The acidity of soil, which is indicated by its pH level, is a function of its concentration of free hydrogen (H+) ions. Soil pH levels that fall below 7 are considered acidic, with a value of 5 and below indicating highly acidic and potentially damaging conditions. In contrast, pH levels above 7 are considered alkaline as they have higher concentrations of hydroxyl (OH-) ions. Both types of soil have different mineral profiles and can affect plant growth in either beneficial or harmful ways.
Generally, most garden plants thrive in slightly acidic to neutral soils (pH 6.0 – 7.5). The acidity levels in practically any type of growth medium can affect how well the roots are able to take up nutrients, which are crucial for plant development and survival. Naturally acidic substrates are the result of hundreds to thousands of years of organic matter decomposition. Moreover, when minerals like calcium, magnesium, and potassium are washed away by stormwaters or irrigation, pH levels can drop further.
The use of fertilizers, particularly those containing ammonia, likewise enhances the acidity profile of substrates. While acidic conditions in wetland regions and tropical zones can be amended with the repeated addition of lime, this can be time-consuming and costly. Instead of fretting over acidic conditions, consider growing acid-loving plants! As they flourish in soil pH levels ranging from 4.5 – 6.0, these “acidophiles” can effectively transform the landscape.
1) Bigleaf hydrangea (Hydrangea macrophylla)
Prized for its relatively large leaves and its heavy bouquets of pastel-colored blooms, the bigleaf hydrangea is a game-changer in partly shaded landscapes. This deciduous perennial produces bushy, highly textural shrubs that range in height from 2 – 6 feet (61 – 183 cm). In the wild, it is found in the acid-rich forests of vegetated coasts and mountain ranges. Its roots are most efficient at taking up ideal concentrations of nutrients in soils with pH levels between 4.5 and 6.0.
One of the most exciting features of the bigleaf hydrangea is its tendency to have inflorescences boasting an array of colors. In acidic soils, where the roots are more likely to hyperaccumulate aluminum ions, its flowers develop blue pigmentation. In basic soils, where aluminum ions are generally less available, the flowers tend to be more purple, pink, or red. As a patch of soil may have varied pH levels, a single hydrangea shrub may simultaneously produce blue and pink flower clusters.
2) Purple rhododendron (Rhododendron catawbiense)
Also known as red laurel, Catawba rosebay, and purple ivy, this evergreen shrub is highly valued as a flowering ornamental. A member of the Ericaceae or heather family, it is known for naturally forming dense thickets in woodland ridges. Equipped with shallow roots, it favors well-draining yet consistently moistened substrates. Shrubs situated in acidic landscapes, with pH levels ranging from 5.0 – 6.0, tend to boast high flowering rates.
A part shade lover, the purple rhododendron appreciates exposure to morning sun. It should not be situated in areas receiving intense afternoon heat. Moist, protected borders along a tall fence or an outdoor water feature would certainly be complemented by the elegant features of this evergreen shrub. Do note, however, that its shoots may grow to a maximum height of 20 feet (6 meters) in optimal conditions. As they also tend to be highly flammable, situating them right next to a home’s foundations may not be the best idea.
3) Winter heath (Erica carnea)
With a whole range of cultivars that have received the RHS Award of Garden Merit, winter heath has many outstanding qualities as an evergreen shrub. Low-maintenance and tolerant of cool conditions, it is often cultivated as a ground cover plant. Its cultivars rarely grow to more than a foot tall, making them the perfect height for naturalizing small slopes, the base of a garden fence, or the exteriors of a woodland cottage.
Though winter heath is attractive all throughout the year, it is especially spectacular in late winter to early spring. During this time of year, its stems burst with racemes of bell-shaped flowers. These may even begin to appear on plants that are partly covered in snow! Of course, the right soil conditions are crucial to enhancing bloom density and encouraging timely bud production. Sandy soils with pH levels between 5.5 and 6.5 are recommended.
4) Bleeding heart (Lamprocapnos spectabilis)
One of the most popular choices for adding color and texture to shade gardens, bleeding heart is an unforgettably striking perennial. Its inflorescences, which come in the form of gracefully arching racemes, appear through spring and early summer. These are distinguished by their thin floral stalks and their evenly-spaced, pendent flowers. As suggested by this herb’s common name, the blooms are heart-shaped. The inner petals emerge from the bottom of each heart to form a teardrop-like shape.
Perfect for adding charm to a fairy garden, bleeding heart thrives best in well-watered substrates and alongside other shade-loving plants (e.g. hostas, ferns, astilbes). Its roots favor humus-rich soils with pH levels ranging from 6.0 to 6.5. Slightly higher pH levels are tolerated by mature specimens, but do note that these may cause the foliage to look yellowish. In regions with intensely warm summers, the leaves and shoots may naturally die back as the roots become dormant.
5) Eastern white pine (Pinus strobus)
If you’re in search of a fast-growing tree to add shade and vertical complexity to a medium-to-large garden, consider growing the eastern white pine. This acid-loving species favors pH levels ranging from 4.5 – 6.0, making it one of the toughest species in areas that are frequently drained of minerals. Best situated in sloping areas with northeastern exposure, it appreciates cool conditions, proper drainage, and ample moisture.
The eastern white pine is capable of growing to 100 feet (30.5 meters) tall in areas where its trunks are left to thrive for dozens of years. Set apart by its bundles of needle-like leaves, slender seed cones, and evenly-spaced whorls of branches, it can serve as an invaluable resource to wild animals. Many forest birds find shelter and protection in its canopy, while small mammals feast on its cones.
