I've used shiny aluminum foil insulation laid on the ground on the south side of my sunroom to add heat and light. Also have a reflective material, Extenday, on the floor of my greenhouse. You also might consider a better covering on the greenhouse. Mine, double poly, probably blocks at least 50% of incoming light.

Just remember more light in means more heat. So you need a good cooling system.

If you are talking reflective plywood on the east and west side it can't be very tall or it will block light early and late in the day. And if your covering material is the real issue, trying to push more in won't help much.

Thanks fruitnut. I only use 1 layer of 6 mil poly but I notice that last year some flats suffered from lack of light and grew unevenly. I have a red brick floor so no light was being reflected back but I will try your suggestion to make the floor more reflective.

Because my greenhouse is situated where the long walls of east and west get the most sunlight in the spring, the panels of plywood shouldn't block anything except maybe 30 minutes of morning sun. I need about 20% more light in the greenhouse being reflected at the plants if I was making an educated guess to the light needs based on what I saw last year.

Flat white paint reflects the most light. The only thing better is mylar, but mylar tends to wrinkle when you glue it on anything, which makes it not work as well.

Make sure you secure the plywood so that the wind doesn't hurl it into your poly.

Thanks Cole. Any particular brand of paint you recommend? I also heard that there was a new paint that was as reflective as a mirror out now but I am afraid of burning a hole through my plastic with it.

Any exterior latex or acrylic should be fine. I doubt the mirror paint would be worth the money, anyway.