I use medium weight row covers to protect strawberry blossoms from frost. I put them on in early spring and just leave them until I see fruit. And last spring, my gailan had just germinated when temps were going to drop to the upper teens. I set up a mini greenhouse with metal hoops and plastic, and underneath, directly on the plants, laid a double row of the row cover. Came through with no damage. I think it depends on what you plan to protect. I also use row covers all season on cole crops--that's against flea beetles and the butterflies. Heat build up is not a problem.

Do you know what you'll be trying to frost protect? Transplants? It's the cool/cold soil temps that would be an issue for summer veggies and your method wouldn;t appreciably warm the soil. Also keeping the air temp above freezing for summer veggies like tomatoes, peppers may possibly keep them alive, but they wouldn't grow nearly as well as with a later planting date. No advantage at all there. On the other hand, alot of late winter/spring crops are very happy with mid-20's and may not need any "frost protection". No need to worry about overheating with those lightweight floating row covers.

1. walls o water the good till 12F

2. real soil heating cables

you could also make sheet wall o water if have vacuum sealer.

There are different grades of row cover thickness and warmth. For spring use you shouldn't have to worry about it getting too warm. To make it hold more heat in make a tunnel out of EMT and put row cover on that and then put row cover at plant level too. I do lots of EMT tunnels and it is amazing how cozy it is in there in both spring and fall. The wind is stopped and it is very nice. However I didn't catch what plant you are talking about. Heat loving plants really hate to sit their roots in cold soil. It almost doesn't make any sense to plant tomatoes early like that unless you are a professional grower. It is all about degree days. Doing it in fall gets you somewhere. Or for people who have really up and down spring temps it might be worth it. If you are going to do heat loving seedlings in spring get it all ready first, plastic the soil, stick a thermometer in there and really get that soil warm before planting.

Mike are you sure that the row cover was the only difference between those 2 areas in your bed? Pic taken in the fall? Might be getting the OP's expectations much too high.

This was a spring planting, and that was the only difference. This raised bed is planted close to the house next in a bit of a micro climate.

just last night I pulled out my copy of Four Season Harvest because I had the same question about row covers. There's a table in the book that charts temp differences with row covers. I'm considering building a double-wall row cover for extra warmth and multiple purposes.

First, two layers of insulation are better than one--especially if separated by a little air space between layers. Second, if I use shade cloth for the first layer, I can pull off the outer, thicker layer during the day and still have shade/bug protection underneath.

I'll probably go with the hoop method and test using a high/low thermometer to measure inside/outside temperature differences to see how much actual warmth I get from it.....'cause I'm a bit of an incurable data junky.

I think that row cover can be useful in case of an overnight light frost. But because of its porosity, it is not a good insulator. So in case of cold it can moderate a bit. I don't think it has much "greenhouse effect" property. Unless it is very tightly woven. I could be wrong.

Seysonn