In my mind, lots of sunlight (or equivalent sunlight) is important to productivity. It isn't essential to growing foliage. If the purpose of your cold frame is to grow seedlings that you can later transplant into direct sunlight, reduced light will be fine. The warmer temperatures will speed up the process. Be aware that most lighting systems that we use to illuminate seedlings (for example, a fluorescent bulb a few inches over a plant) produce illumination that is significantly fainter than direct sunlight.

The ideal of course is to get a balance between the two rather than sacrifice one for the other. So it should be easy to increase the sunlight while retaining the heat just by using a different top cover. But I can't believe that clear poly or even opaque white would block 70-80% of the light, more like 30% max. Most 60% shade cloth is very dark green and 80% shade cloth is coal black.

Dave

Well, if your clear poly is over small windows in a bulky opaque frame, I guess you could block that much light. But yes, clear plastic itself is pretty transmissive. White acrylic sheet, however, does block 70% of the light. Unlike shade cloth, it has no holes.

MY MISTAKE sorry my cover transmits 70% not blocks. I am guessing on this. I used to think that light was more important then heat. I know feel heat is more important. I can see a huge difference in a day with full sun at 60 degrees vs. 80 degrees. I started with cold frames to keep things warm - and soon learned things grew a ot faster - hard to be sure but it seemed a lot faster then same plant same age outside the cold frame - despite cover blocking 30% of light.

see pic, any guesses as to how much light this stuff blocks?

I cannot say enough about the cold frame I built. Looks ugly but it covers 3-4 times the area of my $120 cold frame, will last at least 3-4 times as long and is over twice as tall. It's a no brainer.

Only you can choose which you prefer... light or heat... but growing 'faster' does not mean they are 'better'. There is no controversy that all things being equal, and given adequate amounts of either, light will trump heat. Heat does not produce photosynthesis - you need light. Hence the prefix... 'photo'.

However, there is a point of diminishing returns. Therefore, as mentioned above, balance is required. Plants grown in warm conditions, with less than adequate light, will grow VERY fast as they stretch to find brighter light. That doesn't mean 'better'. Plants grown in too cold conditions - even with more than enough light - will either die, or slow down their metabolic activity. Plants grown under conditions that are too warm can also see changes in metabolic processes - the most common for cool weather crops grown in too warm conditions is that they will bolt.

For tomatoes, at least, seedlings grown with less-than adequate light indeed stretch out, but then you've got a long stem to bury and form roots. If I had a choice of 80F with half-light, versus 40F with full light, I'd take the former quickly to make plantable seedlings.

Good info guys and kind of what I expected. I was hoping for a solid answer but there is not one. I think i have a good compromise - I do not loose much light.