I've heard you can't buy French Tarragon as seed. I googled for this: "French tarragon rarely sets seed. It is propagated by division, or from cuttings. Division is best undertaken during the dormant period - autumn or early spring. Carefully cut or gently pull a piece of root apart from the main plant, making sure there is a dormant bud attached. Cut the piece of root to about 8-10cm (3-4in) in length. Put these to root, laid horizontally or vertically, in a well drained organic cuttings mix. Keep them cool but frost-free. Water initially, but keep the compost on the dry rather than wet side"
French tarragon