When it’s hot and sunny, I try to fall back on the guidelines for container plants in stressful circumstances, clustering them together to form a microclimate, and frequently misting with a fine hose nozzle to temporarily cool foliage. I sometimes cut back stems to ease stress on root systems unable to adequately service limp leaves.
What also makes a difference is mulching the soil surface in each container. No matter if it’s a field of roses or a pot of begonias, covering the soil with organic mulch is the first and best defense against heat and drought. I often use leaves, pine needles and shredded bark to create a two- to three-inch (5- to 8-cm) layer of mulch around the stems in each pot, being sure to extend the materials all the way to the pot rims, and leaving no cracks where sunlight can enter. I learned that the depth of mulch is key to really keeping soil temperature cool, or at least cooler than it might be when fully exposed. A thin mulch (one inch / 2.5 cm or less) really didn’t do much, but the deeper layer resulted in cooler soil. As well, there is significantly less moisture evaporating, meaning less watering is needed.