Found anywhere from a few inches to several feet beneath the surface, hardpan is a layer of soil so compacted that neither plant roots nor water can penetrate it. Though typically associated with desert regions, it can be present in other areas, too. In most cases, it results when soil does not break down in the normal fashion and instead forms compound mineral particles that are fused tightly together. Hardpan may be present throughout an entire garden or only in certain parts of it.
You may not know you have a hardpan problem until you try to grow trees and shrubs with deep, extensive root systems. Because the roots cannot penetrate the hardpan, the plants' growth will be slowed, and they'll fail to thrive. Fruit trees will bear small crops. During spells of heavy rain and strong winds, large trees may even topple.
In vegetable and flower gardens, plants may dry out rapidly, since their roots will be confined near the surface. But rot and fungal problems may crop up too: since water cannot drain through the hardpan, it simply pools on top of it, waterlogging nearby roots.
When faced with hardpan, many gardeners despair, believing they must remove the entire hardpan. This isn't necessary! In beds of shallow-rooted plants, you can construct drainage channels to prevent water from pooling and causing root damage. Around established, deep-rooted plants, punch holes through the hardpan at 4-foot intervals; when planting new shrubs and trees, punch a hole through the bottom of the planting hole. Fill the drainage holes thus created with good-quality topsoil. Or simply construct a high raised bed filled with good soil; this is an effective solution if you're growing vegetables or a cutting garden for flowers.