Celery (cool-season stalk 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
12–10 weeks before the median last spring freeze
Sow 10–12 weeks before the set-out window opens (UNH: time from seed to transplant 10–12 weeks).
Transplant outdoors
from the median last spring freeze to 2 weeks after
UNH: set out after the last frost. Purdue's semi-hardy class allows 2–4 weeks before; the later UNH rule is used (celery is easily vernalized by cold spells).
Seed germination soil temperature: optimum 70°F (optimum range 60–70°F); minimum 40°F, maximum 85°F. Purdue HO-186-W Table 3. Cold soil below the minimum rots seed — a reason windows and soil readings both matter.
Variety assumption: A long, steady season (about 100–120 days from transplant; Penn State lists 120).
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.