Tarbush (Flourensia cernua) is a native perennial shrub closely associated with the Chihuahuan Desert of the southwestern United States and northern Mexico. Also called American tarwort, the plant belongs to the sunflower family, Asteraceae. Its dense branching, small resinous leaves, drooping yellow flower heads, and distinctive tar-like scent make it one of the recognizable shrubs of Chihuahuan Desert plains and lower slopes.
The shrub is especially characteristic of open desert scrub, alluvial flats, bajadas, mesas, and areas where desert grassland blends into shrubland. It commonly grows with creosotebush and can become locally dominant where soil texture, runoff, and other site conditions suit it.
| Characteristic | Tarbush |
|---|---|
| Scientific Name | Flourensia cernua DC. |
| Plant Family | Asteraceae |
| Plant Type | Perennial woody shrub |
| Main Desert Region | Chihuahuan Desert |
| Native Range | Southwestern United States and Mexico |
| Typical Height | About 0.3–2 m (1–7 ft), depending on site conditions |
| Leaf Form | Small, thick, alternate, elliptic to ovate |
| Flower Color | Yellow |
| Main Flowering Period | Mostly late summer through autumn |
| Typical Habitat | Desert scrub, plains, bajadas, mesas, alluvial flats, and desert grassland margins |
Native Range Across the Chihuahuan Desert
Flourensia cernua is centered on the Chihuahuan Desert and adjacent drylands. Its native range includes Arizona, New Mexico, and Texas in the United States and extends widely into Mexico. Botanical records place it in several northern and north-central Mexican regions, including areas of Chihuahua, Coahuila, Durango, Sonora, San Luis Potosí, and Zacatecas.
This distribution closely follows the broad desert basins, plains, foothills, and plateaus that form much of the Chihuahuan landscape. The species is particularly well represented in southern New Mexico, western Texas, and northern Mexico.
Flora of North America records indicate that tarbush occurs from roughly 800 to more than 2,000 meters (about 2,600–6,600 feet) in elevation. Local conditions matter as much as elevation, however. Soil texture, alkalinity, landscape position, and the movement of runoff water can strongly influence where dense stands develop.
Plains, Bajadas, Mesas, and Alluvial Ground
Tarbush is not limited to one desert landform. It occurs on dry plains, mesas, piedmont slopes, lower bajadas, drainage margins, and alluvial flats below desert mountains. It is also found around the edges of shallow depressions and grass-dominated swales where water occasionally concentrates.
Lower bajadas can provide particularly suitable habitat. Water moving downslope during desert rainstorms may briefly increase soil moisture there without creating permanently wet conditions. Tarbush can make use of these pulses while remaining well adapted to long dry periods between them.
Alkaline and Limestone-Derived Soils
The shrub commonly grows in alkaline, calcareous, clayey, silty, or limestone-derived soils. It can also occur on gravelly surfaces and soils formed from other parent materials.
Studies from Chihuahuan Desert sites frequently record tarbush on moderately to strongly alkaline ground. Its connection with limestone landscapes is especially noticeable in parts of New Mexico and western Texas. This soil preference helps explain why the shrub may be abundant on one desert plain yet much less common on nearby terrain with a different geology.
How Tarbush Looks
Tarbush usually forms a dense, many-branched shrub rather than a single-trunked plant. Numerous woody stems rise obliquely from near the base, producing a rounded or somewhat irregular crown.
Plants growing on exposed, dry sites may remain around 30 cm (1 ft) tall, while individuals receiving extra runoff or growing in deeper favorable soils can exceed 1.8 m (6 ft). Some botanical descriptions record plants approaching 2 m (7 ft).
Small Resinous Leaves and Tarry Scent
The leaves are alternate, simple, thick, and generally elliptic to ovate. Typical blades are roughly 10–25 mm long and 4–15 mm wide, although larger leaves occur. Their margins are usually smooth but may appear slightly wavy.
A useful identification feature is the plant’s resin. Leaves and young growth can feel sticky or resinous, and the plant often gives off a characteristic odor commonly described as tar-like. This feature is reflected in the common name American tarwort.
