Garden Variety: Globe thistle, sea holly add unique look
Globe thistle’s pale purple to steel blue flowers, held high on stiff stems above coarse dark green foliage, are stealing the show this summer in gardens where they grow. Sea holly is a close second with lighter blue to white flowers and a similar growth habit. Both plants thrive in the heat and humidity of a typical Midwest summer, are drought tolerant, and are relatively pest-free.
Globe thistle (Echinops spp.) is a general name that references over a hundred species. The most commonly planted and probably best-suited globe thistles for Kansas are Veitch’s Blue (Echinops ritro ‘Veitch’s Blue’) and Blue Glow (Echinops banaticus ‘Blue Glow’).
Sea holly (Eryngium spp.) also refers to a large (more than 250 species) group of plants, and some are referenced by other common names. One in particular — called rattlesnake master (Eryngium yuccifolium) is a Kansas native. Flat sea holly (Eryngium planum) and Sapphire Blue (cultivated variety) are also excellent choices for the area.
Plant globe thistle and sea holly in the hottest, driest part of the garden. They are excellent choices for butterflies and other pollinators, xeriscape (low-water use) gardens, naturalized areas, and tall border landscape plantings. They also make excellent cut and/or dried flowers for arrangements.
At first glance, globe thistle flowers are spherical and about the size of a golf ball. A closer look shows stiff tubular florets emerging from a central point — like pins stuck very closely in a pincushion. Flowers are generally described as blue, although like many “blue” flowers in the gardening world, they may appear more lavender to purple in color.
The coarse, dark green foliage is deeply lobed like an oak leaf and similarly sized. The undersides are thick with woolly white hairs that give the plant a silvery glint.
Veitch’s Blue globe thistle is three feet or a little more at maturity. The variety was selected for its abundant flowers that are darker in color than the species.
Blue Glow globe thistle is very similar to Veitch’s Blue but larger, growing to four feet or more in the right conditions. Flowers of Blue Glow are described as steel blue.
Flowers of sea holly are dome-shaped and a little smaller than a store-bought strawberry. On rattlesnake master, flowers are white with a silvery green to very pale purple tint. Flat sea holly and Sapphire Blue sea holly have steel blue to pale purple flower heads that are similar in color to globe thistle. Florets are larger and less dense than globe thistle and sit on top of a ring of bracts.
Sea holly has very stiff, erect stems and coarse leaves like globe thistle, but the leaves of sea holly are long and narrow with serrated edges. They mostly grow from the base of the plant, with a few leaves emerging in clusters from nodes along the stems.
Rattlesnake master is best suited for the back of a flower bed, as a specimen plant, or in a naturalized area as it grows to four feet or more. Flat sea holly and Sapphire Blue grow to about 3 feet tall and are more compact in appearance. Flat sea holly also has a touch of blue in the stems and leaves as well as the flowers. Sapphire Blue was selected for its excellent blue coloration throughout the plant.
— Jennifer Smith is a former horticulture extension agent for K-State Research and Extension and horticulturist for Lawrence Parks and Recreation.