You will benefit from the solar heat collection of the plastic but without added heat at night the inside will be the same as the outside. Maybe a few degrees warmer. during the day you will need to be sure and vent the structure because it can get too hot very fast.

Mike

What amount of light is transmitted through the cover? Some of those freeze protection weight covers are only for VERY temporary use, and some lighter weight ones allow enough light penetration to leave them on full time. We need to know that in order to answer your question.

I only would put plastic over the rows covered with row cover if an ice storm is forecast, and even then I'd only put it over during the night time and remove it in the morning (unless it is covered in sleet or snow). As Mike pointed out, low hoops covered with plastic during the daytime gets very hot here even in winter. While people in some climates (like Maine, for example) can (and must) put greenhouse plastic over low tunnel hoops to keep their plants alive in winter, their winter temperatures are a lot colder than ours and their sunlight is less intense, which means their plants don't overheat as easily as ours do under plastic-covered low tunnels. You could leave your plastic over your rows if you have snow or ice and the plastic is covered with enough ice and snow to keep the sun from roasting the plants beneath the plastic. In that instance, the plastic would keep them from freezing (and I'd have the row cover on the plants underneath the plastic, and would have an air space between the row cover and plastic) as long as the plants don't touch the plastic itself, and the snow or sleet would insulate the area and keep sunlight and sun-related heat buildup from roasting the plants.

The plastic will conduct the cold temperatures and transfer that cold to the plant foliage wherever the plants touch the plastic. Breathable fabric row covers are preferred because they don't do that. If you have two rows of plants side by side in freezing weather and cover one row only with floating row cover and the other one only with plastic, which one is more likely to survive? As long as the temperatures stay within the range that the row cover protects (6-8 degrees below freezing with the fabric you bought), the row cover plants are less likely to either roast in the sun or freeze at night than plants in low tunnels covered only with plastic. The plastic over the rows can keep plants warm (and even much too warm) on a sunny day, but won't keep them warmer at night at all unless you have a heat source beneath the plastic. In years when I've used plastic to cover a row of plants in late winter/early spring, I've had 5-gallon buckets of water underneath the plastic. The water-filled buckets act as solar collectors during the day and then release heat at night. I don't remove the buckets during the day, but I remove the plastic unless it is going to be very cloudy all day.

Carol's point about the sturdiness of the row covers is a great one. The ones that are lightweight enough to allow excellent light penetration likely will be torn to shreds by winter winds. They will last longer if used over hoops and not just allowed to float loosely above the plants while resting on them. The heavier row covers that can last longer in the sunlight and wind will block a lot of the light, which can slow down plant growth significantly.

I have used floating row covers for almost a decade now, but primarily use them for overnight frost protection during the transition periods in late winter/early spring and again in late autumn/early winter when plants are being subjected to early spring or late autumn frosts and freezing temperatures that can kill them or at least severely set them back. I don't necessarily keep the row covers over winter crops the entire season because they do block some of the light, and the light is weaker in winter already so the plants need all of it that they can get. I just use them during the coldest periods when the plants otherwise would freeze to death.

It also depends on what plants you're trying to grow in the fall/winter garden. Many of the plants that grow and produce in winter actually do a great deal of the growth in the fall when the daylength (number of hours of sun per day) is longer and the temperatures are milder. Then, in the colder winter weather, they are sort of in a holding period where they are harvestable but aren't actively growing much in terms of size. However, at that point, all you're really doing is keeping them alive so you can harvest from them.

I've used two types of row cover for years. One is heavier weight and provides roughly 8 degrees of frost and freeze protection. I can leave it on all the time as it just doesn't rip and tear in the wind. Unfortunately, it blocks a lot of light. The one I've used that is more lightweight (I believe it is Agribon-19) lets a lot more light through, but tears pretty easily in the wind. Some of the Agribon I have only gives 4-8 degrees of protection, but I have put a double layer of fabric over plants on cold nights and have gotten 8-10 degrees of protection from it.

This past spring I bought the third kind of floating row cover. It is Dewitt's Ultimate Frost Blanket and it gives at least 10 degrees of protection, and in my garden it seemed like it gave as much as 12-15 degrees of protection on one very cold night when I had thought the plants might freeze even with it over them. It is very heavy and laughs at the wind. I think I could leave it over hoops over a low tunnel all winter long and it wouldn't tear. However, it really blocks a lot of the light. I left it on the hoops over the rows of tomato plants for five days straight last spring when a cold spell hit late in the season and they stayed safely warm and did not freeze. However, they didn't grow a single bit in height or width during that five days because it blocked so much of the light. They did, however, bloom and set fruit during that period.

Some years I have put higher tunnels (6-8' tall) made of clear 6 mm plastic over rows of tomato plants in mid- to late-spring when a strong cold front has brought very cold temperatures, sleet and snow after the tomato plants already have been in the ground for several weeks. It worked in terms of keeping them alive, but I had to vent the ends every day or remove the plastic altogether so the plants wouldn't roast.

Every type of plant performs differently under row cover fabric and under plastic, and the plant performance varies with the weather. This occurs because of the way that both types of cover affect temperatures and light. Lettuce, for example, is perfectly happy under floating row cover (though I prefer to use hoops to hold the row cover above the lettuce instead of letting it float freely in the wind) but can get too hot under plastic and if it gets too hot it can become diseased, can become so hot that it bolts, or can roast and die. Remember that plastic not only allows heat to build up ridiculously fast, but it also blocks air flow and causes higher humidity to build up as well. That higher humidity and poor wind flow can lead to all sorts of diseases that flourish in those exact conditions.

