Caring for a Lavender Plant?

I finally have a lavender plant (tried many times from seed) that started out very small, but now is about 2 feet in diameter. It has last season's old growth on it. I was wondering if and how I should prune it and if there's any type of "food" it likes or diseases to look out for? It took so long to finally get a lavender plant I don't want to risk losing it. Thanks for your help!

Hardiness Zone: 6b

By kim from Westminster, MD