Yes, they do block light. The thicker/heavier the row cover the more light is blocked.

http://www.rodalesorganiclife.com/garden/use-row-covers-extend-your-season

http://www.motherearthnews.com/organic-gardening/row-covers-zmaz08fmzmcc.aspx

I assume the low hoop is covered with plastic? 70 degree days and full sun can cook seedlings if you don't have it vented in some way.

But how cold are your nights getting? If it's low 30's then you could just use row covers over the hoops instead of plastic.

Rodney

They block a small amount of light, but as they increase heat this would defeat the purpose.

If you have been covering them with shade during the day they aren't getting adjusted to proper sun exposure. They should be gradually acclimated to full sun exposure. Seedlings should not be wilting in 70 degree weather IMHO and do not need protection, even cool weather plants... something is going on.

Thanks, everyone. I do not have plastic over the hoop. I use it in the summer for pink eye, purple hull peas which can get several feet long. This year I didn't plant anything under it for the winter, so never covered it with plastic. I like to use the space under it for spring greens because they will pretty much be over before the peas and up and providing shade. We had a 5 day forecast of moderate night time temps when I planted them, but an unexpected cold front moved through a day or so later, so the heavy cover. As I tried to remove it, they would wilt, almost to the point of looking near death. I just went out and gave them a good watering. The sun won't hit them for another hour. I took the cover off and will keep an eye on them. If they wilt just a little, I will let them be, but if they wilt too much I may cover them but just for a couple of hours until the sun gets lower. I know they need to get hardened off more, was hoping for something between the two extremes I have subjected them to. I may throw a multi-layer cover of row cover directly over them if they wilt too much and just leave it on for a few days. Our nighttime temps are varying from low 30's to mid 50's, daytime temps from 40's to high 70's. Cwazy weather!

First, "row cover" comes in all kinds of weights/thicknesses and different materials in the composition. So you can't make any blanket statements about it with any validity. You have to know the specific brand name/weight rating to be accurate.

Most of the various row covers will let light through but how much varies greatly by brand and weight.

Second, when the weather has no "happy medium" your protection measures can't either. But all of the crops you listed are very cold tolerant, even frost tolerant, so the wilting is far more likely due to them being OVER-protected from the temperature changes, especially the low temps. That radical changing environment under the hoops is keeping them from adjusting, from being hardened off. But without knowing exactly what row cover you are using we can only speculate.

Dave

Thanks again, folks. I couldn't find my floating row cover, hardware store was out. But...the day clouded up and will be for 2 more days, moderate highs and lows, even after the sun comes out Tuesday. My plants are looking very happy this afternoon and may get some rain over the next two days. Maybe before any more near freezing lows or high 70 days they will have hardened off enough to handle whatever comes around.

Susan