Answer: Grass seed can actually be planted before the last frost; you want to get grass seeded early in the season so it has as long as possible to grow before the hot summer weather sets in. (Keep in mind also that early fall is the best time of year to seed a lawn.) You could seed in March as long as there is no snow on the ground, the seed should settle into the soil during frost/thaw periods and then germinate in April when the soil warms up.
Hardy perennials can also be planted before the last frost as long as they have been acclimated to being outside at the nursery -- and have not been forced into premature growth. Otherwise, plant when you think the last frost has passed.
You can check average frost dates for both spring and fall at:
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