Nasturtium (flower) 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
6–4 weeks before the median last spring freeze
UW-Madison states seed can be started indoors in peat pots 4 to 6 weeks before the last expected spring frost, giving 42 to 28 days before last frost.
Transplant outdoors
from the median last spring freeze to 2 weeks after
NC Cooperative Extension (Pender County) names nasturtium among the large-seeded plants to transplant outdoors after the threat of frost, giving a two-week set-out window from the last-frost date.
Direct-sow outdoors
from the median last spring freeze to 2 weeks after
Sources disagree - UW-Madison and Illinois Extension sow nasturtium only once all danger of frost has passed, while Iowa State treats it as a cool-season annual that can go in 3 to 4 weeks before the last frost - and the later, more frost-conservative after-frost window wins, opening at the last-frost date and running two weeks.
Variety assumption: Covers both climbing (T. majus) and bush (T. minus) garden nasturtiums; Illinois Extension notes nasturtium does not transplant well, so the transplant window assumes peat or paper pots planted intact, as UW-Madison specifies.
"Seed can be sown directly in the garden in late spring to early summer once all danger of frost has passed or started indoors in peat pots 4 to 6 weeks before the date of the last expected spring frost."
"Large seeded flowers and vegetables are the easiest to grow and include cucumbers, zucchini, squash, melons, cosmos, marigolds, nasturtium, zinnias, and sunflowers. To get a head start on the season, seeds of these varieties can be sown indoors in mid-March to transplant outdoors after the threat of frost."