Leaf retention varies with weather and location. Tarbush commonly loses much of its foliage after freezing temperatures, yet some plants may retain leaves well into winter, especially in relatively sheltered or better-watered positions.
Drooping Yellow Flower Heads
Tarbush belongs to the same botanical family as sunflowers, asters, and many daisies, but its flower heads look quite different from the familiar large daisy form.
The flower heads are small, yellow, and usually nodding or drooping. They contain disk florets but lack the conspicuous ray florets that form the petal-like outer ring of many members of the Asteraceae family. This gives tarbush flowers a compact and understated appearance.
Across much of its range, flowering is concentrated in autumn. Flora records commonly place the main flowering period around September through November, while observations from some parts of Arizona and Texas extend the season from late summer into December.
Fruits and Seed Development
After flowering, tarbush produces small, dry, one-seeded fruits typical of many members of the sunflower family. They are flattened, hairy, and generally only a few millimeters long.
Fruit development continues into the cooler part of the year. Research from the southern Chihuahuan Desert has recorded seed dispersal during the dry season, followed by germination when summer rains return.
A Root System Built Around Desert Rainfall
One of the most useful adaptations for understanding tarbush is hidden below the ground. Mature plants develop an extensive root system that can use water from more than one soil layer.
Much of the root mass lies relatively close to the surface. Research summarized by the U.S. Forest Service found that more than three-quarters of adult root material at studied sites occurred within the upper 40 cm (16 in) of soil. Horizontal roots can extend about 4 meters (13 ft) from the plant.
This broad shallow system allows tarbush to respond to modest rainfall that moistens only the upper soil profile. Such rainfall pulses are common in deserts, where many storms do not soak deeply into the ground.
A smaller number of roots can penetrate much farther downward. Deep roots give established plants access to moisture that remains beyond the reach of the main shallow root network. The combination allows tarbush to use brief surface moisture while maintaining access to deeper reserves during prolonged dry conditions.
Seasonal Growth in a Summer-Rain Desert
The Chihuahuan Desert receives an important share of its precipitation during summer. Tarbush growth and regeneration fit this seasonal pattern.
Warm-season rainfall moistens the upper soil layers and can stimulate new stem and leaf growth. Young seedlings also tend to appear after summer rains. Established shrubs may remain green through much of the warm season before shifting energy toward flowering during late summer and autumn.
- Summer: rainfall supports vegetative growth and seedling emergence.
- Late summer and autumn: flower heads develop across much of the range.
- Winter: fruits mature and many leaves senesce after cold weather.
- Dry season: seeds accumulate near parent plants and may be moved short distances by runoff.
The timing varies across the plant’s large geographic range. Elevation, temperature, summer rainfall, and local soil moisture can shift flowering and leaf loss by weeks or even months.
Tarbush in Chihuahuan Desert Plant Communities
Tarbush is more than an occasional shrub scattered through the Chihuahuan Desert. In suitable terrain it helps define entire vegetation communities.
Jornada Basin research in southern New Mexico recognizes tarbush shrubland as one of the characteristic habitat types of the northern Chihuahuan Desert. At the Jornada landscape, tarbush-dominated vegetation is associated particularly with silty and gravelly soils on lower piedmont slopes, while other soils support black grama grassland, creosotebush shrubland, mesquite duneland, or playa vegetation.
Relationship With Creosotebush
One of the shrub’s most frequent neighbors is creosotebush (Larrea tridentata). The two species may occur together as dominants or co-dominants across broad stretches of Chihuahuan Desert scrub.
Their relative abundance changes along soil and moisture gradients. Creosotebush often becomes more prominent on warmer and drier ground, while tarbush can be more abundant on deeper soils and lower landscape positions that receive additional runoff.
Other associated plants vary by location and can include tobosa grass, black grama, burrograss, mesquite, acacias, saltbushes, and many smaller desert grasses and herbs.
