Eryngium planum ‘Jade Frost’ – A variegated form of Sea Holly with bluish-green basal foliage edged with cream margins that occasionally show a hint of pink. It bears violet-blue thistle-like blooms midsummer. The flower stems make excellent cuts and can be dried by harvesting when mature and hanging upside down. Grows 24-30″ high by 12-24″ wide. Hardy to zone 5.
Eryngium varifolium – Moroccan Sea Holly is primarily sold as a foliage plant, with the beautiful basal leaves being heavily marbled in white. It bears pale lavender-blue flowers with spiny white bracts that are produced from July to August. This smaller growing species is a good feature plant in rock or alpine gardens. Grows 12-18″ high by 12″ wide. Hardy to zone 5.
Eryngium planum ‘Blue Hobbit’ – A dwarf Sea Holly that is useful for planting in those difficult dry, sandy soils. It features a tap-rooted basal rosette of undulating green leaves with dense umbels of silvery-blue thistle-like flowers in midsummer. This is a good choice as a container accent and will attract butterflies. Grows 8-12″ high by 10-12″ wide. Hardy to zone 4.
Eryngium planum – This native of southern Europe is considered by many gardeners to be a bit invasive, but if you have large gaps in full sun borders that suffer from heat or drought, then this is your plant. It bears masses of small metallic-blue flowers (from June to August) that make excellent long-lasting cut flowers. Grows 2-3′ high by 2’+ wide. Hardy to zone 4.
Eryngium x ‘Big Blue’ – A Blooms of Bressingham introduction with dramatic thistle-like foliage with both silver and blue highlights, depending on the season. It bears up to 4″ wide iridescent blue flowers surrounded by silvery-white bracts from July to September. ‘Big Blue’ is best used as a specimen plant given its architectural beauty. Grows 24-30″ high by 24″. Z4.