Soil type can vary considerably from one location to another. The physical features of a landscape, its proximity to the coast, the amount of rainfall it receives, as well as its existing community of plants can significantly influence the quality of its soil. Areas that generally stay dry and have porous substrates, such as those found in arid regions, tend to have alkaline soils. In contrast, the moist soil in tropical rainforests is likely to be acidic.
Alkaline soil has a pH level that is usually above 7. Slightly alkaline substrates, which can be highly desirable for some hardy plants, have pH levels that range between 7.5 and 8.5. When pH levels are above these values, the soil may be considered poor and may be toxic for most plants. Calcium carbonate molecules from limestone, granite, concrete walkways, shells, and crushed stone usually increase the alkalinity of the soil.
Calcium, magnesium, and sodium ions are some of the key components of alkaline soils. When these are found in high concentrations, they can affect the availability of nutrients and how efficiently these are taken up by plant roots. A high alkalinity can be rectified by mixing organic matter, sawdust, sphagnum peat moss, or sulfur-based compounds into the soil. However, amending soils can be time-consuming and costly in the long run. Instead, try your hand at cultivating these alkaline-tolerant plants!
1) Salt cedar (Tamarix ramosissima)
Salt cedar, also known as “tamarisk”, can be grown as a small tree or as a large shrub. This deciduous species has become popular as an ornamental plant due to its generously dense plumes of pink flowers. In USDA hardiness zones 2 – 8, these appear in late summer to early fall, transforming the appearance of the entire plant. The blooms can last for up to 6 weeks, proving to be a long-lasting spectacle for an unforgettable hedge.
Perfect for coastal gardens or landscapes with poor soils, salt cedar is known for its tolerance for salt and its suitability to infertile substrates with alkaline conditions. In warm regions, its roots can thrive in pH levels of up to 7.9 and can generally survive in higher alkalinities. A rapid grower, its shoots may reach heights of 10 – 15 feet (3 – 4.6 meters). As a single plant may have a spread of 13 feet (4 meters) and can spread on its own to create dense colonies, its unmanaged stands may become invasive.
2) Common yarrow (Achillea millefolium)
Once widely grown as a medicinal plant, the common yarrow is a wonderful perennial with immense wildlife value. This herbaceous species is often cultivated as a garden plant because of its ease of propagation and its low-maintenance requirements. Though it does have the tendency to self-spread via rhizomatous growth, its roots can be restricted to dedicated borders, plots, or containers to create a dense hedge or bed.
This versatile member of the Asteraceae family produces flat-topped clusters of white blooms from spring to early fall. Some of its cultivars are set apart by their lemon-yellow, peach, salmon pink, or red blooms. A quintessential component of grasslands and rocky outcrops throughout its native range, the common yarrow tolerates mildly alkaline pH levels. Its wild stands can be found in both wet and dry coastal zones with good drainage.
3) Lavender (Lavandula spp.)
Horticultural favorites, species of the Lavandula genus are arguably some of the best multi-use plants for semi-arid gardens in temperate zones. These wonderful herbs have naturally pest-resistant and herbivore-repellent leaves. Though their bitter taste and strong fragrance intimidate many animals, their purple inflorescences attract a diversity of beneficial pollinators. Their essential oils are rich in some of the most valuable phytochemicals in the perfume industry.
As lavender species have evolved to thrive in dry areas, where substrates tend to be poor and loose, they can survive in slightly alkaline conditions. Though pH levels from 5.8 – 8 are preferred by most ornamental varieties, they may withstand slightly higher levels as long as they are exposed to full sun and provided with good drainage. Do note that, while direct sun exposure is appreciated by their fuzzy foliage, protection from oppressive summer heat is recommended.
4) Woodland phlox (Phlox divaricata)
Woodland phlox is a clumping, low-growing wildflower that forms the loveliest, self-spreading stands in rock gardens and borders. A recipient of the RHS Award of Garden Merit, it produces mats of fragrant blooms throughout the peak of spring. With shades of lavender, blue, white, or light pink, these attract butterflies, bees, sphinx moths, and even hummingbirds.
