The Hedgewitch of Foxhall by Anna Bright

In this gorgeous stand-alone fantasy romance perfect for fans of Margaret Rogerson and Allison Saft, a rebellious witch undertakes a last-ditch quest to restore magic to medieval Wales-as two princes vie for her heart.
Magic is fading from Wales-choked off by King Offa's Dyke, the enemy earthworks that spans the entire border. Even the dragons have disappeared. And now an attack is imminent.
Prince Taliesin would love to watch magic die. Prince Dafydd fears it, and the throne. But when their father promises the crown to whichever son can destroy the dyke and restore magic to Wales, the brothers are forced into an uneasy rivalry.
Ffion works hedgewitch magic for poor folk, not princes. Unlike the power-hungry Foxhall coven, she uses only what nature can spare. But when the coven's greed costs Ffion everything, she will need power beyond her wildest dreams to get back what she's lost.
So when Prince Taliesin arrives, begrudgingly seeking a witch's aid, Ffion agrees to help him-even if it means walking from one end of Wales to the other with the most use-less peacock she's ever clapped eyes on. Even if it means striking a bargain with Dafydd behind Tal's back. The fate of Wales depends on their quest . . . and so might the fate of Ffion's heart.