Tarbush and Desert Grassland Change
Tarbush is native to the region, yet it has expanded into some areas that historically supported more grass and fewer shrubs. This distinction matters. Increased shrub cover does not mean the species is foreign to the Chihuahuan Desert.
Long-term studies in the Southwest show that transitions from grassland toward shrub-dominated vegetation can involve several interacting influences, including grazing history, drought, soil properties, erosion, redistribution of water, and changes in the abundance of perennial grasses.
Tarbush often becomes abundant on fine-textured or runoff-receiving sites where conditions favor shrub establishment. Once mature shrubs are established, vegetation patterns can persist for many decades.
Reproduction and Seedling Establishment
Tarbush reproduces primarily from seed. Its flowers are adapted mainly to cross-pollination, and research from the Chihuahuan Desert indicates that only a fraction of the many fruits produced contain viable seed.
Seed dispersal is surprisingly local. Although older descriptions sometimes suggested wind movement, detailed field observations found that most seeds drop close to the parent shrub. In one Chihuahuan Desert study, about 90% of seeds landed beneath the parent plant. Water flowing across the soil during storms can then move some of them farther away.
This helps explain why tarbush often forms patches rather than appearing evenly across a landscape.
Germination is closely linked with moisture. Seeds generally remain dormant through the dry period and become more likely to germinate as the summer rainy season develops. Even under suitable conditions, establishment is slow compared with many short-lived desert plants. Tarbush follows the opposite strategy: seedlings are relatively uncommon, but established shrubs can persist for many years.
Recognizing Tarbush in the Desert
No single feature should be used by itself, but several traits together make mature tarbush relatively distinctive in its native range.
- A dense perennial shrub with many stems arising near the base.
- Usually about 1–2 m tall on well-developed sites, though much shorter plants occur.
- Small, alternate, thick leaves with an elliptic or ovate outline.
- Sticky or resinous foliage.
- A noticeable tar-like scent when leaves or stems are handled.
- Dark, often nearly black-looking older stems.
- Small yellow flower heads that droop rather than forming large daisy-like blooms.
- Frequent occurrence on limestone-derived, alkaline, clayey, or alluvial soils.
- Strong association with Chihuahuan Desert scrub and creosotebush communities.
Common names can create confusion because names such as blackbrush are also applied to unrelated desert plants. Flourensia cernua is therefore the most reliable name when identifying the Chihuahuan Desert tarbush described here.
Why Tarbush Is Closely Tied to the Chihuahuan Desert
Tarbush reflects several features that distinguish the Chihuahuan Desert from simpler images of a desert as uniformly sandy ground. Its distribution is strongly linked with limestone geology, alkaline soils, broad basins, alluvial deposits, piedmont slopes, summer rainfall, and long-lived shrub communities.
Its root system captures shallow moisture from relatively small storms while maintaining access to deeper soil water. Its seasonal cycle follows the region’s summer-rain pattern. Its association with creosotebush, tobosa, black grama, and other dryland plants places it within some of the best-known vegetation mosaics of the Chihuahuan landscape.
For this reason, finding Flourensia cernua across a dry plain or lower bajada is often a botanical clue to the surrounding environment: alkaline desert soils, episodic rainfall, open shrubland, and the ecological conditions characteristic of the Chihuahuan Desert.
Sources
- U.S. Forest Service — Flourensia cernua, American Tarwort (distribution, habitat, root system, reproduction, phenology, and Chihuahuan Desert ecology)
- Texas A&M AgriLife — Tarbush, Blackbrush (plant morphology, roots, flowering period, habitat, and associated vegetation)
- New Mexico State University — New Mexico Range Plants (tarbush description, elevation, limestone habitat, and southern New Mexico distribution)
- Jornada Basin Long Term Ecological Research Program — Site Description (Chihuahuan Desert setting and tarbush shrubland habitat)
- SEINet — Flourensia cernua (Flora of North America morphology, flowering season, elevation, and habitat information)
- Royal Botanic Gardens, Kew — Plants of the World Online: Flourensia cernua (accepted scientific name, taxonomy, and native distribution)