6) River birch (Betula nigra)
As the river birch favors moist conditions in a wide range of wetland habitats, its strong preference for acidic substrates comes as no surprise. Some swamps in its native range are thoroughly dotted with its well-established colonies. In these environments, this prominent, flood-tolerant tree can influence many ecological relationships. Its root system aids in preventing high erosion rates, ensuring that floodplains can remain fairly stable throughout the year. Moreover, waterfowl use the tree as both a nesting site and a source of food.
Fairly cold-hardy, the river birch can be cultivated as a specimen tree in exposed rain gardens. As its trunk thickens and elongates, you’ll find that its bark is quite distinct. On young trees, this may appear to be paper-like and may easily flake off. The bark becomes increasingly furrowed and thick over time. Eventually, a healthy river birch can create a canopy that casts dappled shade onto lower-growing shrubs and herbs.
7) Flowering dogwood (Cornus florida)
If you find that the trees above are much too large for your yard or garden, you’ll be delighted to know that this enchanting species (and its many cultivars) rarely grows to more than 33 feet (10 meters) tall. Able to live for up to 80 years, it can serve as a long-lasting focal piece with year-round appeal. In spring, delicate blooms adorn its multiple levels of branches, completely transforming the appearance of its canopy. These are soon followed by clusters of new leaves.
Once the blooms are pollinated, they begin to develop into summer fruits. These small, red-orange drupes are favored by dozens of species of native birds, which serve as vectors for seed dispersal. Any fruits that remain on the tree may complement the reddish, purple leaves in fall. Attractive even through winter, when its stark and highly-textural branches become bare, this acid-loving tree can add vitality to partly shaded, evenly moist locations.
8) Mountain laurel (Kalmia latifolia)
Like many of its close relatives in the Ericaceae or heath family, the mountain laurel is known for thriving in acid-rich conditions that would compromise the growth of other ornamentals. This charming shrub favors pH levels ranging from 4.0 – 5.5. Typically found in oak-heath forests, it tends to form dense colonies over wet substrates. In its wild habitats, some of its thickets are so compact that they are near-impossible to walk through.
In late spring to early summer, mountain laurel shrubs are irresistible to both seasoned and novice gardeners. Their stems become abundantly coated with spritely, eye-catching inflorescences. Composed of showy, bell-shaped blooms with the most alluring shades of pink, white, and red, the inflorescences unfailingly attract bees and butterflies. Several cultivars can be grown next to one another to create a spectacular pollinator garden.
9) Creeping juniper (Juniperus horizontalis)
A low-growing, evergreen shrub with enormous potential in cool climates, creeping juniper is a versatile member of the cypress family (Cupressaceae). In the wild, its trailing shoots can form sprawling mats over cool and rocky, occasionally barren, headlands. Its shoots are able to separate from the mother plant and develop their own root systems. This adaptation allows them to produce clonal colonies and cover vast portions of bare landscape.
Creeping juniper has a pH preference of 4.0 – 6.0. Its tough root system can tolerate a wide range of substrates, but it is usually most successful in well-draining soils with a considerably sandy profile. Low-maintenance, its shoots require little in terms of care and maintenance. They can effortlessly spread over sloping parts of the garden, suppressing the growth of weeds in the process.
10) Common camellia (Camellia japonica)
A member of the Theaceae or tea family, the common camellia has leaves with complex phytochemical properties. Containing caffeine and terpenoids like vitamin E, squalene, and lupeol, the leaves release a rich fragrance when they are crushed or processed. Borne on shoots that grow to about 36 feet (11 meters) tall, they are leathery, deep-green, and have finely toothed margins. From January to March, vibrant blooms add complexity to their backdrop of dark foliage.
In its native range, the common camellia is found in high-altitude forests, where they may weather heavy snowfall. The first blooms appear as soon as the snow begins to melt, heralding the onset of warming temperatures. A high concentration of organic matter, coupled with slightly acidic conditions (pH 5.5 – 6.5) and moderate moisture, facilitates favorable flowering rates.
11) Northern bayberry (Myrica pensylvanica)
The northern bayberry has evolved to thrive in slightly acidic, peat-rich, and nutrient-poor soils. This deciduous shrub is set apart by its leaves and their distinct scent. With a generous concentration of scent glands on their undersides, they emit a spicy aroma when rubbed or crushed. These are borne on increasingly woody stems that may grow to a maximum height of about 25 feet (7.6 meters). Inconspicuous blooms give rise to this species’ key feature – its silvery-grey winter berries.
Though the northern bayberry can develop into a small tree, it is often cultivated as a shrub. Its shoots can be pruned regularly to maintain a height and spread of about 6 feet (1.8 meters). Popular as a landscape plant, it can be grown in rows to create a natural screen over acidic and sandy substrates. Once bayberries are well-established, they exhibit a remarkable tolerance for salts. This makes them ideal for seaside plantings as well.
12) Holly (Ilex spp.)
Known for their vibrant, red berries, and their glossy, serrated foliage, holly shrubs are a captivating group of evergreen plants. These hardy species can be found in a variety of environments, from laurel forests to coastal swamps and sandy beaches, but what truly sets them apart is their affinity for acidic substrates. This preference for pH levels ranging from 5.0 to 6.5 ensures that they flourish in their chosen niches and can successfully compete with other acid-loving plants.
One of the most popular members of the Ilex genus, the European holly (I. aquifolium) tends to be a pioneer species throughout its native range (Europe, West Asia, and Northwest Africa). It can form thickets in shady forests and can quickly adapt to disturbances. This rugged shrub prefers notably acidic substrates that remain relatively moist but not waterlogged. It should make for a festive addition to the partly-shaded edges of a rain garden!
