To answer this question is with the standard response when addressing the whole of Alaska, it depends. To go further into that, it depends on the variety, how your growing it, where you are at, and what you can do to help it winterize.
For cold weather areas, the English Lavender varieties are going to be your safest and best option. There are some such as the Hidcote Superior that has been shown to survive temperatures down to -20 °F during the winter. These are all ones that can be planted and winterized outside and without very much fuss or extra care needed. If you are worried about them then it might be best to plant them next to the house so that the heat helps them over the colder parts of winter. Other varieties include:
- Munstead Lavender.
- SuperBlue Lavender.
- Royal Velvet Lavender.
- Melissa Lavender.
- Buena Vista Lavender.
- Violet Intrigue Lavender.
- Blue Cushion Lavender
You can still grow varieties from the Spanish, French, or Italian lavender groups, however they will need a bit of a different take in growing them.
Should you choose one of the non-English varieties or live in an area that gets colder than -20 °F during the winter, pot raising might be your better option. Thankfully, lavender does very well in pots as long as there is good soil drainage and you bring them in before the first frost.
All Lavenders enter a state of dormancy over winter and do not require much water (water them once every 4-6 weeks during winter) but the do need to be exposed to sunlight and ideally avoid the more humid rooms of the house such as the kitchen or bathroom where they could encounter lots of steam (Lavenders prefer drier conditions).
Place the potted lavenders in the sunniest window in your house and enjoy the fragrance over winter. Alternatively they can place them, over winter in a heated green house, as long as the temperature remains above freezing.
Pruning lavenders every year will help the plant survive winter and will serve to prolong a lavenders life in general. The lower stems at the base of the lavender become more woody over the course of its life.
Regular pruning in the early spring and late summer help delay the formation of woody stems which is necessary as it is the new shoots that produce flowers and not the old wood. Not to mention the wood is less resilient and prone to splitting in winter thanks to snow, frost and water damage.
When woody stems break over winter they do not usually grow back, so that lavender remains damaged.
Regular pruning of lavender will:
- Prevent lavenders from splitting and maintain a more attractive shape.
- Promote new growth and therefore produce more blooms
- Extend the life of lavender
- Pruning is best done in the early spring where you should remove about a third of the soft green growth and aim to shape the lavender into an even mound so the foliage is nice and compact.
More compact foliage will prevent snow and ice getting into the lavender and therefore resist the effects of winter better.
Typically you will see the best results from a late summer prune where the spent flower stems are trimmed back for a more tidy, uniform appearance ready for winter. This is followed by a Spring prune early next year to promote the growth of new stems that will bear this years flowers. Cut the soft green growth back by a third but do not cut back into the woody stems as they are unlikely to produce any new growth and you could kill the plant.