well, since i cant find your town (gwinn? MN) on a map, its hard to tell where you are located... however, i live in northern MN zone 3, and we can normally grow onions tomatoes, strawberries... also many many others; corn, squash, pumpkin, carrots, beans and peas, and even broccoli, cauliflower, lettuce... USUALLY we can plant by the middle of may, which is now, but the unusually cool weather this year might push that into the middle of june. i KNOW that in past years (the past 3 anyway) my husband and i have bought flowers in the beginning of may and started our flower gardens by now. the shop we use opens the first of may and we are always really anxious for her to open... and this year we arent even thinking about it yet cuz its so cool.
so i would start the middle of june, or when temps start to stay above the 45 degree mark at night. i think thats probably a good night temp.. although higher would be preferrable. you can start seedlings now, but, like mine, they may grow too big for their seedling pots and you might have some trouble with that. i have TONS of seedlings that are waiting to get in the ground, and ive lost many because of lack of water and more soil. theres only so much one can do!
good luck! you should probably contact local greenhouses for more detailed information on the area you are in. they are the ones who would know the most about the planting time and what to do with this cool weather!