Answer: I wonder what is going on with your viburnum! Air pockets within the rootmass would explain the afternoon wilting. I suspect that there is a gopher or mole tunnel directly beneath the root system of your viburnum which is where the water is draining. You are probably filling up the hole when you water, and once the roots get moisture, the plant perks up. I'd wait until late fall when growth slows and dig the viburnum up. I bet you find some kind of tunnel or hole beneath the shrub!
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