Row covers come in various weights and sizes. I have two, both are from the agribon company and provide 6-8 degrees of protection and can let 50% light transmission. And to me that is the benefit of using row cover vs another kind of cloth, I *KNOW* without a doubt what kind of protection I am getting and how much light can get through during the day. Now I have used a plain white sheet in the past, and I still use weed blocking landscape fabric for a shade cloth in the summer. My current frost blankets are dark green and from home depot. I say, if you know you are going to only use them at night, so light is not an issue then use it. I have heard of people using old quilts and the plants are ok.

I have both summer weight and winter weight row cover fabric. The summer one is light and you can see thru it. The winter fabric is a very tight weave and it appears to be a synthetic blend. It's white so light gets through it but it is insulating. I got mine on-line at gardeners supply. I just took it off of my onion bed. I used it there to protect them against the changes in our little micro-climate.....and to keep the stupid cats out of the bed also.

The reason agricultural textiles sold as row cover, floating row cover, frost bankets. summerweight row covers or seed germination fabrics are useful is because they are air, light and water permeable to some degree, allowing them to achieve the purpose for which a gardener is using them while not interfering too much in normal plant growth. They also are extremely lightweight. Also, they are tested so we know what degree of light transmission occurs at different weights of fabric and what sot of frost/freeze protection should occur with each specific weight.

Most textiles sold as row covers are classified by their weight, expressed in ounces per square yard. There are several different brands including Reemay, Agribon, DeWitt and Typar Xavan, but I've mostly used only Agribon and DeWitt so my experience is with them, and I'm sure of the numbers given below being accurate in our climate.

The most lightweight row covers are only 0.45 oz./ sq. yd. and are used only as seed germination blankets,or to provide light shading or protection from insects (by the fact that if they are tacked down tightly against the ground, insects cannot reach the plants under the row covers). Agribon+AG-15 Insect Barrier is an example of this weight row cover, and it allows 90% light transmission but does not provide frost protection. It is considered an extra-light row cover. This weight is so lightweight that it can be tacked down with earth staples or another method of your choice and allowed to rest/float directly on top of the plants. To achieve total insect exclusion, the fabric at every point must be down tight enough against the ground that insects cannot crawl underneath them. A gardener still can have issues though, with overwintering insects that emerge from the soil beneath the row cover after it is put down, since those insects are trapped beneath the cover with the plants.

The next heaviest weight group (though generally still considered a lightweight row cover) is the 0.5 oz./sq. yd. and 0.55 oz.sq. /yd. fabrics. They generally provide 4 degrees of frost protecion and allow 85% (Agribon+AG-19) to 87% (DeWitt Deluxe) light transmission. Sometimes I have used a double layer of AG-19 to get maybe 6-7 degrees, but not necessarily 8 degrees, of cold protection on a cold spring night. You can let these float loosly over sturdy plants, but it is safer to suspend them over hoops (wire, EMT, PVC, etc.) because even their light weight in combination with a steady wind can hurt young seedlings by repeatedly rubbing against them.

Moving up a step you have the 0.9 oz./sq. yd. (Agribon+ AG-30) to 1.0 oz./sq. yd. (DeWitt Deluxe Plus) group that is considered a medium weight row cover product and give 4-6 degrees of frost protection, depending on the manufacturer and allow 65% to 70% (varies with manufacturer) light transmission. Some people let this weight float, but I always use hoops.

At 1.5 oz./sq. yd. you have the heavy weight group, which includes DeWitt Supreme (6-8 degrees of frost protection and 50% light transmission) and Agribon+AG-50 (8 degrees of frost protection and 50% light transmission). Due to their weight, these should be used only with hoops to avoid plant damage caused by their weight.

Finally, there's the extra-heavy weight group, which includes Agribon+AG70 and DeWitt Ultimate, which give 8 degrees (Agribon) to 10 degrees (DeWitt) of frost protection. Both allow only 30% light transmission. DeWitt Deluxe now is 2.5 oz./sq. yd. (I think it was 3.0/sq. yd. when I first started using it, then it was changed to 2.5) and Agribon is 2.0 oz./sq/ yd. Hoops must be used with these extra heavy weight textiles.

