The middle of the day is when the sun is at it's peak. As we all know, this time of day is very hot. If your garden was watered in the morning, the vegetable greens should be able to withstand the heat. If you must water at this time, water at the root and avoid wetting the leaf as this will cause wilting.
If your veggies are regularly wilting and are receiving enough water in the morning and evening, consider a partial shade cover such as a shade cloth. Some greens such as lettuce need a little shade.
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