"I’ve seen VERY LITTLE in the way of freeze protection. Those same
tomatoes – outside temp was 31, inside temp 34. It kept them alive but
that’s it."

When it comes to frost protection the goal is not to generate heat. It is just to prevent frost on the plants, to get that +5 degree increase over the outside temp. Lots of other variables like sun exposure, soil temps, materials used, etc. also have to be factored in.

Although, depending on how the spun polyester row covers are used it is possible to get more than a 5 degree difference. First because it comes in several different thicknesses, second it blocks wind, and it can be multi-layered, lends itself to creating internal air pockets around the plant that can trap additional warm air as it rises from the ground, etc.

All in all low tunnels have proven to be a tried and true method for spring season extension.

Dave

I agree with both of you. Most years you can plant up to a month early with low tunnels and harvest a week or two earlier than without the cover. Every now and then you will lose the crop.

I used spring row covers for many years and I eventually migrated to high tunnels and now I even have permanent heat sources in all but one of my high tunnels. Now if I plant a month early I can almost guarantee not only live plants in any Spring weather but also a month earlier harvest.

I still use large area floating row covers but in the Spring it’s mostly just for covering blooming strawberries, peonies or other sensitive crop that normally might have excaped frost damage. However in the Fall I use covers regularly to cover a variety of crops when frost is forecast. In the Fall light transfer is not important so the heavy fabric works well unless it is very windy. I normally can reuse Agribon 50 for 5-6 years but last Fall the wind was so bad that I’ll need to use new fabric this year.

So try your hand with low tunnels if you like taking chances. I attended a seminar presented by a horticultural researcher from Egypt and he showed pictures of hundreds of acres of low tunnels in the farms near Alexandria that were very successful. For all the work involved I would expect much better results in our fields.

I use a plastic covered low tunnel to raise all of my spring seedlings. Even on overcasts days when the temperature does not rise above freezing, it stays about 45 F inside. Of course, I need to bring them in at night during cold spells such as that, but it generally doesn't freeze inside until below 28 F or so outside. On warmer days, say in the 40's, and even when overcast, it stays consistently in the 70's inside. This is perfectly fine for cool weather crops, and they can take a freeze no problem. By mid to late March, when most warm weather crops (tomatoes, peppers, etc.) are sown, we no longer have those extended below freezing spells, and it's plenty warm inside during the day, even if temperatures outside are in the 40's. If it's too cold at night they can always be brought in.

Now, for season extension, say, getting lettuces in early, my understanding is that the covers keep most of the elements themselves off. For example, your plants may get a freeze (though it will not be as hard a freeze outside), assuming it freezes inside at all, but cool weather crops specifically can handle that. The cover keeps the elements off, that often accompany the freeze and kill the plant. There is minimal wind chill, and there is no cold wind to dry out your plants when it whips through on those especially cold nights. It does trap some warmth, though air temperatures in the 40's are fine for cool weather crops. I believe that the low tunnels are more for warming the soil than anything. I'm keeping my fabric cover right on top of the seedlings with no hoops, and they push it up by themselves. It does the job of keeping the elements off, and it also warms the SOIL, which is often the challenge of growing cool weather crops this early, not the air temperature!

There are many ways of using a low tunnel. Keep in mind that if using plastic, you need to vent it when warmer than, say 40, on a sunny day. It got into the 50's today, so I was able to move my seedlings outside of the low tunnel to give them some exposure to winds, making stronger plants. Inside, where no plants were kept. It hit almost 120 F this afternoon.