The secret to overwintering lavender is proper planting. Most lavenders die during winter because the soil they are planted in becomes waterlogged or soggy. Make sure your lavender is planted in a spot that has fast-draining soil, and work organic matter in the form of compost or peat moss as well as horticultural sand into the planting site.
Northern gardeners who have little success in overwintering lavenders should try planting lavender on the south side of buildings or walls. Soil stays 5 to 10 degrees warmer on the south side of buildings and the buildings block some of the snow, ice and wind that would otherwise compromise the health of the lavender. Given the proper planting conditions, though, lavender is extremely easy to winterize.
Remove your lavender's flower stalks when the flowers begin to fade. Cut the stalks off as close to the leaves as possible. If your lavender has outgrown its spot you can reduce its size by one-third. Do this pruning no later than 3 weeks before your first frost date.
Wait until the first hard freeze to winterize your lavender. Doing this will ensure your lavender becomes fully dormant.
Cover your lavender with straw or chopped leaves. For large lavenders (taller than 3 to 4 feet) cover the base or crown of the plant with straw or chopped leaves. Pile straw or chopped leaves around the bottom half to two-thirds of the plant.
Remove the straw or chopped leaves in spring after all danger of frost has passed.