Tomatoes (warm-season fruiting vegetable) is classed tender in the cited extension guidance. The timing rules below are relative to your local freeze dates — enter a location to turn them into calendar dates.
The rules this site applies
Method
Frost-relative window
Rule & reconciliation
Start seeds indoors
7–5 weeks before the median last spring freeze
Sow 6–8 weeks before the outdoor transplant window opens (UNH: time from seed to transplant 6–8 weeks; Missouri G6570: 6–8 weeks in packs).
Transplant outdoors
1–3 weeks after the median last spring freeze
UNH: safe to set out 1–2 weeks after last frost. Purdue classes tomato as tender — plant after the average last spring frost. Latest-starting rule (UNH) used; span extended to 14 days.
No direct-sow outdoors window: Direct seeding is not recommended timing practice in the cited guides for most U.S. climates; tomatoes are grown from transplants.
Seed germination soil temperature: optimum 85°F (optimum range 60–85°F); minimum 50°F, maximum 95°F. Purdue HO-186-W Table 3. Cold soil below the minimum rots seed — a reason windows and soil readings both matter.
Variety assumption: Assumes an average mid-season variety (roughly 65–80 days from transplant).
Hardiness classes: hardy = plant 4-6 wk before last spring frost; semi-hardy = 2-4 wk before; tender = after average last frost; very tender = at least 2 wk after. Table 2: fall planting at least 4-8 wk before first fall frost.