The plant should be sufficiently dormant to cover with mulch. If a good snow cover comes early and stays, that's the best and cheapest mulch you can get. However, we can't plan for or depend on getting an early snow cover that will last all winter long. So, we have to annually apply mulch late each fall. The type of organic mulching material is important.
Mulch hay is the worst because it is usually full of weed or grass seeds.
Using mulch hay can result in all kinds of weed problems. Clean wheat straw is an ideal material. Oat straw, wood chips or shavings and pine needles are materials that can be used if available at reasonable costs. Old leaves are not recommended because they mat down and can smother the strawberry plants. Spread the mulch uniformly over the planting.