I have a bed thats 12x4 feet with pvc hoops, intended for starting seeds of perennial plants, and holding containerized plants. I want it to function like a cold frame, but have a plastic top for protection from hard rains. I tried row cover fabric but that seems to let in too much rain, the plug trays I used fill up with water, some small seeds wash out presumably, and then the soil compacts. Now I want to switch to plastic but I dont want to bake my seedlings. If the ends of the tunnel are completely open will that provide enough ventilation? Would you recommend full length side vents or ridge vents too? I dont want it to be warm in the winter time either, dormant plants need to stay dormant, and seeds need to stratify. Thanks

Maybe a double decker? Try setting up a lean-to cover of greenhouse plastic film over a shade cloth covering on the hoops. The film can protect from hard rains and the shade cloth will lower the heat. Keep the film up and off the shade cloth. You'll have to experiment with the distance.

Henderman said: Maybe a double decker? Try setting up a lean-to cover of greenhouse plastic film over a shade cloth covering on the hoops. The film can protect from hard rains and the shade cloth will lower the heat. Keep the film up and off the shade cloth. You'll have to experiment with the distance.

I appreciate the reply. Is there any reason not to put the film on the hoops with lots of vents and the shade cloth on top of that?

You expressed concern about not baking your plants and ensuring adequate ventilation. Mounting the plastic on the hoops is going to increase temperature and decrease ventilation. A low tunnel doesn't provide much room for natural airflow and/or mounting a fan so I suggested a structure that keeps it more open and provides more room for natural ventilation.

Most folks who haven't tried it don't realize how hot it can get in a tunnel/hoophouse/greenhouse. Even with lots of venting my fullsize hoophouse can easily get to >130° if I pull the fans and shadecloth. Those temperatures are too high for gardening.

Thanks that makes sense. I'll have to put some thought into this so it doesnt turn into a sail.

How about make the hoop house so you can "roll up" the sides a little or a lot, plus have the ends open. That way you will still have a roof but with plenty of ventilation.

My neighbor has a hoop house (or HAD a hoop house) He did not have a sail problem but a snow problem. This past winter heavy snow collapsed it. It's still laying flat in his yard. This was a big one, about 12' wide by 20' long.

Leave ends open, or skirts partly rolled up, or add fans. Thats all I got for fixes.

Heres what i came up with. I cut the old 12' hoop house in 2 since I had to move it to a different area anyway. I covered the arches in insect netting then put the white plastic on attached with pvc snap clamps and 1x2 batten boards to 2x3 at the bottom. I am thinking now I could have made a more traditional cold frame out of wood framing and polycarbonate panels, and it would probably last longer, but this should work for a while.