Pretty sure you should be good to go by may 15. Find your last frost date and subtract 2 weeks...

Yup soil temperature is the best, but for some I use just a little by guess and golly. If a seed pack says as soon as the ground can be worked Then plant early. As a general rule a cold frame will give about 1 month of early growth, of course that growth will not be as fast as later planted seeds. I would not start any heat loving plants in the cold frame, but use it to harden off some of those seedlings. I am on my second batch of cold frame radish The first planted 3/23 and boy did they grow slow, my cabbage was started from seed in late March. I left spinach over winter under the snow. It restarted this spring and I have three pickings off it. My seeded spinach is still a month from harvest and will bolt by harvest time.Look at each plant very carefully even tomatoes have very different temperature adaptability I hope this was of some help to you. Oh and you can gain about a month in the fall too. I need a bigger frame and will have one come fall.

Curt#