Most plants thrive in soil that has a neutral pH, but there are some that prefer acidic (low pH) or alkaline (high pH) soil. It is important to test your soil before planting to ensure that you select plants that are suited to the soil. Neutral pH is considered to be between 6.5 to 7, while anything below a pH of 6.5 is considered acidic and anything above 7 is considered alkaline. There are many plants, ranging from trees to shrubs to flowers, that are most at home in either acidic or alkaline soil.
Slightly Acidic-Tolerant Plants
There are many plants that will tolerate and thrive in slightly acidic soil, including fir, pine, spruce and magnolia trees. Shrubs that do well in acidic soil include blueberries, hydrangeas and dogwood, and perennials that thrive in acidic soil include butterfly weed and most varieties of ferns. The most planted hydrangea, the bigleaf or common variety (Hydrangea macrophylla), produces a blue bloom in acidic soil and a pink bloom in alkaline soil.
Highly Acidic-Tolerant Plants
Members of the rhododendron family, including azaleas, are the best-known plants that thrive in highly acidic soil. In general, these plants also need well-draining soil, filtered shade and ample water. If you want to plant a member of the rhododendron family but have alkaline soil, you may build a raised bed of about 2 feet that has rich, organic soil and some sand to help with draining. The pin oak tree (Quercus palustris), which is native through much of the north and northeast, is an acid-friendly, fast-growing tree with droopy lower limbs that is often planted along streets, according to the 1997 Sunset National Garden Book.
There are fewer plants that thrive in alkaline soils than acidic, but among the best bets is arrowwood viburnum (Viburnum dentatum), which is an evergreen shrub that can grow to 10 feet and works well as a hedge. Arrowwood viburnum has dark green leaves that change to yellow, orange or red in the fall and cream-colored flowers in late spring. Small, bluish-black berries dot this shrub in fall. Arrowwood viburnum is hardy in zones 3 to 8, can tolerate very hot or cold weather and likes moist soil. Other alkaline-tolerant plants include boxelder (Acer negundo), a maple tree that grows to 60 feet; locust (Robinia), a fast-growing tree with pink blossoms; and mockorange (Philadelphus), a shrub with fragrant white blooms in spring.