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perennial

Flat Sea Holly

Eryngium planum

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Flat Sea Holly (Eryngium planum) at Ashcombe Farm & Greenhouses

Eryngium planum flowers

Eryngium planum flowers

(Photo courtesy of NetPS Plant Finder)

Flat Sea Holly (Eryngium planum) at Ashcombe Farm & Greenhouses

Eryngium planum in bloom

Eryngium planum in bloom

(Photo courtesy of NetPS Plant Finder)

Plant Height:  24 inches

Flower Height:  3 feet

Spacing:  15 inches

Sunlight:  full sun 

Hardiness Zone:  4b

Other Names:  Tetra Petra

Description:

A sea holly with unusual and show-stopping prickly steel-blue flowers with darker blue stems, thistle-like; tolerates hot dry sites; attractive to butterflies; pair up with softer textured plants, beautiful in dried arrangements

Ornamental Features

Flat Sea Holly features bold clusters of blue ball-shaped flowers with bluish-green bracts at the ends of the stems from mid summer to early fall. The flowers are excellent for cutting. Its attractive serrated heart-shaped leaves remain dark green in color throughout the year. The fruit is not ornamentally significant. The blue stems are very colorful and add to the overall interest of the plant.

Landscape Attributes

Flat Sea Holly is an herbaceous evergreen perennial with a mounded form. Its relatively fine texture sets it apart from other garden plants with less refined foliage.

This plant will require occasional maintenance and upkeep, and is best cleaned up in early spring before it resumes active growth for the season. It is a good choice for attracting butterflies to your yard. Gardeners should be aware of the following characteristic(s) that may warrant special consideration;

  • Self-Seeding

Flat Sea Holly is recommended for the following landscape applications;

  • Mass Planting
  • Hedges/Screening
  • Rock/Alpine Gardens
  • Border Edging
  • General Garden Use
  • Container Planting

Planting & Growing

Flat Sea Holly will grow to be about 24 inches tall at maturity extending to 3 feet tall with the flowers, with a spread of 18 inches. When grown in masses or used as a bedding plant, individual plants should be spaced approximately 15 inches apart. Its foliage tends to remain dense right to the ground, not requiring facer plants in front. It grows at a medium rate, and under ideal conditions can be expected to live for approximately 10 years.

This plant should only be grown in full sunlight. It prefers dry to average moisture levels with very well-drained soil, and will often die in standing water. It is considered to be drought-tolerant, and thus makes an ideal choice for a low-water garden or xeriscape application. It is particular about its soil conditions, with a strong preference for poor, alkaline soils, and is able to handle environmental salt. It is highly tolerant of urban pollution and will even thrive in inner city environments. This species is not originally from North America. It can be propagated by cuttings.

Flat Sea Holly is a fine choice for the garden, but it is also a good selection for planting in outdoor pots and containers. Because of its height, it is often used as a 'thriller' in the 'spiller-thriller-filler' container combination; plant it near the center of the pot, surrounded by smaller plants and those that spill over the edges. Note that when growing plants in outdoor containers and baskets, they may require more frequent waterings than they would in the yard or garden.

 
 
Hardiness Zone Plant Height Minimum Sunlight Soil pH Preference
Characteristics
Massing  Screening  Rock  Edging  Garden  Container 
Applications
Flowers  Foliage Color  Texture  Bark  Attracts Wildlife 
Ornamental Features