A baking slope combines nearly every hot, dry challenge at once: water runs off before it penetrates, topsoil is often thin, wind exposure increases moisture loss, and there's rarely any shade relief. Plants that handle heat in a flat bed often still struggle here because the drainage is so aggressive.
The most common mistake on a slope is planting the same way you'd plant a flat border, individual plants spaced apart with bare soil between them. On a slope, that bare soil erodes, dries out, and radiates heat right back up at the plant crowns. The goal on a slope isn't just to find plants that tolerate heat and drought. It's to achieve ground coverage as quickly as possible.
Mass planting beats individual specimens every time on a slope. Ten plants of the same variety planted closer together create a shared microclimate between them. More shade at the soil surface, more moisture retention, more root competition against weeds. A single plant surrounded by bare soil struggles; a mass of the same plant thrives.