Steve, The way you broke down the dates and the success rate really helps me understand some of my own losses and successes. I knew last year that bare root roses had no chance in the june heat. I didn't think about why potted plants struggled.
When our heat got so extreme last summer, I decided to stop watering. The roses stopped blooming or growing, but they didn't turn brown. They weren't really mulched well at that time. None of them died from the heat or drought. Our nighttime temps stayed high, which usually is a killer. I have some roses that never ever get supplemental water. If it doesn't rain, they just go on "hold". I have not replaced the perennials i lost in the heat last summer. It's looking like this summer will be just as hot, so I would expect the same types of plants to die again. Maybe i'm finally learning, or i'm tired of experimenting. Plants that are pushing the zone end up looking like they're struggling.
