Which Scabiosa do you mean? I would have thought that deadheading would keep most flowering longer, although I never bother as I like mine to seed around (Scabiosa columbaria, Knautia arvensis and Centaurea scabiosa). Or do you have Cephalaria gigantea? I don't think any of them are biennials.

i have scabosia atropurpea tall 3ft type not purple i think light shades of ice blue,white pink.i never label things much and plant them not knowing what they are sometimes but i put these in mid summer and a lot flowered and some came thru winter to flower again