If you have slow draining, heavy clay soil you may have noticed that plants like lavender and penstemon do not do well for you.
No lavender I’ve planted ever persisted well for me until I realized that I needed to plant it on slopes, in raised beds or in berms, to improve the drainage through run-off.
It is also a good idea to amend heavy clay soil by mixing in sand and vegetative matter. Fast draining sandy soils are called “sharp” soils by English gardeners. The disadvantage of fast draining soils, of course, is that plants need to be watered more frequently than those growing in heavier moisture-retentive soils. (So if you have been complaining about having clay soil, check your water bill in dry weather, and it may make you feel a little better.)
You will notice that the plants that do not droop in hot, dry weather are frequently those with deep roots, like butterfly weed and coneflower, and those with grey foliage such as yarrow and Russian sage.
Make a point of planting moisture loving plants in the areas of your garden that do not drain quickly and where there is afternoon shade. In full sun, plant those that are described as drought tolerant such as coneflowers, sedum, and herbs.