Agree it’s Rhizo; but you don’t need to limit yourself to plastic films. For low tunnels you might want to consider row cover which tends to be cheaper than 4 or 6 mil greenhouse grade films.

Agree completely. Painters sheeting is designed to last as long as it takes to paint. Not a growing season. You should understand that even "UV-resistant" plastic sheeting will only last a few years under intense sunlight. UV-resistant plastic has compounds added that specifically absorb the UV radiation, so the plastic absorbs less of it. Works a lot like sunscreen.

Alright. Thanks for the replies.

So, to outline what I'm doing, I had originally intended to create a cold frame for raising seedlings next spring, but I already have low tunnels built. I use AG30 fabric on them for cool-season crops. These low tunnels are being attached to a bottom wooden frame, so the whole thing can be lifted up and moved easily. Now, next spring I can move it and take the fabric off, put plastic on, and raise seedlings under it rather than pay the extra money for a cold frame or greenhouse.

What kind of UV-treated plastic would you recommend for this purpose?

Does anyone have any recommendations as to where they buy their plastic?

Seems like garden supply stores are the way to go. But if you want to make an investment in stronger, longer lasting stuff, you might look into "clear marine vinyl", which is what they make industrial exterior covers and windows out of.

I guess we are blessed in PA to have many produce supply centers that provide many choices. Garden centers of Walmart, Lowes or Home Depot are utterly lacking in this area. Sadly the shipping costs can be very high for these materials which are rather bulky.

Good quality 6 mil film can last for many years if handled and stored properly. I leave mine on my high tunnels (usually 2 layers) and I usually replace between 8 and 12 years, once went 14 years. You can tell by touch when it begins to get a bit brittle.

I've been using the same sheets of plastic that I cut from a larger roll for years. I move it from one low tunnel set up to another--early spring then late fall. I bought the stuff at my local hardware store in a very rural area. Nothing fancy here. The tunnels are very low--metal hoops set up over 4 foot wide beds and only a few feet long.

I have the luxury of a high tunnel type of structure. We're in NH so my husband built a wood frame rather than hoops. Yesterday it was 40 outside and sunny. The high tunnel temp was 71. But, at night, it gets as cold inside as outside except the ground never freezes. My question to jacoblockcuff is what kind of seedlings do you plan to grow? In zone 6a, I'm not sure I would trust things like peppers and tomatoes. I grow these indoors on a very inexpensive DIY seed starting rack. Lights are old, used office fluorescent fixtures hanging on chains so they can be adjusted. If you are planning cool season crops, the outdoor low tunnels should be fine but I would still be concerned about overheating during the day. Even with sides rolled up and front door open, my high tunnel can easily heat to 100 deg F in warmer weather. I have seen plastic covered low tunnel type structures where you can tip the covers off during the day. You might get lucky with greenhouse plastic. I would keep checking Craigslist for someone who either has remnants or maybe removing torn plastic from a large high tunnel.

Mainly cool-season crops such as cabbages, defrost. I am interested in doing some tomato experiments inside the low tunnels as well, carefully, as I suspect temperature dips at night as well. Yeah, I'll have to watch the venting for sure. Last year I grew tomatoes started in March outside during the day, bringing them in at night. Highs occasionally being in the 30's throughout that period, and they seemed to take it. Not frost though. I think a low tunnel would make for an easier time as I wouldn't have to carry seedlings back and forth unless it's an especially frigid night. Thoughts?

Many say that tomatoes like the longer daylight hours of starting under a light, but mine seemed to catch right up with the commercially grown tomatoes bought at a local nursery, eventually beating them out in growth- until the septoria leaf spot hit of course.

The overnight lows are my only obstacle, but I've seen many people fill black jugs up with hot water during the day, and they let them heat up over the day and provide a minimal heat source at night. During my seed-starting time, usually March and April (last frost mid May) temperature lows are usually in the low 20's for us, highs in the 40's (warmer in April). With this I suspect I'd have to vent well, but the frost, I'd imagine, could be kept out by providing a heat source as I described above and/or layering cheap plastic sheeting from Lowes on top of my greenhouse film just for overnight protection? Easy 5 minutes every evening. What do you think?

Thanks everyone for the recommendations. Craigslist is a good idea! I'll keep the thread updated as to what I get.

You can do some reading. I've read of tricks like putting Christmas tree lights inside to keep things warm. I'm pretty sure the market gardeners here in NH are using supplement heat like propane in their large high tunnels.

My husband got a 4 way thermometer so we can see what temps are indoors, in the high tunnel, outdoors and in the garage. This has been very handy so I can see if temps are too high or too low. With a low tunnel, I don't know if the ground would freeze or not. It doesn't in my high tunnel which is something like 16 x 20 or thereabouts. (Sorry, I never remember numbers.) Our last frost is late May. According to a local garden writer, he does not plant tomatoes out until early June based on a number of things. He said his tomatoes catch up.

We are lucky to have a large garage so I put my seedling plants in my garden cart and wheel them in and out during the hardening off period. In 2016 I started some peppers very early because they needed 90-110 days to maturity and I was very pleased with results. Peppers were flowering by late May.

Despite 100 deg temps, both peppers and tomatoes are healthiest in the high tunnel. I had a terrible time with Opalka paste tomatoes planted outdoors this summer. Did not get much of a crop and I'm not sure if it was septoria or something else. One thing, the high tunnel tomatoes etc don't get rained on but I do use an oscillating sprinkler.

One thing I like to do is follow some of the market gardeners on facebook. One of the local ones posts photos of what they are doing so I use them for incentive and timing. Maybe you have something in your area who does that.

I have two of the Eliot Coleman books for advice. There is also a small commercial grower in NH who supplies a large variety of heirlooms to area chefs. He has the strangest homemade liquid fertilizer. Check out Rusty's Tomatoes.

Look around at your local big box store for exterior poly-film. This is the 4 mil super clear plastic film used to create outdoor storm windows over your regular windows.

I picked up a few rolls of the stuff from Menards a few weeks back while winterizing supplies were on sale- 8'x25' rolls for 5 bucks each. Don't know what the regular price on them was.