When can I plant these out?

I've started petunias, impatiens, vinca, verbena, bacopa and lobelia indoors. My impatiens and some of my vinca have started to get big and are developing flower buds. When is it safe to plant them outside? Our last frost date is April 15th. Thanks so much!
--Pam

If you want to be safe, then I'd wait until your last frost date. But it wouldn't hurt to start gradually getting them hardened off and used to the outdoor temperatures, sunlight, etc now. That way when 4/15 hits they'll be ready to go in the ground.

Our average last frost date is also 4/15 but we almost always have a surprise hard freeze that damages the perennials. I get spring fever and get impatient waiting to plant, but I don't like to take a chance damaging my plants just to get them in early so I wait until at least May 1.

I agree with ecrane 3. April 15 isn't very far away. Put them outside during the day, and take them in at night if the temp will be too low. That way they will be hardened off and ready to be planted outside when it is safe in your zone.

I have lived for years in both MA and NY so familiar with surprising cold spring weather. In MA. Our safe planting date was May 20 there. One year we had snow first week in June.

I'll start hardening off the big guys. Thanks for your help.