At first, I didn't think I had any answer. Earlier this spring we started using a side shade for my 'monster' planter with lettuce and other greens. In fact, we used a welding sheet, if that's a clue to the sun's intensity. This was on the side of the planter. I noticed the plants just inside the planter along the edge of the welding sheet caught the most shade. They did not grow as fast, but they were terrific anyway. When everything got big enough it seemed the crowded greens shaded the soil and the welding sheet wasn't necessary. We also put it there to help block the drying wind. As with the heat, the mature foliage seemed to help keep the soil moist. And all this is on the south side of my house which receives dappled shade in the heat of the day. It was worth the trouble.

I would play around with it. You could unclasp one end of the sheet, fold it back and expose some of the plants for longer periods of the day while comparing its growth to those under continuous shade.

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You can see the welding tarp hanging off the back end, here. Boy, I cannot wait until it's filled again. Maybe I should tarp on top and start now.