Looking for a striking purple-blue flower that's as low-maintenance as it is attractive? Take a look at sea holly. Sea Holly plants (Eryngium) look like glowing small thistles. They are very similar to globe thistle (Echinops), but sea holly flowers have a distinctive collar and make a compelling addition to your garden. They are very tolerant of dry conditions and also handle salt spray with ease. Although they can be more difficult to grow in areas with cold, wet winters, if you can get them established, you should have good luck with them.
Sea Holly plants can have green or silvery-blue stems. The leaves can be long and thin and not much to look at, deeply lobed, or even round. It all depends on the variety. The flowers are unique and arresting - green or blue cones with bract "collars” in silver, white, green or bluish-purple. The colors often look almost metallic and painted on and can change in the sunlight. As you would expect from looking at them, some sea holly flowers can be coarse to the touch and will scratch you, but not so much that handling them is onerous. Others are actually quite surprisingly soft.
Mature Plant Size
Size depends on the variety you are growing. The small common sea holly (Eryngium maritimum) only gets about 6 – 18 in. tall. Giant sea holly, (Eryngium giganteum), can grow as tall as 4 ft. Giant sea holly also goes by the name "Miss Wilmot’s Ghost", because of its shadowy coloring. It is named for an English gardener, Ellen Wilmot.
A full day of sun will give you’re the strongest sea holly plants and the most blooms. They can handle a bit of partial shade, but the plants will be floppy.
Flowering starts in mid-summer and will continue into the fall. Frequent deadheading of the flowers will prolong their bloom time.
Sea Holly Growing Tips
- Soil: Sea holly is not particular about soil pH. Anything around the neutral range would be good. However, the plants need good drainage or they will die off. Sea holly has a long taproot. This makes it very drought tolerant, but it also means it is getting its water from deep in the soil and surface moisture will just allow the crown to rot.
- Starting from Seed: Most sea holly varieties can be started from seed. They do best if stratified first. The easiest method is to direct sow in the fall and then be patient and wait to see what sprouts in the spring. But you could start sea holly seeds indoors if you chill them for about 4 weeks in the refrigerator and then move them out to germinate in about 2 - 3 weeks.
- Propagation: Because sea holly has a tap root, it does not divide easily. You can, however, take root cuttings in the spring.
- Planting: Sea holly transplants easily as seedlings, but it gets more difficult as the plants mature, which is why you don’t see a lot of them in plant nurseries. If you want to move volunteers, do it early.
Caring for Sea Holly Plants
- Water: Once established, sea holly is very drought tolerant and won’t need additional water unless subjected to a prolonged, hot, drought.
- Fertilizer: Sea holly is not a heavy feeder, 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 with compost in mid-season.
- Maintenance: These are low maintenance plants. They don’t like to be moved and they don’t like excessive water and fertilizer. Sea holly will bloom longer if you deadhead the spent flowers, but they look wonderful long into winter, so leave the fall flowers on the stems. The birds will thank you.
Pests and Problems Affecting Sea Holly
- Problems with sea holly are usually caused by damp conditions either in the soil or air.
- Slugs are about the only insect to break through sea holly’s defenses. If your plants are in a dry, sunny spot, slugs should not be a major problem.
- Sea holly is susceptible to root rot and powdery mildew. Rot root can be controlled if you have well-draining soil. Powdery mildew is tougher to control, but you can try a baking soda or milk spray.
Using Sea Holly in Your Garden Design
This is a great flower for all those spots in the garden where the hose doesn’t reach or in that infamous hell strip between the sidewalk and the street. But don’t confine it there. The blues and silvers blend well with just about every color, especially yellow and orange. Pair it with rudbeckia, coreopsis, zinnia, and cosmos. The tall varieties can need some support and planting them behind sturdy plants, like coneflowers, will help keep them standing.
Sea holly is popular with bees and butterflies, but not with deer and so far, not with rabbits. The flowers last several days in a cut flower arrangement.
The Best Varieties of Sea Holly to Grow
It’s getting harder and harder to recommend varieties because new introductions come out every year and claim the shelf space of older types. They’re all lovely, but some grow better in different conditions, so check to see which are successful in your area.
The names will try and outdo each other with sprinklings of "glitter" and "sapphire", but it’s hard to know how they’ll perform in your garden. There are also some new varieties, like Eryngium planum "Jade Frost", that have variegated foliage. "Jade Frost" also has pink margins and veins. You may find a wider variety of choices in catalogs.