First of all, unless you're at really high altitude (I mean like, 10,000 feet or above), forget about UV. It's irrelevant. I don't think that too much light is an issue either. As I said, it's mostly about temperature. If it's hot, and the plant is in full sunlight, the plant will feel hotter! Again, remember, the AIR temperature in full sun is lower than the SURFACE temperature for anything dark, like a leaf, or asphalt. My plants get 100+F air temperature in full sun for four weeks here in Central Texas. How do I know when to shade them? Well, if they droop even when the soil is moist, they're too hot. They're not pumping water fast enough to keep up with the evaporation by the heat. I can't do much about the air temperature, but I can keep the sun off of them if I need to. I'd like them to have as much sunlight as possible (as Seysonn points out), but not if it means they're going to get overheated. I guess that means that the way the plants look is more important than what your light meter says. Actually, I should say that the only time I've ever shaded any plants was when I was planting a fall crop of beans in August. Those seedlings were delicate enough, with shallow enough roots, that they needed some protection. I think I also once put shade over some squash, but they were on their last legs anyway. Cukes might get stressed by the heat. Tomatoes, probably not, unless they are leggy, with very long stems. Peppers LOVE heat and sun. Now, I don't live in a dry desert climate, but I can presume that evaporation rates will be higher there, meaning that plants could get stressed with somewhat lower air temperature. But again, just look at the plants. They'll tell you if they're unhappy. Hope that helps.

Well my first guess from the description would be a compacted soil, but most bonsai soils are very porous. Not watering for a long time could have the same effect, but you said you have been watering it. Many gritty soils can be difficult to wet properly because the water tends to channel and run straight down rather than spreading throughout the soil. Maybe this is what you're seeing? If you water all over the soil, allow the soil to absorb the water for a bit, then water again, the soil should be more completely saturated. You can also submerge the pot in water (bottom watering), just do it slowly and no deeper than the soil surface (you don't want the soil floating and disturbing fine roots). Also, these plants do best with direct sunlight, so the more light you can give it, the better.

Here's proof that nothing is wrong with the Canna. Less than 2 hours later, twilight, reduces direct sunlight and temperatures and ... Amazing - eh? Isn't that just too cool?! I have also noted that Bird of Paradise curled their leaves today, but less pronounced as they were under shade cloth in my nursery.

Starting at the front of the house with all sorts of light conditions, here's my 6 largest-all bloom age going around the perimeter of the house. All are in different stages of leafing and blooming. Full Shade-Miami Rose (Cliff) Full Shade NOID Pink w/orange Center (Helen) Afternoon Sun Aztek Gold (Linda Leaf) All Day Full Sun Aztek Gold (Lady) All Day Full Sun NOID Wine Full Sun Morning NOID Wine