I would think that the lesser percentages of shade would do - I remember my sister used to use the black fiberglass window screening when she lived in Rhinebeck NY, with good success.

Hi blueberrier - because of the temp reduction shade cloth affords (10-15 degrees), the recommended % is tied directly to whatever the current air temps. Here in Z6 I keep 40% shade over it all the time after about May 1st-10th because we are well into the 80's by then.

If I am having 90 degree days I add another layer of 20% and for those July/August days where we range into the 100+ degrees, I add yet another layer of 20%.

I grow mine in a 8'x8' square bed so it is easy to rig shade cloth and I cant the cover just enough to block the worst of the noon-4pm sun.

Good luck. ;)

Will floating row cover do the job as well?
-O

Organica, not really - one of floating row cover's virtues is that it passes a lot of light and a fair bit of heat. It will help a little, but not as much as actually shading the plants.