I bought some black shade cloth (60%) that is scheduled to arrive on Tuesday. I'm thinking as soon as the temps get into the high 90's it's time for the shade cloth. But I'll be curious to see what other experienced AZ gardeners say. I really don't have an answer for you, but am interested in the answers you get!

I almost got the black in 60% but then I was going to have vines growing underneath it so was worried that it would be too hot for them.

I read elsewhere to put the shadecloth up by the end of May, but that seems late to me. I will probably get too worried and put it up the end of this month. I guess it will depend on how the plants look.

I didn't need much cloth, live in Central AZ (105/summers)
but priced 50% Black vs. 50% White at Gemplers
White added 75% to the cost...
White seems like it would be an advantage against radiating heat
An Austrailian guy says it's EZ to just 'white wash' the black cloth
I am going to try and save that 75% cost!!
I plan to:
Water down some exterior white latex (25-30% water)
then 'dry roll' it against some plywood
then hang to dry
2nd coat it if necessary.
Should work for us do-it-yourselfers!
Cheers!

I have had shade cloth up protecting my tomatoes from the afternoon sun for at least three weeks now. Not sure if they needed it yet but they are thriving. The price I paid at Gemplers was lower than Catalog Clearance.

i've had my 60% tan shade up for 4 weeks...i know it was early but we had that heat wave, and all my veggies are growing crazy!