Sweet potatoes (warm-season tuber) is classed very 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
Transplant outdoors
(plant slips)
2–4 weeks after the median last spring freeze
Purdue: very tender (vegetatively propagated) — plant slips at least two weeks after the average last frost.
No direct-sow outdoors window: Sweet potatoes are grown from slips, not seed.
Variety assumption: Assumes an average variety (about 100–120 days of warm weather from slips).
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.