I don't know of any data to quote .. of course plants are temperature sensative so one might be careful using VERY cold water on plants that are from very warm regions.

Unless you keep your gardening water in the freezer before watering your tropical ferns I don't think there is much to worry about.

I run the hose a bit before watering my Cactii in the summer to avoid using the burning hot water in the hose .. makes sense .. cooked roots don't work well.

Use common sense ... watering green house tropicals at 80 F with 40 F water maybe not a great idea ?? I don't know .. I have never had a problem with cold water working outdoor ornamentals out doors.

Good Day ...

I run the hose a bit too when watering outside-the water does get really hot sitting in the hose. Never had a problem with cold water for outside plants.
For inside I only use warm water for no other reason than I dislike having cold hands and working with really cold water. Some people have written saying they use ice cubes for watering and don't report any problems.

Some indoor tropicals don't like air temps lower than 50-55 f. I can't imagine they would tolerate water colder than that for any length of time.

Then there is the water spotting on african violet leaves said to be cause by cold water. I read somewhere that it is not the cold water, rather the difference in temperature of the water vs. the leaf. Seems to me it was something like 15-20 degrees or more that did it?

Hi
What type of plants are you referring to?? If your referring to tropical "house" plants Yes it is quite harmful. Some that come to mind are Episcia.they will wilt at 50 and 45 os fatal.!! Darlingtonia requires cold water to thrive while air temps must be above 50.Most epipytes particularly orchids fall into either warm or cool requirements.
When i was in Costa Rica I was fascinated by the extrene microclimates set up by altitude change,saltwater intrusion and raifall. Rain fall would be in the 80's while mountain streams would be in the 40's The demarcation of what grew where was so sharp it looked liked it had been carefully planted. Another was total amount of rainfall one side of a mountain would get over 150 inches while the other side would get 10.!! From the crest you could see a sharp line exactly where the rain was falling and how much.
Another was salt marsh that was frequently washed by heavy rain or rising and falling flood waters. The ferns would tell you exactly how far up the salt would rise.
Was so fascinating I could have looked at it for years!! lol
gary.