How to Grow and Care for Sea Holly

Sea holly (Eryngium) is a striking and hardy perennial plant that thrives in coastal conditions but can be successfully grown in gardens away from the sea as well. Known for its spiky, metallic-blue flowers and unique foliage, sea holly can add texture and a bit of drama to any garden. Proper care and attention will ensure it grows to its full potential. Here’s a guide on how to grow and care for sea holly.

1. Choosing the Right Location

Sea holly thrives in full sunlight, which is essential for the best flowering and foliage color. Ideally, plant it in a spot that receives at least six hours of direct sunlight per day. While it can tolerate partial shade, it may not flower as profusely.

Sea holly does well in well-drained, slightly acidic to neutral soils. The plant is drought-tolerant once established and prefers dry to medium moisture levels. It is especially suited to sandy, gravelly, or poor soils, which makes it a great choice for coastal gardens. However, avoid planting it in areas with heavy, clay-based soils that retain water, as the roots are prone to rot in soggy conditions.

2. Planting Sea Holly

To plant sea holly, follow these steps:

• Timing: The best time to plant is either in the spring, after the last frost, or in the fall before the first frost. This gives the roots time to establish before extreme temperatures hit.

• Spacing: Sea holly can grow quite large, so space your plants 18-24 inches apart to allow for proper air circulation. This spacing helps prevent overcrowding and supports good growth.

• Soil Preparation: If your soil is heavy or lacks good drainage, amend it by adding sand or gravel to increase drainage. If the soil is too rich in nutrients, it can cause the plant to become leggy. Sea holly prefers poor to moderately fertile soil.

• Planting Depth: Dig a hole large enough to accommodate the plant’s root ball, which should sit level with the surrounding soil. After placing the plant in the hole, fill it in with the soil mixture, and water thoroughly to settle the soil around the roots.

3. Watering and Fertilization

While sea holly is quite drought-tolerant once established, it requires regular watering during its first year of growth. Water deeply to encourage the roots to grow deep into the soil. However, avoid overwatering, as the plant is susceptible to root rot in soggy conditions.

After the first year, watering can be reduced, as sea holly can withstand dry spells. During periods of drought, it’s beneficial to water once a week to prevent the plant from becoming stressed.

Fertilizing is generally not necessary for sea holly, as it prefers poor soil. If the soil is particularly depleted, you can apply a balanced, slow-release fertilizer in the spring before the growing season begins. Avoid over-fertilizing, as this may lead to excess leaf growth at the expense of flowers.

4. Pruning and Deadheading

Pruning is not typically required for sea holly, but it can benefit from occasional deadheading to encourage additional blooming. Remove faded flowers as they start to dry out to prevent the plant from diverting energy into seed production. This will allow the plant to focus on producing more blooms.

In late fall or early spring, you can cut back any old, dead foliage to make room for new growth. However, it’s not necessary to trim the plant aggressively. Sea holly’s natural form is often attractive, so unless there are damaged or overcrowded areas, light pruning should suffice.

5. Managing Pests and Diseases

Sea holly is a relatively low-maintenance plant in terms of pest control. It has few natural pests, though some insects, such as aphids, can occasionally cause issues. If aphids appear, you can treat them with insecticidal soap or by spraying the plant with a strong jet of water to dislodge them.

Sea holly is also resistant to most common plant diseases, but it can occasionally suffer from fungal infections, especially if the soil is too moist. To prevent this, make sure the plant has good air circulation and avoid overhead watering that can leave moisture on the leaves for extended periods.

6. Propagating Sea Holly

Sea holly can be propagated by seed, division, or cuttings:

• By Seed: The easiest method of propagation is by collecting seeds from your plants. Let the flowers mature and form seed heads. Once the seeds are fully dried, they can be harvested and stored for planting. Plant the seeds in a well-draining soil mix in early spring. Sea holly seeds often require cold stratification for a few weeks before they will germinate. Place them in the refrigerator for a few weeks before sowing for the best results.

• By Division: Another way to propagate sea holly is by dividing established clumps. This can be done in early spring or fall when the plant is not actively blooming. Carefully dig up the plant and separate the root ball into smaller sections, each with some roots attached. Replant the divisions in their new spots, ensuring they have adequate spacing.

• By Cuttings: While it’s less common, you can also propagate sea holly by taking softwood cuttings in early summer. Cut a non-flowering stem with a few leaves, and remove the lower leaves. Dip the cutting in rooting hormone and place it in a pot with a well-draining mix. Keep the cutting moist and place it in a warm, bright spot.

7. Overwintering and Winter Care

Sea holly is hardy in USDA zones 4-9, making it suitable for a variety of climates. However, it can benefit from some winter care in colder regions. While the plant can survive cold temperatures, mulching around the base of the plant can protect the roots from freezing. Apply a layer of mulch in late fall to help insulate the soil, but avoid covering the crown of the plant to prevent rot.

In areas with extreme cold, it may be helpful to cover the plant with a frost cloth or provide extra protection during harsh winters. Once the plant is established, it can generally handle the winter weather with minimal intervention.

8. Varieties of Sea Holly

There are numerous varieties of sea holly that offer different color variations and sizes. Some of the popular varieties include:

• Eryngium planum (Flat Sea Holly): Known for its flat, spiky, blue flowers and attractive silvery foliage.

• Eryngium alpinum (Alpine Sea Holly): Smaller in size with rich blue flowers and silver-gray leaves, making it ideal for rock gardens.

• Eryngium giganteum (Miss Willmott’s Ghost): Known for its striking silver-blue spiky flowers and distinctive grayish foliage.

Each variety has slightly different care requirements, but they all thrive in similar conditions, so it’s easy to mix and match them in a garden for a unique look.

9. Design Ideas with Sea Holly

Sea holly’s architectural form makes it a perfect addition to a variety of garden styles. Here are a few ideas for incorporating it into your landscape:

• Coastal Gardens: Sea holly thrives in coastal environments, so it’s perfect for beach-themed or coastal gardens. Its spiky flowers complement other hardy coastal plants like lavender, yarrow, and ornamental grasses.

• Cottage Gardens: The striking flowers of sea holly can be a great contrast to the softer forms of traditional cottage plants. Pair it with other perennials like daisies, coneflowers, and foxgloves for a wildflower look.

• Cut Flower Gardens: Sea holly’s spiky blooms make great additions to cut flower arrangements, and it has excellent vase life, making it ideal for floral displays.

• Rock Gardens: Due to its drought tolerance and preference for well-drained soil, sea holly works beautifully in rock gardens, providing texture and interest among low-growing succulents and other tough plants.

Conclusion

Sea holly is a hardy and low-maintenance plant that can thrive in various garden settings. By choosing the right location, providing proper care, and managing any pests or diseases, you can enjoy its striking flowers and foliage for many years. Whether you’re designing a coastal, cottage, or rock garden, sea holly’s unique beauty will be an eye-catching addition to your landscape.