It's funny that you should ask about ground preparation, Applestar. That is this weekend's project. I'll go out and put black plastic down on the garden soil and make sure its secure. Then I put my row covers, or I guess they're more "hoop houses" as they are a heavy clear plastic and are closed on the ends, down on top of the areas where my future rows will be, and secure them.
Then I let the sun do that voodoo that it do so well. At about a week before I am ready to put my plants in the ground, (around the middle of march for my cold stuff and between the 20th and 25th of March for my hotties) I take everything off and start tilling. By then, the average high is 60 degrees and the average low is 40 degrees here. In New Jersey I'd guess it would be about 3 degrees more than that. With the combination of the plastic and the temps the tilling goes pretty smoothly.
When I have my rows tilled how I want them, I chose a calm day and start transplanting. When a row is in I put 3 liter pop bottle cloches over each plant, sinking them to the depth of the plant root to keep everything warm. Then I put the row covers back on and secure everything.
By now it's the end of March/ beginning of April and with both the cloches and row covers covering the plants, it's not the cold you have to worry about, it's the heat. I have to make sure things don't cook, so I get the caps off the bottle cloches if it gets over 55 that day and raise the plastic on the row cover if it gets over 60.
It does seem like a lot of work. But you have to remember, I'm well compensated for it those first few weeks at the Farmer's Markets.
You were asking about how I am able to water? I use the pvc clips on my hoop houses, so I can just remove a couple clips, raise the plastic and water.
For your dilemma, I would suggest potting up on your cold plants, putting black plastic or cardboard (your local appliance store is begging you to take their boxes) down under that 4' of hoop, put your pots on top of that and cover the hoops up with the heavy clear plastic. If you are worried that won't be enough, use cloches too. With that much protection for your cool weather plants I just don't see a problem. It takes quite a freeze to nip most of those cold plants, and in March we just don't have those long freezes. Sure it dips down there periodically, but it usually goes right back up the next day.
I hope that was of some help. I lurked on here for quite a while before I started piping up, and in that time I've seen you cheerfully help scores of people. So it gives me great pleasure to assist you and return some of that kindness.