To raise the quality of your crops you can place fertilizer on the plant while it is growing. To carry the Star Rank over to the next generation of crops, you will need to use the Seed Maker shed to convert your crops back into seeds for planting. The blueprint for the seeding shed is for sale at the Silk Country vendor for 40,000 G from the start of the game. You will need to combine 10 Small Lumber, 15 Lumber, 5 Brick, and 5 Turnip Seeds (any quality). If you don't have access to the Safari or the ability to buy Brick, you can find it by diving in the rivers.