Answer: I'm afraid that an area on the south side of your building is simply too hot for larkspur. While they will take full sun in an area without reflected and radiated heat, they won't do well in a really hot spot. In fact, I'd plant them on the opposite side of the building where they get morning sun and shade from the hot afternoon sunshine. For the area you describe, a heat and drought tolerant plant would perform better. Try Purple Coneflower, Liatrus, Black Eyed Susan, Catmint, Globe Thistle, or other "xeriscape" type plants in the site.
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