John Polk wrote:Yarrow (Achillea millefolium) is a medicinal herb, that greatly improves any plant that is raised for the essential oils.
The yarrow triggers an increase in oils in nearby plants, hence more flavor/aroma.
If you want mint that will 'knock your socks off', plant it amongst yarrow.
Most culinary herbs, likewise will benefit from the yarrow.
It is hardy to about zones 2-3.