How to Grow Sea Holly

sea holly

The Spruce / Adrienne Legault

Sea holly plants are low-maintenance perennials with striking purple-blue flowers that look like small glowing thistles. While they are not related at all to the true hollies, they are very similar to globe thistle. Native to Europe, 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 their stems can be green or silvery blue, depending on species.

Sea holly plants are fairly specialized and therefore are normally purchased through specialty seed providers or as mature plants potted in containers. Spring is the best time to plant either seeds or mature plants—sea holly will grow quickly, and semi-mature nursery plants will flower in their first season.

Botanical Name Eryngium planum
Common Names Sea holly, blue thistle, blue devil, blueweed
Plant Type Herbaceous perennial
Mature Size 2–3 ft. tall, 1–2 ft. wide
Sun Exposure Full sun
Soil Type Sandy, well-drained
Soil pH Neutral to acidic
Bloom Time Summer, fall
Flower Color Green, blue, blue-purple, silver, white
Hardiness Zones 5 to 9 (USDA)
Native Area Europe
sea holly in a garden setting
The Spruce / Adrienne Legault
closeup of sea holly
The Spruce / Adrienne Legault

Sea Holly Care

Sea holly plants are pretty easy to care for, as long as you nail the proper soil and sun conditions. You'll want to plant sea holly in dry to medium sandy soil in a location that boasts full sun. Once planted, it will require little tending. Flowering for most varieties starts in mid-summer and will continue well into the fall.

These low-maintenance plants don't like to be moved, so be sure to position the plant somewhere you're looking to have it long term. It's a great option for all those spots in the garden where the hose doesn’t quite reach, or for more difficult spots, like a strip of grass between the sidewalk and the street. The blues and silvers of sea holly blend well with just about every color, especially yellow and orange—consider pairing it with rudbeckia, coreopsis, zinnia, and cosmos.

Tall sea holly varieties often need some support so planting them behind sturdier varietals, like coneflower, will help keep them standing. Additionally, sea holly will bloom longer if you deadhead the spent flowers, but they also look wonderful long into winter, so you always have the option to leave the fall flowers on the stems. Sea holly is popular with bees and butterflies, but not with deer and rabbits, and the flowers last several days in a cut arrangement.

Light

A full day of sun (at least eight hours) will give you the strongest sea holly plants and the most blooms. While the plants can handle partial shade, the reduction in light may lead to weaker stems, which may make it necessary to stake the plants to keep them upright.

Soil

The best planting medium for sea holly plants is dry, sandy soil that is still rich with organic matter. Although these plants will grow in poor soil, it's best to amend your mixture with a little compost or peat moss before planting. Additionally, sea holly is not particular about its soil pH—anything around the neutral range is sufficient. However, the plants do need good drainage or they will be susceptible to root rot and can die off.

Water

Thanks to their long taproot, sea holly plants are very drought tolerant once established and won’t need additional water unless subjected to a prolonged, hot drought at the peak of summer. That being said, excess surface moisture can cause the flower's crown to rot, so sea holly plants should be somewhat segregated from other plants that requiring more water. If and when watering is necessary for the plant, aim the water source at the base of the stem to reduce the risk of built-up moisture.

Temperature and Humidity

As long as it's planted in the proper hardiness zones, sea holly does not have any additional temperature or humidity requirements. It will do better in the cooler days of spring and fall but certainly will not die off in warmer weather.

Fertilizer

Sea holly plants are not heavy feeders, but you should still make sure your soil has plenty of organic matter in it. If not, you will need to feed or side-dress your plants with compost in mid-season.

Sea Holly Varieties

New introductions of sea holly come out regularly and claim the shelf space of older types. While they're all lovely, some do grow better in certain conditions, so check to see which are successful in your area.

  • 'Jade Frost: A varietal that boasts variegated foliage, with pink margins and veins
  • 'Blue Glitter': A bountiful version, with dozens of blooms per plant and gray-blue foliage
  • 'Sapphire Blue': A classic favorite varietal with striking blue flowers and leaves
  • 'Tiny Jackpot': A petite varietal that grows to just 14 inches tall and looks good at the front of a border
Giant sea holly also known as 'silver ghost'
 
sapphire blue sea holly
Trudie Davidson / Getty Images

Propagating Sea Holly

Sea holly's taproot can make it a bit difficult to propagate. To do so, carefully dig up the plants in late summer (taproot and all), cutting away nice fat sections of roots—try to never cut away more than one-third of the plant's total root mass. Replant the parent clump immediately, then 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.

How to Grow Sea Holly From Seed

Most sea holly varieties can be started from seed, and they will do best if stratified (chilled to stimulate the growth cycle) first. The easiest method to planting is to directly sow the seeds in the fall, then patiently wait to see what sprouts up in the spring.

If you want to get a head start on your planting, you can start sea holly seeds indoors; chill them for about four weeks in the refrigerator, then plant them outdoors. As with many perennials, these seedlings may take most of the first growing season to establish good root systems—in subsequent years, you can expect them to flower at the expected time.

Common Pests/Diseases

While sea holly is generally free of many different kinds of pests, it is susceptible to both root rot and powdery mildew. Both fungal diseases can arise if the plant is kept too moist, too humid or if it's planted in soil that does not drain properly. Set yourself up for success by watering the plant at its base and keeping ample distance between the plants, each other, and other flowers in your garden to promote good air circulation.