Retaining moisture is good; too much heat is bad. Lettuce won't germinate well in warm soil so be careful that the plastic doesn't warm the soil and not at all if you cook it. I would suggest a row cover material to help retain moisture instead. I like the box idea. I've made legs for 4 similar boxes that I grow bunching onions in. Getting the plants to a more managable height saves your back and makes drainage easier.

plastic or any kind of row cover, during the night is ok but it is not going to make a significant difference. But if you make a plastic hoop over your veggies(24/7) it can help more. Eventually, in tour climate, your cool crop will come to stand still and you are better off to harvest them, even when the frost won't kill them.

Yeah - I thought it was pretty interesting when I read the "tip" last Fall. And I figured it was worth the investment in a package of seeds to experiment! I planted the seeds right after Halloween, and then added a fresh layer of compost to the raised bed. It seems like I only got 50% germination of the seeds. But then - this was a pretty long hard winter. Next fall, I'll probably plant spinach in most of my raised beds. We love spinach. Anything we won't eat fresh, I'll freeze for use all year long. And I really like the idea of getting 2 crops out of 1 raised bed. Kinda like doubling your production!

Schastapop, You're welcome. With a low tunnel, I'd probably put the frost blanket over it and leave it there as long as it allows enough light penetration. Then, if cold weather threatens, I'd add the layer of plastic on top of it, but I'd remove the plastic in the morning once the temps have warmed up above freezing. The problem with a plastic-covered low tunnel is that it will warm up incredibly hot and incredibly fast on a day like today when the highs are in the 70s or 80s. I have a high tunnel greenhouse and, without open vents and shade cloth, it was over 100 degrees in there on sunny February days when the air temperatures were in the 20s or 30s at night and in the 50s through the 70s during the day. Before we put the shade cloth on the high tunnel, the temps were over 100 degrees in there every day. Now, with shade cloth and 4 open vents and 2 open doors, I can keep the high tunnel only a little warmer than the outside air temps, which is what I want. Normally, the larger a covered greenhouse type structure is, the easier it is to keep it from getting too hot and the smaller it is, the harder it is to keep it cool. That's why plastic over a low tunnel isn't as good of an idea at this time of year as floating row cover over a low tunnel---when it is hot during the day, the row cover allows air to flow from the inside to the outside and vice versa so the heat buildup is less. The plastic can be needed at night though when temps are in the 20s and 30s. If you apply the plastic in late afternoon, some heat buildup will occur that will help keep the plants a bit warmer that night, and the plastic helps hold in the heat that has been soaking into the ground all day. Plastic is just hard in our climate because of our weather's erratic nature. Now, if we were in a consistently cool spring climate like you might see in New Hampshire or Maine, I bet the plastic could stay on 24/7 with the row cover underneath it. George has written an excellent thread on hand-pollination of squash, and when the time comes that people need that info handy, one of us will find it and link it, as we do several times every year. It isn't hard to do, but you do have to watch the air around you while you're doing it, or a squash vine borer moth will fly in and lay eggs on the plants while you have the cover removed temporarily so you can hand-pollinate the flowers. Dawn