Eryngium planum, the blue eryngo[1] or flat sea holly, is a species of flowering plant in the family Apiaceae, native to the area that includes central and southeastern Europe and central Asia.

It is an herbaceous perennial thistle growing to 50 cm (20 in) with branched silvery-blue stems, and numerous small blue conical flowerheads surrounded by spiky bracts in summer.

[2] Eryngium planum is used in European folk medicine as a diuretic, a stimulant, and an appetizer owing to its essential oils, and bioactive compounds,[3] and in this usage it may be known as Eryngii plani herba or Eryngii plani radix.

The groups of bioactive compounds present in E. planum are phenolic acids, triterpenoid saponins, flavonoids, coumarins, and essential oils.

Qualitative and quantitative determinations of the phenolic acids by reverse phase high-performance liquid chromatography (RP HPLC) show relatively small amounts of rosmarinic, chlorogenic, and caffeic acids in the basal leaves and the roots of intact plants, and greater concentrations in E. planum from in vitro cultures.