After satisfying the tributes from the water temple, cleaning up the undersea garbage, upgrading the museum and donating more than 100 exhibits, the town's rating will be raised to C level, and Sam's Supermarket will provide a variety of new seeds, including winter tribute seeds.
Many people's rating will be stuck at C level in their first year in the town and cannot be upgraded to D level (you need to enter the innermost room of the community center to check the rating). This resulted in Winter Tribute seeds being unavailable in stores.
To be promoted to C level, the following four conditions need to be met:
Complete all tributes in the Water Temple except the Winter Tribute.
Clean up most of the trash on the ocean floor, at least as close as you can get to the Mermaid Palace.
Upgrade the museum and donate more than 100 exhibits (there is no counting in the game, you need to count it by yourself).
It is not possible to upgrade to C level in the first year, but after meeting the above three conditions, it will be automatically upgraded in the second year (to be confirmed).
After upgrading to level C, Sam's Supermarket will provide a variety of new seeds, including winter tribute seeds.