The first time I bought 15, but thought it was a little thin. I then bought 500 feet of 19, which is light enough to leave in place for insect protection but strong enough to put over hoops. I have also used it on cold nights, but it really doesn't give a lot of degrees of protection. One year I grew some things very late into the Fall and winter season in a low poly tunnel and on a few really cold nights, I also just laid row cover over the tops of the plants for a little extra protection under the tunnel. At some point I may buy some heavy duty stuff, but I tend to plant tender crops a little later because I am so far north (Grove). I have a lot of flower pots that I can put over things if I have an occasional cold night or hail threat. If I lived further south, I would probably try to plant earlier, but in my area it makes more sense to wait.

I would buy whatever weight I think I needed based on the timing of when I plant and what sort of temperatures are likely after that. Agribon AG-30 gives you 6 degrees of cold protection. Agribon AG-50 gives you 8 degrees. The number of degrees assumes low wind. If wind is high, you may lose some protection. I am in a relatively low wind area since I'm in a valley, so I don't feel like the wind generally causes my plants to have less protection than expected with a specific weight of row cove.

AG-30 still can float loosely over plants or can be suspended over hoops. AG-50 is heavier and needs to be suspended over hoops or some sort of structure so it doesn't crush the plants. I wouldn't start out with the heaviest row covers like DeWitt Ultimate Supreme, which gives 10 degrees of protection, because it is very heavy and can cause overheating issues that burn plant foliage if you don't remove it early enough in the day in our climate. While it is a great heavy-duty row cover, I don't think it is a great choice for someone's very first row cover since it holds in heat so well that plants can overheat and there's a certain learning curve with using it.

With both AG-15 and AG-19 you can leave them on day and night if you wish, but they don't provide as much cold protection.

I have several different weights now---AG-15, AG-19, a Reemay that gives 6 degrees of protection, DeWitt Supreme and DeWitt Ultimate, and in my tool box but not used yet, MicroMesh and Biothrips Proteknet, both of which are for protecting plants from insects. I like to tailor whatever I choose to use in a given situation to the weather, but in the beginning I bought a really lightweight row cover and then kept moving up progressively to heavier ones. I use the heavier ones more now than the lighter ones, but I think a mid-weight is ideal for someone's first row cover. If you're expecting colder temperatures than your row cover can protect plants from, you can use a double layer, toss plastic over the row cover, or toss a bedsheet or blanket over the mid-weight row cover in a pinch.

`now i am dealing with 50 to 80 days and 25 to 45 nights would AG-19 give me protection from frost and freezing to 30 and additional cover for lower temps. Would I need to remove it during day. I guess I am asking what is the heaviest cover I can use and leave it on.

Because we can have 25 degree nights and 80 degree days all at the same time, I wouldn't leave anything too heavy on all day. You can get an excessive heat buildup with just a couple of hours of sunlight with the heavier weights. I try to get my 10-degree row cover off the plants as soon as the temperature is approaching 40 degrees.

To leave the row cover on all day, which is something I only do with Agribon AG-15 or AG-19, I wouldn't use anything heavier than Agribon AG-30. Our days get too hot too quickly for me to even consider leaving AG-50 on all day. I left it on all day a couple of times for a couple of types of cool season plants when we stayed at or near freezing all day with 100% cloudy weather and a chance of sleet or a wintery mix in the forecast and still felt uneasy about all the light I was blocking. I wouldn't leave the AG-50 on all day with any degree of sunshine because of the heat buildup you can get in our climate. In somewhere like Maine, I bet they can leave the AG-50 on all day, but they don't get our extremely warm and sunny winter and spring days combined with frosty or freezing nights.

Understand that with the heavier weights, you lose a substantial amount of light and the lower light will slow down or stop plant growth. With AG-30, you're losing 30% of the light and with AG-50, you're losing 50% of it. While plants might do fine on 30% less light at this time of the year, they grow much more slowly with only 50% of the light. Because less light equals less growth, I prefer to remove the row covers as early in the morning as possible so I'm not slowing down plant growth.

The only times I leave row cover over plants 24/7 is when using Ag-15 or AG-19 over broccoli and other cole crops to keep the cabbage loopers and imported cabbage worms off of them. Those two are so lightweight that they don't block enough sun to slow down plant growth substantialy nor do they cause a heat buildup. Unfortunately, they don't offer much protection from the cold.

There is a stiff learning curve with using the heavier weight fabrics. I have left them on too long on some sunny days and had plants burned because of the heat buildup, so I am extra careful not to do that any more.