Chris,
I have a Valerian plant in my bog garden. The rest of the garden is
very sandy and free draining. This year I have had seedlings appear
everywhere, in plant pots with my bay tree, in the strawberry bed, even
amongst my overwintered perennials in pots.
My treatment would be to sow now expecting to see them germinate next
year after receiving a cold treatment over winter. The RHS book on
propagating plants says sow in spring at 10=B0C