The Centaurea cyanus, commonly known as cornflower, is an annual flowering plant native to Europe. It used to grow in cornfields, hence its name.
Nowadays, it's traditional for Austrian MPs to wear a flower in their buttonholes at the opening of parliament. The colour of the Freedom Party, a right- wing populist party, is blue, so they have been wearing a cornflower at the opening of the Austrian parliament since 2006.
The blue cornflower was one of the national symbols of Germany mainly due to the story that when Queen Louise of Prussia was fleeing Berlin, pursued by Napoleon's forces, she hid her children in a field of cornflowers and kept them quiet by weaving wreaths for them from the flowers. The flower thus became identified with Prussia (it was also the colour of the Prussian military uniform). Maybe for this reason it was her son´s (Kaiser Wilhelm) favourite flower, too. This flower was used by pan-German nationalists in the 19th Century and after the unification of Germany in 1871, and it went on to become a symbol of the country as a whole.
Between 1934 and 1938, when the Nazis were a banned party in Austria, it was the secret symbol they used to wear in order to recognise each other.
Cornflower blue is a defined colour and one of the favourites of the Dutch painter Vermeer (ultramarine was the finest and most expensive blue used by Renaissance painters made by grinding lapis lazuli, a semi-precious stone into a powder).
It is worn in France as "le bleuet de France", a symbol of remembrance, similar to the poppy in the UK (symbol of the 11th November 1918 armistice). After World War II it was also used in France on May 8, the date of the capitulation of Nazi Germany.
In France, it is linked to a poem, similar to In Flanders Fields
« Les voici les p’tits « Bleuets »
Les Bleuets couleur des cieux
Ils vont jolis, gais et coquets,
Car ils n’ont pas froid aux yeux.
En avant partez joyeux ;
Partez, amis, au revoir !
Salut à vous, les petits « bleus »,
Petits « bleuets », vous notre espoir ! »
— Alphonse Bourgoin, extrait de Bleuets de France, 1916
Cornflowers are often used as an ingredient in some tea blends like the famous Lady Grey.