When is the best time to start seeds? Start some seeds too soon and you will end up with lanky plants under the lights. Too late and the plants will be too weak when transplanted to the garden or may not mature in time to produce before your frost. Develop a seed starting schedule so you know the optimum time to start your seeds. A seed-starting schedule provides a guideline of when to sow seeds and when to transplant seedlings the vegetable garden.