If the soil in a shady garden site is decent, all you need is to add shredded leaves and compost to each plant's planting hole. Top that off with a thick layer of mulch, and plants should do just fine, provided they remain evenly moist. Soil conditions may vary widely in shady spots. In time, by adding several inches of finely shredded leaves around the base of a tree, you'll create the soil conditions needed to support the growth of numerous shade-tolerant plants. The best solution is to plant something that grows in dry shade, such as English ivy or Japanese pachysandra, both of which are easy to find and grow.
Dogwoods are one of the best kinds of shade-tolerant trees.
They're true understory trees that in the wild grow in the shade cast by larger trees. The most common dogwood is Cornus florida , native to the United States, and it's a great tree. An Asian species, C. kousa , flowers about a month later and thrives equally well in shady spots. Japanese maples also do nicely in all but the densest shade, but red varieties may not color as well without a few hours of sun, preferably morning sun. When it comes to shrubs, the list includes short, tall, and medium plants as well as deciduous and evergreen varieties.