Soil
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Whatever style of garden and penstemons you select, you should have soil that is well aerated and drains quickly. This may mean digging deeply for a perennial bed or building a raised bed above the surroundings with a mix of coarse sand and fine gravel. It should be about 6" deep if you live in an area where precipitation is high or if your soil is an impervious clay. Many eastern and mid-western gardeners grow a great variety of penstemons, even species from arid sites, by building beds of sand and gravel above their normal soil or by creating slopes. Roots will travel down to anchor the plant in the soil, but the crown, the most vulnerable part of a plant, will be protected from standing water. Even in arid locations, the garden area selected should have good drainage, and often the plants will perform better and bloom longer if some humus and fine gravel and/or coarse sand are incorporated into the soil. Fine gravel will reduce capillary action in the soil and slow evaporation from the surface. Soil amendments such as peat moss and manure should be avoided, especially in dry climates. The best amendment is homemade compost (leaf mold and mushroom compost are also used) but it should be only a small percentage, except where you are growing hybrids. The American and Canadian hybrids do well in average to good soils but the large-flowered hybrids from milder climates require soil that is porous and holds enough moisture that plants never totally dry out. Most penstemons do not seem to be fussy about the soil pH, although slightly alkaline soils seems preferred by many species and hybrids. If your soil is quite acidic, it would be good to add some, pumice, ground oyster shell or something else that would bring it nearer to neutral.
Selecting, Growing and Caring for Penstemons
From Chapter 41