Again with the blue.

I can’t help it; it’s in the blood. I see blue and I’m immediately at home, at peace, at ease. So when there’s another plant that promises blue, in almost any form, sign me up.

Blue in the garden comes in so many tones: Picture the rich, cobalt hue of Caryopteris flowers as well as the soft pastels of hydrangea blooms (in the appropriate soil) and the vast array of purples that sometimes pass for blue. But it’s not always easy to find a true, steely blue.

Image Courtesy of iStock | macroart

Eryngium planum (sea holly) offers this and more. Tolerant of drought but showy in a rather eerie way, sea holly is an excellent choice for drama throughout the seasons. Spring is rather quiet, when the plant develops a rosette of dark green, toothed basal leaves reaching about 6 inches tall and spreading to about 12 to 14 inches. A sturdy, stiff, violet to blue stem emerges and begins to branch, eventually reaching nearly 3 feet tall.

In summer, abundant thistle-like flower heads are borne atop the stem. Emerging violet-blue, each flower head is a spherical to ovoid umbel composed of very small, tightly packed flowers. At the base, each flower head is framed by a spiky ring of bluish green bracts that somewhat resembles a creepy Elizabethan collar. The appearance of the flowers, supported by raggedy, almost threatening bracts and foliage, is the stuff of cartoon evil. It virtually hisses, “Don’t touch.” But they’re utterly fascinating, and if you can see beyond the perceived threat to the incredible beauty, you’ll be richly rewarded.

The flowers are highly prized for use in cut arrangements – just be careful when handling them.

Soil should be well-drained, leaning toward the dry side. Sandy soils work well, and this tough – and tough looking – plant will tolerate relatively poor soils. If situated in a rich, fertile soil, in fact, sea holly may tend to sprawl. (The same is true if it’s not offered full sun.) At any rate, it’s best to avoid overwatering and be glad that you’ve found a plant that shines in dry conditions.

Choose the planting site carefully, as E. planum resents being moved once it’s established. A strong tap root keeps it firmly and happily growing where it lands, and transplanting is difficult at best. Site several plants if you want a collection, or be patient and wait for the slowly spreading Eryngium to multiply over time. The plants I grew took nearly eight years to reproduce, and by that time, a neighbor’s growing tree had provided enough shade to challenge their vigor.

Name: Eryngium planum

Common name: Sea holly

Hardiness: Zones 5 to 9

Mature height: 24 to 36 inches

Mature spread: 12 to 24 inches

Classification: Herbaceous perennial

Landscape use: Eye-catching as a single, dramatic specimen; at the back of perennial beds and borders; best used in dry, sunny sites

Ornamental characteristics: Dark green basal leaves support strong, rigid, bluish stems topped by several thistle-like, steely blue to violet-blue flower heads; plant provides a unique, architectural focal point

It’s not surprising that Eryngium is bothered by few pests; both insects and mammals tend to steer clear of its prickly nature. Leaf spot diseases may occur, but my plants suffered only from lack of sun. They’re fussy about moisture, and too much may lead to root rot.

There are a number of cultivars on the market, each featuring an enriched color, habit or variegation.

  • ‘Blue Hobbit’ is a true dwarf, offering the stunning blue of its taller relatives but packing an ornamental punch at only about 10 to 12 inches tall.
  • ‘Big Blue’ offers nearly iridescent blue blooms and silvery leaves.
  • ‘Blue Glitter’ produces steel-blue flower heads that appear to gleam and glitter.
  • ‘White Glitter’ features the characteristic spiked flower heads, but produces them in a ghostly, silver-white.
  • ‘Jade Frost’ boasts unique blue-green leaves highlighted with creamy white margins that appear pink if they’re exposed to cooler night temperatures; flowers appear violet-blue.