If cucumber plants are growing well, wilt very badly in the sun, revive quickly in shade and over night, is the wilting harming them? Their soil tests okay for water. I am in northern California and the sun gets blazing hot with temperatures up to, and sometimes over 100 degrees Fahrenheit (~38 °C). I fear bitter cukes will result from this scenario. Should I throw cheese cloth over them or screen them in some other way?
Wilting is a sign of stress, and not harmful in itself, as long as it doesn’t dry to the point that the vascular tissue is damaged. When that happens the water column is broken and transpiration stops.
I had a professor in college who was a plant physiologist, his field of study had to do with the physiology of plant vascular systems. That was a very boring class.
You would basically not be able to avoid this in some Cucurbitaceae, as they just don’t have the root system to support their above ground parts, even if they are sitting in a puddle — it is just too much water loss to keep them turgid (that is the term they use).
The best things you can try to reduce transpiration:
Shade cloth — reduce the amount of heat hitting the leaves, ground and air.
Mulch — increase the amount of water that stays in the soil.
Mister — increase the humidity around the leaves.