As an ornamental ground cover, woodland phlox can be used to naturalize areas with poor soil in mildly alkaline landscapes. As its shoots rarely grow to more than a foot tall, they can also be used to add texture and color to the base of larger, alkaline-tolerant shrubs. To enhance the quality of their leaves and blooms, place them in areas receiving dappled shade and ample moisture. When this species’ shallow root system has become well-established, it should survive through brief dry spells.
5) Chrysanthemums (Chrysanthemum spp.)
“Mums” belong to a remarkably diverse group of flowering perennials and their highly-valued cultivars. Prized by horticulturists for their showy blooms, many of them have received the RHS Award of Garden Merit. The most popular types are perhaps Chrysanthemum x morifolium and C. indicum, which produce massive floral heads with a pompom or anemone-like appearance. These come in a wide range of pastel colors to enliven the brightly lit parts of a well-manicured garden.
Though chrysanthemums tend to fare best in neutral conditions, their well-established stands can tolerate pH levels that go up to 8. Hardy to USDA zones 5 – 9, they grow beautifully outdoors and can provide year-round color and texture. They generally grow better when their leaves are exposed to several hours of morning light. In temperate zones with mild temperatures, their blooms are most likely to appear in fall.
6) Purple-flowered rock rose (Cistus x purpureus)
The purple-flowered rock rose, also known as the orchid rockrose, grows as an eye-catching shrub with evergreen leaves. This lovely cultivar, one of the most popular among the rock roses, measures around 2 – 4 feet (0.6 – 1.2 meters) tall. Its shoots bear pairs of simple, oppositely arranged leaves with greyish-green to light-green tones. In late spring to early summer, these are pleasantly contrasted by short-lived, bright pink blooms with yellow centers.
Hardy to USDA zones 7 – 10, the purple-flowered rock rose is capable of budding vigorously in plots with well-draining substrates and mildly alkaline conditions. It is suitable for loamy parts of the garden that rarely receive supplementary irrigation. It can also be grown as a specimen shrub in coastal areas, particularly in zones where poor soils tend to dominate the landscape.
7) California lilac (Ceanothus spp.)
California lilacs, also referred to as soapbushes and buckbrushes, are known for their capacity to fix nitrogen. These members of the Rhamnaceae or buckthorn family typically grow up to 10 feet (3 meters) tall. There are some outliers, such as the blueblossom ceanothus (C. thyrsiflorus) and the island ceanothus (C. arboreus), which can be cultivated as small trees (up to 23 feet or 7 meters tall). Generally, these plants are most breathtaking in spring and summer – when their branches become densely covered in blue inflorescences.
The blue tones of the California lilac truly come in an assortment of shades. Some species sport indigo and cobalt blooms, whereas others have the most delicate sky-blue hues. In the wild, the blooms add seasonal color to coastal zones and the edges of rocky forests. Unsurprisingly (for an alkaline-tolerant plant), their contrasting leaves and shoots have high concentrations of calcium.
8) Border carnation (Dianthus caryophyllus)
Symbolic of motherly love, the border carnation is often prized for its vibrant, pink to red flowers. These have distinctly fringed petals, contributing to their overall texture and complexity. They also emit a cinnamon-like scent and are thus often added to homemade potpourris. The blooms are typically produced in late spring to summer, but note that they are seldom able to tolerate scorching heat.
Border carnation favors well-draining and gritty substrate mixtures, particularly those that have mildly alkaline pH levels. Popular cultivars have shoots that measure up to 18 – 24 inches (46 – 61 cm) tall in optimal conditions. Their leaves may persist through winter in regions with mild temperatures. As their blooms tend to last quite long, they make for fantastic cut flowers to brighten up a home’s interiors.
9) Sweet pea (Lathyrus odoratus)
Frequently cultivated as a form of vertical interest on low fences and trellises, sweet pea is a flowering climber. Though it is an annual, its well-established stands can grow quickly enough to reach a height of 6 feet (1.8 meters) within a single growth period. Of course, these require ample support around which their tendrils and trailing shoots may gain footing. Nowadays, dozens of its cultivars are used by horticulturists for spring interest.
