When can vegetables be planted outside
Mille Lacs County Minnesota possible springtails
If you are talking about seeding plants like lettuces, peas, radishes etc. now is the time to do so.
If you are asking about started plants, May 15 is the earliest time to put them in the ground. We can still get very cool weather, and even frosts. If the cold weather doesn't kill the plants, it will severely set them back.