Teucrium hircanicum 'Purple tails'

To quote Beth Chatto:

A selected form. A useful vertical for virtually any soil or situation. Rather course, sage-like foliage. Each stem producing a spike of dusky purple-red flowers during mid to late summer. Associate well with silvers and greys of different shapes and textures. The seed heads stand well although at the risk of over abundant seeding. 

Photo by: Salicyna, Wikimedia Commons