Answer: Clay soil should be prepared as any garden soil would be prepared, by loosening it down about twelve inches and adding copious amounts of organic matter such as compost, old rotten leaves, throughly rotted stable manure/bedding, old rotted mulch, pine fines, or milled spagnum peat moss. The old rotted mulch and pine fines can be an excellent addition because the particles are relativley large. Then if it is heavy clay, you can also work in some coarse builders' sand or fine grit as well. These materials will help the soil hold air as well as water, thus improving the drainage.
One problem with amending the planting hole only is that it can work like a sump and collect excess water in the bottom -- your mix drains faster than the surrounding native soil. So it is better to amend a larger area to lessen the interface issue.
Besides the soil type, your planting site location also affects the drainage. In a low spot, or in an area where water naturally drains or collects, the site is a poorly drained one. For better drainage, plant at the top of a slope or along a slope. If your property is flat, you can try making slightly raised planting beds to improve the drainage. Even just a few inches will help.
Hostas require a shady site and do best in evenly moist soil -- but not saturated or sopping wet with standing water. Roses require a well drained site. Salvias do best in a site with perfect drainage. Generally speaking, if you have heavy clay soil you might want to consider selecting plants that do well in clay rather than plants such as salvia that would prefer a sandier soil with perfect drainage. Your local professional nursery staff should be able to help you select some, or your county extension will have suggestions.
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