I am gonna try it this year with plants that like cold.. lettuces, cruciferous veg and sugar snaps, delphinium and larkspur on a whim. but honestly I'm not sure what to expect. I'm zone 7a. I'm also planning to place then surrounding my dog's kiddy pool (which I should have drained but didn't) hoping the water will help regulate temperatures.
The one thing I do balk at is Basil and tomatoes.. that's bonkers. They don't like 50, nevermind chance of frost. I always start early, much to my detriment, butNever tomato or basil. I'll buy the plants early, as soon as Home Depot has them on sale, but they never go out until the soil is warm, which can mean June! I hate anecdotes instead of data, but Grandma Rosie always told me putting tomatoes out before it's hot will stunt the plants. Even if you don't see the damage, even a brief period of cold, or simply soil that is hasn't warmed up yet, will take them weeks longer to produce fruit. She always started them inside, and even if they got way stringy, she'd just bury them up to their crowns. Even though she lived 10 degrees colder than me, she always had tomatoes before I did.
I also notice tons of pictures of set ups full of dirt, some sprouts, but nearly none of healthy transplantable seedlings, so I am going to take this with a grain of salt
