I planted some non-stops in a planter on the front of my house. The planter is north facing so it gets a few hours of direct sun from roughly 10:00 to 1:00. I planted them in a mix that I make from orchid bark and peat moss. I added some slow release fertilizer according to package directions. The planter drains extremely well.
The problem I'm seeing is severe leaf burn. Large areas of the leaf turn brown/black, then wilt and dry out. Today I noticed that the plant is even releasing many of these leafs at the stalk. There are new leafs that are growing from the base of the stalk and even some of them have black spots on the leaf.
I'm no expert but obviously this is leaf burn of some kind. I'm a little sceptical that it's from excessive sun or heat. It's been reaching a maximum of 80 degrees of late but since there is reflection from the wall it's probably much higher for a few hours. When I added the fertilizer I think I actually mixed in roughly half of the recommended amount to be safe. This fertilizer has mostly slow release forms but it did say it had some freely available nitrogen also. I did not add much mulch for fear of causing mold so the top 1" or so of the soil is fairly dry/hot at times. I check the soil before watering and then water fairly heavily if the soil is nearly dry at 2" down.
I have some large flowered begonias that I'm growing in the back yard in dappled sunlight using virtually the same potting mix (except I used osmocote fertilizer which is a slow release form only) and they show no signs of stress whatsoever.
I'd really like to get to the bottom of this - I was thinking a would pull a few of them. Some I would repot in my mix that I use for the large flowered types and place one in dappled sun, one in full sun. Another one could be potted in the existing mix and removed to dappled sun.
What do you think, too much sun or fertilizer burn?
Leaf burn on non-stops
My vote is the sun with the extra heat bouncing off the walls as you said, I don't think I have seen them grown in mid day sun anywhere, only morning sun. All mine only get morning or dappled sun.
Try your experiment of moving them and see if they improve, perhaps with a drink of Superthrive to help them recover.
Tuberous begonias root from the top. If you are letting the top 2" of soil dry out your plants may be dying of thirst. Try keeping the soil evenly moist but not soggy.
I also wonder about your planting mix. I use a light weight mixture like Pro-Mix. Some of my tuberous begonias get mid-day sun for a few hours (no more than 3 hours) and do fine but I never let the soil dry out. In my experience tuberous begonias are not nearly as fussy about moisture as rex begonias although they will rot if the soil stays soggy.