Silk Seed

Description
Silk Seeds come from Silk Cocoons, which can be found in snowy environments, such as Hintertown and Pale Vale. The seeds must be planted in their environment in order to grow. They'll grow faster on earth that has been tilled by a hoe, but can be planted on a dirt block as well.

Here you can see the progress of seeds planted in the garden versus dirt block, with a new seed in the middle for comparison.