English lavender varieties, while cold hardy, are native to drier Mediterranean climates and are typically grown annually in northern states. The issue with harsher, colder winters and slower evaporation rates is that thawing, damp, cold soils retain lots of moisture around the lavender roots, leading to root rot. This condition will often kill lavender, rather than the chilly air.
One way to prevent this is by planting lavender on mounds. These mounds, when properly constructed, can help drain excess water away from the roots, reducing the risk of root rot. Consistently removing the snow from the dormant plants is also important to allow the water from melting snow to drain through the soil past the roots. Even with these precautions, we cannot promise that plants will survive the winter.