Just in case anyone is growing this from seed as I am this year, be aware that they don't take kindly at all to 'pricking out' as I've found to my cost. Out of roughly 60 seeds sown into a small propagator and kept warm for about two months, I started moving my strongest plants (these, Hollyhock, giant sunflowers, Globe Thistle and Teasel so far) into 49 cell windowsill ones, there were 12 Viper's Bugloss that germinated (out of the ~60) and only two of these 12 have survived!
So if you do try, don't use my technique of gently lifting the plant out holding a leaf and teasing the soil away from the roots (worked OK for the vast majority of the other plants I transplanted..) rather try and grab the roots and soil whole and move it that way.
I've planted another 'emergency' batch so hopefully there'll be enough time for these to come through. You live an learn I guess.
Anyone had any luck with this variety? They're supposed to be brilliant for insects but finding them quite tricky, despite the fact they grow in poor conditions out in the wild.