Tomato Growth Timeline from seed to harvest We start seeding our tomatoes around March, we are in zone5b. The timing for starting tomato seeds is often determined by the last frost date in your area. As a general rule, start your tomato seeds indoors 6-8 weeks before the expected last frost date. You can also search for a planting calendar online that is more specific to your area. SEED STARTING MEDIUM: Coco coir is produced from the husk of coconuts. On its own it does not contain any nutrients