Sea holly, or Eryngium, is an attractive plant with interesting stiff blue-green foliage and bluish-green cone-like flowers. Because perennial varieties do not bloom the first year if planted from seed, 1-year-old plants usually are purchased and planted after all danger of frost. This allows for maximum bloom production the first year after planting. Although some varieties are perennial to USDA Zone 4, their survival rate is unpredictable. Sea holly flowers, leaves and stems are harvested and dried in a cool dry and dark area to preserve their color. They are used in a variety of garden crafts and projects.
Locate an area of the garden with average to rich, well-drained soil that gets at least six hours of full sun each day of the growing season.
Clear the area of all rocks and weeds. Spread with a shovel or by hand 1 inch of compost over the planting area along with the recommended amount of granulated organic fertilizer, according to the fertilizer label.
Work the fertilizer and compost mixture into the top 6 inches of soil with a hoe to create good drainage, as the compost helps keep the soil loose by adding organic matter to the soil. Rake the area smooth.
Plant the sea holly plants into the improved garden soil. Dig a hole that is sufficiently wide and deep enough so the sea holly plants are planted at the same level they are planted in the containers.
Handle the root systems and the plants very carefully. Take special care not to damage the tap root, the long central root that grows downward from the root system. Do not attempt to straighten or change the direction of the tap root, as it will straighten and grow downward if the sea holly is planted at the correct depth.
Cover the root base of the sea holly with the enriched soil while adding water to the planting mix to prevent any air pockets from forming around the roots. Avoid moistening the foliage when watering by placing the water source at the base of the plant.