Here's a general guideline for seed-starting dates in the Twin Cities area. (Move the date a week earlier for every 100 miles south of the metro area, and a week later for every 100 miles north.)

EARLY TO MID-MARCH

Ageratum

Bells of Ireland

Celosia

Seed dahlias

Dianthus

Gomphrena

Nicotiana

Salvia

Snapdragons

Sweet alyssum

Vinca

EARLY APRIL

Hollyhocks

Cleome

Gloriosa daisies

Helichrysum

Marigolds

Annual phlox

Verbena

Broccoli

Cabbage

Eggplant

Head lettuce

Peppers

Tomatoes