"In Sussex and Devonshire superstition, yarrow was used for finding one's real sweetheart. [...] In a similar tradition in Wicklow, girls would pick yarrow on Hallow Eve and recite:

Thou pretty herb of Venus' tree,
Thy true name is yarrow;
Now who my bosom friend may be,
Pray tell thou me to-morrow.

then retire for the night without speaking and go to sleep with an ounce of yarrow sewn in flannel under the pillow." (from Wikipedia)