Pruning the flowers should be enough........it's now & wait & see.....they love rock gardens with good drainage. If you get freezing weather in the winter, you can mulch around the roots. Lavendar does not winter well in our area........too many night under zero. But it was suggested to me that choosing a sunny area in a winter spot to plant it to protect it especially in a inside corner against your home will give it warmth for the winter. Hidcote & Munstead are the hardest lavenders to grow.