Row cover fabric isn't perfect, but because it allows good air flow, plants in our climate, even in winter, do better underneath it than they do under plastic in the long term, if we're talking about simple rows of plants under low tunnels. (High tunnels are different from low tunnels because of their size.) Plants under low tunnels can be fine under plastic as long as it is really cloudy, but can roast in just a few hours of sunlight in our climate even in winter.

Think about how hot the much larger and taller greenhouses get in winter sunlight than the outdoor weather. I have to be sure to open the greenhouse doors and vents around sunrise every single sunny morning in winter or my greenhouse can hit 140 degrees in winter by 9 a.m. and that's with 50% shade cloth covering the greenhouse. The rapid rise in temperatures and the accompanying heat buildup in the greenhouse can cause cool-season plants to get so hot that bolt and go to seed if I forget to open the doors and vents to prevent heat buildup. With a smaller low tunnel, the heat buildup can be even more of a problem because of the smaller size. Low tunnels covered with plastic will get hotter and they will get hot faster than a high tunnel or greenhouse. The plants can be cooked to death on a sunny day under plastic that doesn't breathe if you don't vent the heat out of the low tunnel.

Last year I had a great winter garden. Mostly I left it uncovered. If the forecast was for temperatures to go below 25 degrees, I covered up the lettuce with floating row cover. The lettuce we planted in late summer/early autumn of 2012 produced until about March or April 2013 (some varieties lasted longer than others), and it probably spent a night or two a week under floating row cover each week. I never covered the rows of lettuce with plastic. I didn't cover up the more cold-tolerant kale, Swiss chard, turnips, collard greens, etc. unless the temperatures were going down into the teens. Some of them eventually did freeze when left uncovered, but others didn't.

Covering up your rows or beds of plants with plastic wouldn't be overkill as long as you can remove it every single day before the sun hits it and starts cooking the plants. I just don't find enough benefit from using plastic throughout the winter for me to think it is worthwhile in our climate with its often sunny and fairly warm winters. It would be different if you had the plants in a high tunnel with doors and vents or sides you can rise and lower so that you could control air flow and lessen the build-up of heat on warm, sunny days. The smaller a plastic-covered area is, the more prone it is to overheat (and much more quickly than you'd expect).

This is one of those areas where the weather on any given day determines how well the row covers and plastic will work, so you mostly just learn it by trial and error. I am in far southern OK and some winters we don't have enough cold weather to make floating row cover necessary at all over cold-season crops for more than just a scattered night here or there. If you are in central OK, western OK or northern OK, you likely would need to use row cover more than I do, and if you're in extreme southeastern or eastern OK, you probably could use it even less than I do. If you are an area with lots of snow and ice in a typical winter, plastic might work for you, but you still have to be able to remove it on sunny days. If you have the kind of job where you leave the house very early in the day, then you're going to have to make tough decisions some days about whether to leave the plastic over the beds and risk roasting the plants if the sun comes out or whether to leave it on the plants and just hope the sun stays behind clouds that day. Personally, I'd rather use floating row cover than plastic. I do always keep a row of 6 mm clear plastic in my potting shed so I can use it if needed, but most years I never use it once. I just have it there for insurance in case a freak snowstorm is forecast for late in the season in spring, and even then it is more for protecting the warm-season plants than the cool-season ones.

Hope this helps,

Dawn

Thank you all, for your thoughtful input and answers.

And Dawn, bless you - you're such a sweetie to put so much time and effort into this forum at all levels! It just blows me away just trying to imagine how you manage your level of gardening and still find lots of time and energy offering your expertise and opinions! Secondly, a belated thank you for the warm welcome you extended to me on my first posting.

After posting this query, I stumbled upon an older post you'd written, detailing all the pitfalls of a large greenhouse. I think you made enough of a case that even a small greenhouse could not be justified for what would be involved, in my case. Which kind of put ME in the "winner's circle" as far as my SO's argument of "having" to have one.....LOL

For the record, I'm in central OK and know how erratic and unpredictable winter can be. These past few winters, I was like "WHERE is winter?" Plus, I can still remember one year that I had tender annuals still in bloom until the first week in December.

I'm a transplant that grew up with hard winters. My third year here we got 6-inches of snow and I couldn't stop laughing at how it was considered a 'blizzard.' The not-so-funny part was when everything seemed to shut down and I was out of work for 3-days!

Now at least, we've got emergency snow routes and a few plows to go around. I can handle the snow but dread ice....like any sane person should.

As far as my gardening ambition goes, a few fresh veggies is what I'm *hoping* for. I realize that it's basically a crap shoot, but I'm willing to put some time and effort into at least trying a fall/winter crop. If it fails, I have a backup plan for starting up in early spring.

The row cover material that I got is the 'supreme' version, and appears to be good quality. The reviews on it were extremely good, too. My original thoughts were to use it for temporary protection and transitioning between growing seasons. Then the light bulb came on and I was like, "Why not use it on some hoops, too?"

For the time being, I've got my fingers crossed that my tender seedlings (direct sow) will grow and toughen up before any nasty weather or temps get here.

A week ago today I planted:
beets
radishes
carrots
peas
spinach
Chinese cabbage
Swiss Chard

I had radishes pushing up on day 3, beets and Swiss Chard on day 4, spinach, Chinese cabbage , peas on days 5 and 6.

Since then I've planted 2 types of leaf lettuce, some Lisbon bunching onions and cauliflower. None of which I really expect to grow, but I have lots of seeds to gamble with.

At any rate, my first attempt at veggie gardening - overall, has been very educational and somewhat rewarding. I enjoy challenges and learning and at least have some pretty pictures to show for what I accomplished. I do look forward to a better year in 2014 and the ability to access Garden Web forums, daily.

Till next time, hoping y'all have a wonderful evening!
Many thanks again.

Here is a link that might be useful: Row Cover Supreme