Though sweet pea can thrive in USDA hardiness zones 2 – 11, it is best grown in regions with mild climates. Its leaves and shoots may die back in excessively warm and humid areas. The fragrant blooms, which range in color from white to pink, purple, and blue, are most profuse in cool and crisp conditions. These attract bees and butterflies. Note that their seeds are not safe for animal or human consumption due to their neurotoxic properties.
10) Japanese honeysuckle (Lonicera japonica)
Another vine with fantastic ornamental qualities, the Japanese honeysuckle has a knack for thriving in a wide range of ambient conditions. For this reason, it may outcompete native plants and form invasive colonies. This trailing plant should only be cultivated in enclosed gardens, where its self-spreading shoots and roots are unlikely to escape into sunny, wild habitats. Escaped specimens tend to become dominant in disturbed sites, such as logged forests and along roadsides.
Though this vigorous climber can thrive in slightly acidic substrates, it can also become well-established in moderately alkaline soils. Its shoots grow remarkably quickly when they are provided with complex support structures and have access to a high concentration of organic nutrients. Its inflorescences, which consist of vanilla-scented, white to yellow flowers, are a source of both nectar and nutrients for a diversity of wildlife.
11) Leek (Allium porrum)
Closely related to garlic and onion, leek is set apart by its incredibly flavorful leaves. A delectable garnish in many traditional dishes throughout its native range, its above-ground features have a fan-like appearance. This is due to the ordered manner in which its arching leaves have sheaths that thoroughly cover the stem. Those who are unfamiliar with this plant may mistake its wild stands for members of the grass family.
Best cultivated in well-draining and loose substrates, leek can thrive in moderately alkaline pH levels that go up to 8 – 8.3. This popular vegetable has a higher tolerance for cold temperatures than many of its bulbous relatives. It is thus considered a cool-season crop and is suitable for growth in the northern reaches of the US.
12) Asparagus (Asparagus officinalis)
Another globally popular vegetable, asparagus is best known for the culinary uses of its young shoots. Though it is one of the most notable members of the Asparagaceae family, many of its consumers are actually unfamiliar with its mature features. A fully grown asparagus specimen has branching stems with whorls of leaf-like cladodes. Its shoot is anchored by an adventitious root system, technically termed the ‘crown’ of the plant.
Asparagus is best cultivated in sandy substrates with a neutral to moderately alkaline pH. Good drainage is crucial for the health of its rhizomes, which are able to self-propagate to form clonal stands. Its blooms, usually appearing through June and July, are yellowish-green, small, and somewhat inconspicuous. In contrast, its fruits are bright red, hard, and contain mildly toxic phytochemicals.
13) Prickly pear (Opuntia ficus-indica)
Many plants found in deserts have a tolerance for mildly to moderately alkaline conditions. This is largely due to the poor, well-draining substrates they have evolved to colonize. The prickly pear cactus, currently known as the most commercially important succulent, is no exception. Able to produce stands boasting heights of up to 16 feet (5 meters), it grows vigorously when rooted into substrates with pH levels ranging from 7 – 8.9.
The spatulate stem pads, or cladodes, of this species are often harvested, skinned, and sliced into “nopalitos”. In Mexico, these are served with jalapeños or are processed further to create spicy jams and fruit juice. The fruits of this species have a water content and taste which is often likened to that of watermelon. This would be the perfect, low-maintenance plant for an edible, desert garden!
14) Baby sage (Salvia microphylla)
Baby sage is a great choice for mildly alkaline landscapes in USDA hardiness zones 7 – 10. This unforgettably charming member of the Lamiaceae or mint family is also known as “blackcurrant sage” due to the blackcurrant-like scent of its ovate, oil-rich leaves. Its shoots can grow to about 4 feet (1.2 meters) tall in optimal conditions and can reach their maximum size after just 2 years of proper care.
Able to produce a wealth of blooms in both spring and fall, baby sage now comes in many horticultural hybrids. Many of these are recipients of the RHS Award of Garden Merit due to their drought tolerance and their capacity to thrive in various soil types. Most varieties prove to be generous bloomers in areas with neutral to moderately alkaline substrates. When provided with ample sunlight and moderate moisture, pH levels of up to 8.5 are tolerated.
