Sea holly plants (Eryngium spp.) are low-maintenance perennials with striking purple-blue flowers that look like small glowing thistles. They are not related at all to the true hollies (Illex spp.), but are very similar to globe thistle (Echinops. Sea holly flowers have green or blue cones and a distinctive bract collar in silver, white, green or bluish-purple. The colors have a painted-on, metallic sheen that changes in the sunlight, and the stems can be green or silvery blue, depending on species. The leaves can be long and thin, deeply lobed, or even round, depending on the variety. These plants are very tolerant of dry conditions and also handle salt spray with ease.
There are many species within the Eryngium genus that go by the name of sea holly, but most of the named cultivars are derived from one species—Eryngium planum, which typically grows 2 to 3 feet tall and is hardy in zones 5 to 9.
The sea holly species are fairly specialized plants that are normally purchased as mature plants potted in 4-inch or even gallon-sized containers. Spring is the best, and most common planting time since this is when plants are most likely to be available at nurseries. When planted as semi-mature nursery plants, you can expect them to flower in their first season at the expected time.
|Botanical Name||Eryngium species, especially E. planum|
|Common Names||Sea holly, blue aryngo|
|Plant Type||Herbaceous perennial|
|Mature Size||6 inches to 4 feet tall; 1- to 2-foot spread (varies by species)|
|Sun Exposure||Full sun|
|Soil pH||6.5 to 7.5 (neutral)|
|Bloom Time||Summer to fall (varies by species)|
|Flower Color||Green, blue, blue-purple, silver, white|
|Hardiness Zones||3 to 9 (USDA), varies by species|
|Native Area||Central and southeastern Europe|
How to Grow Sea Holly
Plant sea holly in dry to medium-moisture sandy soil in a full sun location. Though the soil can be dry and sandy, best performance is achieved if the soil has a good ratio of organic material in it. Good drainage, though, is critical. Once planted, sea holly requires little tending.
Flowering for most varieties starts in mid-summer and will continue into the fall. These low-maintenance plants don't like excessive water and fertilizer, and they also don't like to be moved. Sea holly will bloom longer if you deadhead the spent flowers, but they look wonderful long into winter, so leave the fall flowers on the stems. The birds will thank you.
Sea holly is not prone to many diseases or pest problems, but root rot can be a problem in wet soil conditions.
A full day of sun will give you the strongest sea holly plants and the most blooms. These plants can handle part shade, but weaker stems will result, making staking necessary to keep the plants standing upright.
The best planting medium is dry, sandy soil that is still rich with organic material. Although these plants will grow in poor soil, it's best to amend with a little compost or peat moss before planting. Sea holly is not particular about soil pH. Anything around the neutral range is sufficient. However, the plants need good drainage or they will die off.
Thanks to its long taproot, sea holly is very drought tolerant once established and won’t need additional water unless subjected to a prolonged, hot, drought. But excess surface moisture will cause the crown to rot, so sea holly should be somewhat segregated from other plants requiring more water.
Temperature and Humidity
Propagating Sea Holly
Because sea holly has a tap root, it does not divide easily. You can, however, take root cuttings in the spring.
In late summer, carefully dig up the plants, taproot and all, and cut away nice, fat sections of roots. Never cut away more than one-third of the plant's total root mass. Replant the parent clump immediately.
Plant the individual root sections in a mixture of compost and vermiculite in a small pot, making sure the cut end (crown side) of the root segment is facing up. The root segment should be entirely buried, but the cut tip should be just barely under the surface.
Overwinter the pots in a sheltered location. In spring, the root segments should begin to generate new roots, which you will see poking out of the bottom of the pot. When a network of roots is established and green shoots are emerging above the soil line, you can transplant your new sea holly into the garden.
Growing From Seeds
Most sea holly varieties can be started from seed. They do best if stratified first—chilled to stimulate the growth cycle. The easiest method is to direct sow in the fall and then be patient and wait to see what sprouts in the spring. But you could start sea holly seeds indoors if you chill them for about four weeks in the refrigerator and then plant them outdoors; they will germinate in two to three weeks. But as with many perennials, these seedlings may take most of the first growing season to establish good root systems and flower. In subsequent years, you can expect them to flower at the expected time.
This plant works well for those areas of the garden that get constant, direct sun. It does well either as a single specimen plant or grouped in masses. Sea holly also makes a good cut flower for arrangements.
This is a great flower for all those spots in the garden where the hose doesn’t reach or for growing in that strip of grass between the sidewalk and the street. But don’t confine it there. The blues and silvers blend well with just about every color, especially yellow and orange. Pair it with rudbeckia, coreopsis, zinnia, and cosmos. The tall varieties need some support and planting them behind sturdy plants, like coneflowers, will help keep them standing.
Sea holly is popular with bees and butterflies, but not with deer and rabbits. The flowers last several days in a cut flower arrangement.
Varieties of Sea Holly
New introductions come out every year and claim the shelf space of older types. They’re all lovely, but some grow better in different conditions, so check to see which are successful in your area.
- 'Jade Frost' has variegated foliage and pink margins and veins.
- 'Blue Glitter' has dozens of blooms on each plant, with gray-blue foliage.
- 'Sapphire Blue' is a common favorite with blue flowers and leaves.
- 'Tiny Jackpot' grows to 14 inches tall and looks good at the front of a border.
- Eryngium apinum (alpine sea holly) has the deepest blue colors and has a peak bloom period during July and August.
- E. amethystinum (amethyst sea holly) has good cold hardiness (to zone 3), but the growth habit is a little straggly. It grows 12 to 18 inches tall.
- E. bourgatii (Mediterranean sea holly has blue-green flowers with silver bracts; the coarse spiny leaves have white veins.
- E. giganteum (giant sea holly), frequently known as Miss Wilmot's ghost, is a3- to 4-foot tall plant with heart-shaped leaves and extra-large flowers.
- E. yuccifolium (rattlesnake master) is a native plant to the eastern U.S., with button-shaped chartreuse flowers and strappy leaves.
- E. maritimum (common sea holly) is a small plant, growing 6 to 18 inches.
Holly Diseases and Insect Pests. Clemson Cooperative Extension Home & Garden Information Center
Isermann, Maike, Rooney, Paul. Biological Flora of the British Isles: Eryngium maritimum. Journal of Ecology,102,789-821, 2014, doi:10.1111/1365-2745.12243
Wolkis, Dustin, Blackwell, Steve, Kaninaualiʻi Villanueva, Shyla. Conservation Seed Physiology of the CIénega Endemic, Eryngium Sparganophyllum (Apiaceae). Conservation Physiology, 8,1 2020, doi:10.1093/conphys/coaa017