Valerian Plant Harvest

I have a valerian plant in my garden and would like to know the proper time to harvest it. Do I chop it down before it flowers? Or after? Are the roots good?

Deni Bown, in her book "Encyclopedia of Herbs and Their Uses" writes that the roots are harvested in the second year after the tops have died down in the late fall. They are then washed and later dried for 10 to 12 days at 25 to 30 degrees Celsius. Temperatures of 40 to 60 degrees Celsius destroy the active ingredients.
The roots are used for their sedative properties. They can be taken as a tea or made into a tincture. Valerian extracts are among the most used herbal remedies in Europe. For more information on usinf and preparing valerian, please see Andrew Chevalier's "Encyclopedia of Medicinal Plants". Both books mentioned are available from Richters.