English lavender (Lavandula angustifolia), with its aromatic leaves and sweetly fragrant flower spikes, is often considered an herb. But it is actually an herbaceous perennial with a semi-woody growth habit. It's often grown to dry its leaves and flowers to make sachets and potpourri, and for its oils used in sleep aromatherapy. It is also an attractive garden plant with vibrant purple flowers appearing in late spring to early summer.