Eryngium alpinum, also known as alpine sea holly or blue star, is a perennial plant that is native to Europe. It is generally found in the valleys of the Alps and Jura mountains and features metallic blue flowers; bluish-green stems and leaves; and silvery bracts. The bracts protrude from the base of the flower head, which is shaped like a thimble. The upper leaves have three lobes while the leaves that are closer to the ground are heart shaped, but both have slightly-toothed edges. When fully grown, the Eryngium alpinum can reach a height of 3 feet (0.9 meters), with a spread of about 2 feet (0.6 meters) wide.
The blue star usually grows in hardiness zones three to eight. The cold temperature of the lower zones don't normally hinder the growth of Eryngium alpinum, but the lack of direct sunlight may affect its development. Eryngium alpinum should be planted in an area that is exposed to several hours of sunlight each day. Gardeners also should take into account the shade produced by nearby trees and structures.
Eryngium alpinum can thrive in poor soil, but it is recommended to add peat and compost to improve soil conditions. The soil should be well draining and sandy for optimum growth, and adding fertilizers that are rich in nitrogen can aid the development of the leaves and bracts. Generally, liquid fertilizers should be added to the soil every 15-20 days, while slow release fertilizers typically are added to the soil every 3-4 months. Eryngium alpinum is capable of thriving in drought conditions and generally prefers less water — three glasses of water every three weeks is usually enough for Eryngium alpinum.
The blue star can live for many years if it is protected from insect and fungal damage. To protect the blue star from insect damage, especially from aphids and scales, apply insecticides at the beginning of spring. The best way to manage fungal growth is to prevent it from colonizing on the blue star by spraying preventative fungicide on it. Gardeners typically can find insecticide and fungicide products at nurseries and garden centers.
Once the blue star is established in an area, it is difficult to transplant or propagate by division because of the taproot. Root cutting and seed sowing are easier methods to grow new Eryngium alpinums. Once the flower blooms, it can be cut and displayed for several weeks or dried and stored. The stem from which the cut is made also will bloom new flowers.