Diane, How about a simple grain cover crop? Oats or wheat or rye should grow well in the winter. It would be green throughout the winter and look fresh and healthy. It would add organic matter to the soil and improve the tilth. Then, before its time to plant the dahlia tubers, till the cover under and enjoy healthy dahlias the following season. Or would this solution produce weak growth in the dahlias? Dahlias are not my special interest so I don't know exactly what the require. Other ideas? Brook