It depends on which plants (some will tolerate no frost at all) and which Z8 you are in. This Z8 is 35F right now at dinner time and we have 2" snow predicted tomorrow morning.

Do you know your last approximate average frost date? There has been light frost here May 1 more than one year. OK for a hardened off marigold, death to something like impatiens and at very least quite damaging to a tomato.

And anything you have started indoors, even if a cold hardy perennial, will have to be gradually acclimated to the outdoors over a period of several days. For most plants started inside, it's too early/too risky to begin the process, but tell us which plants you are considering....

Check your Farmer's Almanac to see when the average last frost date is. Add one week to be "safe." That's what I do & it hasn't failed me yet. :)

I move my seedlings outside everyday that the temperature is above 50 and bring them in at night and put them under my fluorescent lights. It is a lot of work but results in stocky, sturdy transplants that are fully hardened off.

I am growing purple coneflower, milkweed, annual phlox, zinnias and monarda thanks guys : )