Choose your lavender varieties carefully when planting from seed, and choose one that is suited to your climate. If you have your heart set on a certain type, you can always grow it in a pot that you can move inside during harsh weather. For example, Munstead is best grown in USDA zone 5 while fernleaf is better in zone 8.
Plant seeds 1 inch apart in flats. Place the seeds on potting soil and cover with 1/8 inch of soil. If you want to plant them straight into the ground, dig shallow holes for the seeds at least 1 or 2 feet apart. When growing outside, plant six weeks before frosts generally occur.
Dampen the soil and check it daily to see if it needs more water. Seedlings can take up to a month to germinate. Keep the soil damp to help establish roots.
Transfer small seedlings to larger pots if you're growing them in a tray. Move them again to the outside once they reach 3 inches. Lavender does not need a nutrient-rich soil to survive, but does need a well draining one. If the soil around your home is made of something like clay, improve it before planting. Dig up the ground in a large radius around where you are planning to plant the lavender and add a liberal amount of sand to increase drainage.