leaves marked with silver, and gray-blue or gray-green flowers in summer. Zones 5 to 9.
- Eryngium giganteum: Grows to 3 feet tall with spiny green leaves and metallic blue flowers in summer. A short-lived species. Zones 5 to 8.
- Eryngium planum ‘Blue Diamond': Grows to 3 feet tall with spiny leaves and clusters of metallic blue flowers in summer and autumn. Zones 5 to 9.
- Eryngium yuccifolium: Grows to 4 feet tall with spiny blue-green leaves and bluish flowers in summer and early autumn. Zones 4 to 8. Native to areas of North America.