The best soil mix for winter sowing is a light, airy mix that drains well to help prevent the seeds from rotting or damping off. Some good options include: 1. A potting mix created specifically for starting seeds.A potting mix designed specifically for starting seeds. 2. Promix(a soilless potting mix that is made from peat moss, perlite, and vermiculite) 3. Miracle-Gro Seed Starting Mix (a popular brand of potting mix that is also a good option for winter sowing) is made from peat moss, perlite, and vermiculite and contains a slow-release fertilizer.