Psorothamnus spinosus (Smoketree)

Psorothamnus spinosus or Smoketree, previously classified as Dalea spinosa, is a shrub or small tree species that grows primarily in the deserts of the southwestern United States, specifically within the Sonoran and Mojave deserts. The plant is distinguished by its hardiness in extreme climates, where temperatures can soar and water is scarce. Known for its vibrant purple flowers and dense, spiny branches, Psorothamnus spinosus is often seen growing in rocky hillsides, dry washes, and desert flats.

Smoketree is a member of the Fabaceae family, which also includes popular plants such as peas, beans, lupines, redbuds, and orchid trees.

Like many other members of this family, Psorothamnus spinosus is a nitrogen-fixing plant, meaning it has a symbiotic relationship with bacteria that helps it convert atmospheric nitrogen into a form that can be used by plants. This adaptation plays a vital role in the ecology of desert soils, which are often nutrient-poor.

Psorothamnus spinosus – Smoketree: An In-depth Look

Psorothamnus spinosus is a nearly leafless shrub or small tree with a distinct, irregular form. It typically grows with an open, sprawling habit. Its stems are slender and covered with small, spiny, or prickly projections that help protect the plant from herbivores that may be attracted to its delicate leaves and flowers. When in bloom, the plant has a unique appearance, with clusters of vibrant purple or lavender flowers that appear in late spring to early summer. These flowers provide a beautiful contrast against the muted desert landscape.

Native Habitat

The Smoketree is native to the southwestern United States and parts of Mexico. It is most commonly found in the deserts of southern California, Arizona, Nevada, and parts of Baja California. It thrives in dry, rocky soils and is typically found in areas such as dry washes, hillsides, foothills, and desert flats.

This plant is particularly well-suited to the harsh conditions of the Sonoran and Mojave deserts, where rainfall is limited, temperatures are extreme, and the soil is often rocky or sandy.

Plant Type and Habit

Smoketree is typically a shrub or subshrub, occasionally growing as a small tree. It features a short, twisted trunk with intricate branches adorned with smoky gray or silvery stems. Like the Paloverde, the trunk, stems, and branches perform photosynthesis, contributing to the tree’s energy production. The branches are also covered in spiny twigs.

Plant Size

Typically, the Smoketree grows 5-25 feet (1.5 to 7.5 meters) tall and 5-10 feet (1.5 to 3 meters) wide. The size and shape of this desert plant can be influenced by environmental factors such as water availability, soil type, and exposure to the sun. In arid regions, the plant tends to grow slower and with a more compact form.

Flowers

The most striking feature of the Smoketree is its vibrant flowers, which bloom in dense clusters at the tips of the branches. The flowers are pea-like, dark purple, violet or indigo-blue, creating a dramatic display in the landscape.

The flowering period for Psorothamnus spinosus typically occurs from late spring to early summer, with the most intense bloom happening in May and June.

Fruits

Following the flowering period, this desert shrub or desert tree produces legume-like seed pods that are about 2 to 4 inches (5 to 10 cm) long. These pods are initially green but turn brown as they mature. Once the pods ripen, they open up to release their seeds, which are then carried away to new locations by the wind or animals.

Foliage

Smoketree is often leafless for much of the year, with its leaves falling early in the spring. The few remaining leaves are green or silvery in color. From a distance, the silvery or smoky hues of the leaves create a hazy appearance, giving the plant its common name, “Smoketree.”

Bark

The bark is smooth and grayish-brown, with a slightly rough texture. The bark helps protect the plant from temperature extremes and prevents excessive water loss.

Uses

Smoketree is valued in desert landscaping for its beauty, hardiness, and ability to withstand drought. Its vibrant flowers and unique form make it an eye-catching addition to xeriscape gardens, especially in dry regions where water conservation is important. The plant’s nitrogen-fixing abilities also enhance the fertility of the soil, making it an excellent companion for other desert plants.

