Is there a spot in your yard where nothing will grow? No matter what the problem, you can rest assured that, whatever that conditions, there are perennials to suit that site.
If the problem is a low spot where water often stands, choose perennials with roots that aren't apt to rot in wet ground. In full sun try hardy hibiscus (pictured, below), queen-of-the-meadow (Filipendula ulmaria), swamp milkweed (Asclepias incarnata) or yellow flag (Iris pseudacorus). If your wet area is shady, use cardinal flower (Lobelia cardinalis), globeflower (Trollium chinensis), marsh marigold (Caltha palustris) or turtlehead (Chelone lyonii).
Soil too dry? Good candidates for a hot, dry spot in full sun include blanket flowers (Gaillardia x grandiflora), maiden pink (Dianthus deltoides), purple coneflower (Echinacea purpurea) and Russian sage (Perovskia atriplicifolia).
For the dry shade beneath a large silver maple or other shallow-rooted tree choose ground covers such as bishop's weed (Aegopodium podagraria 'Variegatum'), lungwort (Pulmonaria saccharata), red barrenwort (Epimedium x rubrum) or yellow archangel (Lamiastrum galeobdolon).