(Image from: https://hudsonvalleyseed.com/products/munstead-lavender?_pos=1&_sid=e544a07ea&_ss=r)

Growing Instructions for Munstead Lavender: 

Start indoors 8-10 weeks before last frost, sowing the seeds shallowly as light aids in germination. Transplant outdoors into well drained soil after the danger of frost has passed, taking care not to transplant too deeply. Clip back all spent flower stems in late fall.

(from Hudson Valley Seed Company website)