I don't know if you know how many oz. per square yard are found in your pond underlayment material, so if you use it, you'll just be guessing about how much cold protection it provides. There's not necessarily anything wrong with that. Before I started using row covers, I used everything under the sun (whatever I could scrounge up around the house) from old sheets, blankets, curtains and tablecloths to sheet plastic and even landscape fabric). It always was hit and miss with regard to how well they worked under a specific temperature range or weather condition. You could experiment with your underlayment by putting a Min-Max thermometer underneath the pond underlayment fabric when you use it. By comparing the minimum temperature recorded beneath it with your minimum overnight temperature reading outdoors in the open air, you'll see how well it works. The fabrics provide the frost protection by holding in the warmth of the earth which keeps the air covered by the agricultural textiles generally in the range given above. On a very windy night, they may provide slightly less protection than their rating indicates. If I am expecting a much colder night than the fabric is rated for or if it is going to stay very windy overnight, I sometimes use a double layer which doesn't necessarily offer twice the protection of a single layer but offers a bit more warmth than a single layer does.

The agricultural fabrics sold as row covers generally are made from spunbond polyester or spunbond polypropylene, allowing them to capture warmth while allowing various amounts of light transmission. All allow some degree of water to permeate, and they all provide some degree of protection from wind, and also from hail, sleet or snow. Well, I don't know if any of them would tolerate baseball-sized hail, but think that maybe DeWitt Ultimate would, as it tolerates cats using it as a hammock as long as it is suspended on hoops (not wire hoops, but sturdier EMT or PVC hoops).

I'm all for using what you have handy, but remember that there's a higher risk in using something that hasn't been tested and proven to provide a certain amount of cold protection. If in doubt, use a double layer. As long as you remove the underlayment before the sun gets too high in the sky, there shouldn't be a heat buildup issue. I have to use breathable fabrics because we sometimes, at this time of the year during fire season, get paged out to fires in the overnight or early morning hours, and I might not be able to get home to remove the row covers until the sun has been up for ages. This isn't an issue with air-permeable row covers, but it was back when I used clear plastic in our early years here.

Even when you use air-permeable row covers, I prefer to always have them suspended over hoops that allow an air space between the bottom of the fabric and the top of the plant. Because our weather here can be eratic in late winter and early spring, it is not all that unusual for us to hit 76-78-80-82 degrees during the day and then drop down into the mid-20s to low-30s at night. I'd rather have a white, air-permeable fabric over the plants if I leave at night or in the early morning while we're still below freezing, and then the temperatures rise rapidly once the sun rises. Having open air space as described above provides a bit of a cushion that helps prevent overheating if there is some reason I cannot uncover the plants promptly in the morning. I have had plants get roasted and damaged if I leave the extra heavy weight over them without much air space once the sun is pretty high in the sky (say, 10 or 11 a.m.). It only happened once, because after that I was careful to leave a larger open air space.

This is probably more than you want to know about the various weights of row cover, but it took me a while to figure out all the differences between the different weights so in the beginning I didn't know what to buy, and I think the first one I bought was a Reemay product that gave 4 degrees of protection. I probably should have bought a more heavyweight product in the beginning, but I was so new to using them that I didn't even know that. Nowadays I mostly only use the heavy weight and super heavyweight for frost protection and the extra lightweight for insect exclusion. I have learned that in my cold microclimate (a low-lying creek hollow in the already low-lying Red River Valley), where we routinely go 4-10 degrees lower than forecast at this time of the year (not every night but often enough to keep me on my toes), I need to use the heavier weights when cold is an issue because we often go colder than forecast and only rarely stay warmer than forecast. Nothing teaches like experience, and my experiences here with floating row covers have been painful learning experiences at times, but now that I know how much to trust/not trust our forecast low depending on various conditions, I know more about exactly when a heavy or super heavy weight row cover should be used jn my garden.