Here are some general rules to follow in North America (according to hardiness zones outlined by the United States Department of Agriculture) about when to start your tomato seeds. This seed starting timetable allows 8 weeks for seeds to germinate, grow at least two sets of leaves (or more), reach transplant size, and harden off before setting in the garden.
Zones 9 & 10: Start seeds indoors in early to mid January
Zone 8: Start seeds indoors in early February
Zone 7: Start seeds indoors in mid February
Zone 6: Start seeds indoors in late February
Zone 5: Start seeds indoors in early March
Zones 1-4: Start seeds indoors in mid to late March
What’s a “plant hardiness zone”?
The USDA Hardiness Zone Map divides North America into 11 separate zones, called “plant hardiness zones.” Each zone is 10°F warmer (or colder) in an average winter than the adjacent zone. The map was developed to provide climate information important to horticulture. Most gardeners in North America rely on the USDA Hardiness Zone Map as they plan gardens and choose plants.
Gardeners in other parts of the world use plant hardiness zone maps that follow the USDA model but are modified for their particular climate. For example, Africa’s map is divided into 13 hardiness zones. Australia’s temperatures are mild across the continent; its hardiness zone map is divided into just 5 zones and noted in metric units.
Tomato seed starting timetable at a glance
When to sow tomato seeds indoors
6-8 weeks before last freeze date
When to transplant tomato seedlings to the garden
1 to 2 weeks after last spring frost
When to sow tomato seeds outdoors
After last spring frost (if your season is long)
Other tomato seed sowing details
Germination time: 6-14 days, depending on variety
Germination temperature: 70-85º F
Sowing depth: 1/4 inch
Days to maturity: 53-90 days from transplant, depending on variety