Hardiness

Psorothamnus spinosus is hardy in USDA zones 8-11, making it well-suited for cultivation in desert regions of the southwestern U.S., including parts of Arizona, California, and Nevada. It thrives in dry, well-drained soils and can withstand high temperatures, often exceeding 100°F (38°C) in summer. It is well adapted to harsh conditions, including drought, wind, and poor soil.

Wildlife

The Smoketree is a valuable resource for desert wildlife. Its flowers provide nectar for pollinators such as butterflies, hummingbirds, moths, flies, honeybees, bumblebees, and other insects. The seeds and pods are consumed by various desert birds, including Gambel’s quail and other small mammals, such as rodents and rabbits.

Toxicity

Psorothamnus spinosus is not toxic to humans or animals. However, its thorns can pose a hazard to people and animals, particularly when handling the plant. Care should be taken when working with the plant to avoid injury.

Drought Tolerance

Psorothamnus spinosus is highly drought-tolerant, making it an excellent choice for low-water landscapes. Its deep root system allows it to access water that is unavailable to other plants, while its ability to shed leaves during dry spells helps conserve moisture. The plant can go without water for extended periods and still survive in the harshest conditions.

Invasiveness

Psorothamnus spinosus is not considered invasive and is native to its desert habitat. It plays an important role in maintaining the health and balance of desert ecosystems and does not typically spread aggressively. The plant’s nitrogen-fixing abilities help improve the soil for surrounding vegetation.

Benefits

• Wildlife Support: The flowers, seeds, and pods attract various pollinators and provide food for birds and small mammals.
• Water Conservation: The plant’s drought tolerance makes it ideal for xeriscaping and low-water landscaping projects.
• Erosion Control: The deep roots of the Smoketree help stabilize desert soils and prevent erosion, particularly in areas affected by heavy rain.

Smoketree Care

Light Requirements

Choose a spot that receives full sun throughout the day. Smoketrees thrive in bright, direct sunlight, which promotes healthy growth and blooming.

Soil Requirements

Smoketrees prefer well-drained, sandy or rocky soil. While they are tolerant of poor soil conditions, they do best in slightly acidic to neutral soils (pH 6.0–7.5). The soil should be able to drain quickly to prevent waterlogging, which can cause root rot.

Watering

When first planted, water Smoketree regularly to help establish its roots. However, allow the soil to dry out between waterings. Once established, Smoketree is extremely drought-tolerant. Thanks to its deep root system, it can survive long periods without water. During dry spells, occasional watering will suffice. Be sure not to overwater as it can lead to root rot.

Fertilizing

Smoketrees are adapted to nutrient-poor soils and generally do not require much fertilization. If desired, you can apply a slow-release fertilizer in early spring, but this is typically unnecessary unless your soil is severely lacking in nutrients.

Avoid over-fertilizing, as this can encourage excessive growth at the expense of flowering and overall health.

Pruning

Smoketree does not require heavy pruning but can benefit from light shaping to maintain a tidy form.

Prune after flowering to remove any spent blooms, dead, or damaged wood. If needed, prune any unwanted growth to prevent the tree from becoming too leggy. This is best done in late winter or early spring before new growth begins.

Propagation

Smoketrees can be propagated from seed. To start, collect seeds from mature pods once they have dried.

Scarify the seeds by soaking them in hot water (not boiling) for 12-24 hours before planting to break the hard seed coat and improve germination rates.

Plant the seeds in well-draining soil in early spring or fall, either in shallow pots or directly in the ground. Ensure that the seedlings are protected from harsh conditions until they are established.

Pests and Diseases

Smoketree is generally pest and disease-resistant. However, like many desert plants, it can be susceptible to pests like scale insects, mealybugs, or aphids. Keep an eye out for these pests and treat them promptly if needed, using insecticidal soap or a horticultural oil.

The plant is also resistant to many common tree diseases, but fungal issues like root rot can arise if the tree is overwatered or planted in poorly-draining soil.