This spring!! 80° F in Canton on March 21st; 20° on March 27th. It makes predicting the last sub-freezing temperatures of the season more difficult than usual. And missing your guess by a day or two is heartbreaking when you watch seedlings, babied in your house for weeks and then carefully hardened off outside, collapse limply after a frigid spring night.
Living in the Tri-lakes where every frost-free day is precious, I do what I can to hedge my bets. If I can possibly restrain myself, I do not transplant all of each type of vegetable at the same time. This works even better if I've staggered the date of starting seeds. If all of the plantings survive, the late planted seedlings will catch up to the earlier ones with favorable growing conditions or, as I've heard it, "two weeks under lights is equal to two days in the field."
Clear, still nights are the most dangerous for tender seedlings, and covering plants to trap warmth helps them ride out late spring frosts. I often stretch floating row cover on wire hoops for a few days after transplanting any crop; plastic film or even tarps can be pulled over the hoops on cold nights. Those teepee-like water filled plastic rings are effective for protecting individual plants – the water acts as a thermal mass to dampen temperature swings. It's also worth noting that some plants are more tender than others. I've had tomatoes survive 30° F, but basil leaves can be damaged by temperatures well above freezing.
Another way to avoid jumping the gun on early spring planting is to procrastinate getting seeds started. Among the seeds that await a spot on the lighted shelves in my living room are tomatoes, squash, melons, cukes and sunflowers. These are all large, vigorous plants which, with the exception of tomatoes, should be transplanted no more than four weeks after germination. (Tomatoes can be kept up to eight weeks.) As they are all frost-sensitive, and as the average date of last frost around here is June 1st, my laziness is keeping me right on schedule. Even if your frost-free date is a few weeks sooner, I urge you to hold off a bit longer so that the seedlings you transplant will not become root bound in their containers. (Again, tomatoes are the exception – they can be started earlier and replanted to larger containers as they grow.)
Meanwhile, the coveted space underneath the fluorescent lights is given over to cool weather starts which will soon go outside,
or slow growers like celery root which need to ten to twelve weeks inside before being planted in the sun-warmed soils of June.