Strictly speaking, you do not have a cold frame. You have a hoop house. A cold frame is sunk with respect to the soil level, resulting in better heat retention.

A hoop house is not nearly as good at keeping heat, and you have to seal it to avoid drafts. One little crack on a breezy day, and you will have no difference inside and outside. My hoop houses are perfectly sealed only when there is snow on the ground.

3 mils is plenty adequate, and not different from 6 mil, when it comes to thermal performance. I have used plastic as thin as 2 mils. Another thing that limits the performance of hoop houses is mulch. You have to have bare, moist soil in there for maximum thermal ballast.

In my experience, hoop houses (with their inevitable drafts and leaks) are best used to preserve established plants, such as you are doing. During the day they will trap solar heat and the veggies will thaw and get to grow a bit more in the 70ish temps that will develop by mid afternoon on a sunny day. The soil will rarely freeze even in the dead of winter.

Your chard and lettuce should shrug off a little frosting. One zone north of you, here, I can overwinter cold hardy lettuce (for spring consumption), as well as the regular deep winter veggies (kale, radicchio, corn salad, arugula). Keep the hoop houses sealed as best as you can, except for harvesting and the rare watering.

Thank you!!! So things may still be ok :)

It's getting up into the 60s/70s during the day still, so can I keep it sealed up all the time? Or will everything fry in the sun? I leave at 8am and don't get home until after 6-7pm, so I'm having some trouble figuring out how to take care of everyting. Like this morning, there was still frost on the ground, but I opened it just a touch because it will get close to 70 today. Was that the correct thing to do?

I didn't use mulch, so at least I did that right. I thought I had the plastic down well enough, but now I realize that I don't.

I've found lots of instructions on how to build these things, but little on how to actually USE them, so I'm going with trial and error, and help from here :) which is FANTASTIC!

Thanks
Leisa

You can combine two techniques if you want and use floating row covers inside your hoop house, giving you an extra boost in frost protection.

if it is 70 during the day, and it is in full sun, it does not seem like you need the hoop house yet, at least for chard and lettuce. They actually grow best when the temps are around 70. So I would open it, and not close it until the max temps are in the 50s. It does make a difference to be or not to be in full sun, whether the soil is very moist, and whether it is sunny or cloudy. Here, I put them up on the Thanksgiving weekend and take them out around April 1 (though leaving them up until May 15, so as to start melons early, is also a possibility).

When I use just plastic, it's no warmer in the garden than outdoors. When I use blankets under the plastic, there's a big difference. This is my first try at season extending. There are more details on what I've done on the Rocky Mtn Gardening forum (see link below).
I'm in Colorado, and it's usually sunny here, and the sun is intense. I don't know what it's like where you are, but just keep in mind it may be different. My garden is on the south side of my house, and gets direct sun for most of the day.

Here is a link that might be useful: RMG link

Well, next problem: WIND! The wind blew the plastic right out from under the bricks. How do you keep the plastic down? It turned into a big sail... LOL!

I suppose I just need biggers bricks, and a lot more of them.

Good news is the plants are still alive, though :)

Leisa

That is a good example of the law of unintended consequences, the bugaboo facing all inventors. Your sense of humor is invaluable in such situations. A fiasco provides a laugh, if nothing else. LOL!

But seriously, it isn't more bricks you need. It is a better grip between the bricks and the plastic.

Jim

So, how do you create better grip?

And yes, humor is a beautiful thing :)

I use concrete blocks that are 2X2X16. I found them at my previous house, and brought them all with me when I moved. They are substantially heavier than bricks, and they grip the plastic much better. Because they are long and narrow, they fit snugly in a narrow trench, and being 4 ft for 3, they close perfectly the ends of my 4ft wide beds. They will keep the beds sealed in 60mph winds.

Cinder blocks, red bricks, and concrete chunks are no match to these things. If you can not find them, you can cast them yourself with concrete.

Having a lot of experience with hoophouse blowouts, the secret is in avoiding small leaks to begin with. The blowout often starts at one end. My hoops are about 4 ft spaced and 3 ft high, and one of these "bricks" per gap is enough, if the ends are carefully sealed. Not only 3 bricks per end, but also careful folding of the plastic under the bricks so it does not inflate.

Oh, and of course, 3 clamps per hoop, either side and top, and five clamps at the first and last hoop. Clamps can be found on the Territorial Seed website. I have 200 of them.

Very helpful -- thanks!!! More things to experiment with :)

I just put up a hoop house this year - it is about 5.5' tall and 20 feet long and wide outside washington DC. After following glib's advice your performance should be about 15 warmer then outside on a cloudy day and up to 40 degrees warmer on a sunny day (mine matches Eliot Coleman Winter harvest book). So ventilation is a must with most current daytime temps. I just open the front of mine. At night right now I have been seeing about a 8-10 degree increased temp (at around 32 degrees) in the hoop house. I am using 6 mil. plastic from lowes.

Wow, I just re-read my previous post, and I left out a few details... When I said it was no warmer than outside with just plastic, I meant at night. During the day, I have to be sure there is ventilation, as it will be between 85 and 90 under the plastic in the sun.