I asked the guy at the garden store what would do well around my mailbox in the full sun, and he suggested zinnias. Well, I planted them and they seemed fine and good but yesterday after our first 90° day I noticed they were withered and scorched. Can they be saved? Are they not, after all, suited to full sun? And if not, what would be?

Maybe they just need more water, Lex. Is the soil they're in too well-drained? (Is it at the top of a slope?) Maybe you can move them into a container (that has a drainage hole) and dig it into place so that the soil level in the container is the same as the soil level outside, then water it well daily, without getting water on the leaves. (They're subject to mold.) Zinnias should be able to take direct sun and 90-degree heat.

The container would work similarly to this:

I had some wisteria on a slope and it wasn't doing too well, so I mixed up a little concrete and made a barrier (with rocks in it, so it looked nice) that would dam the water and keep it close to the plant long enough to soak into the soil. The plant improved immediately.

My favorite 'hot, dry, full sun' plant is rose campion. Its grey fuzzy leaves are just as pretty as its flowers. It does grow tallish, so a clump of it looks pretty if surrounded by low-growing plants.