Good question. No doubt that it can damage plants.

But off hand the only time I can think of when I might use it would be before foliar feeding or applying any sort of sprays that could damage the foliage if the UV index is high. But then I always do them very early in the AM anyway when the UV rating would be at its lowest.

Dave

http://www.gardenguides.com/132133-plants-ultraviolet-light.html

I don't pay much attention to them but It might also come in handy when hardening off plants especially for the fall garden when trying to harden plants in July and August.

I pay some attention to it when taking hardening off. I am at about 6500 ft above sea level so I assume that it is going to be high any way, but when there is a UV warning, I try to make a point of taking my most tender seedlings out in the morning or evening, never mid-day or afternoon. Also, even for hardened off seedlings, if there is a UV warning, I try to put transplants in the garden with a milk jug and not a clear plastic soda bottle. I think the milk jugs diffuse the light better, making the transplant shock a little less.

The UV index is actually a linear scale, proportional to the amount of UV-B. So an index of 10 corresponds to twice the incidence of UV-B than an index of 5, and an index of 0 mean no UV at all. The atmosphere transmits UV poorly, so the index is by far most dependent on the angular elevation of the sun. In the winter, the sun is always low, so the index is always small. So high UV index is a summer solstice mid-day proposition. You don't need a forecast to tell you that. To plants, UV-B is largely an agent of damage. Plants do not use UV for photosynthesis. Now, altitude is a factor, as noted. High altitude means less atmosphere above you, and the UV index will always be higher at altitude.

As Dave noted, this issue is more about putting stuff on plants. Bt, for example, is broken down by UV.

But really, the main reason to pay attention to it in gardening is to protect the gardener.

i Live in Brisbane Australia and rent a room; the entire block is entirely covered in shade except a tiny narrow section that gets 4 or maybe 6 hours sun per day (in our winter). I was going to grow in concrete blocks ( Breeze blocks ) , NOT in cement Cinder BLOCKS !!!

So a small area of average 5 hours fair sun or else heavily shaded any ideas of vegetable to try and grow, i am going with (not-moving) grow bags now.