Tharp's Bluestar

Tharp’s bluestar is adapted to the seasonal temperature ranges and precipitation patterns of the northern Chihuahuan Desert, featuring hot, long summers and mild winters with brief periods when temperatures may fall below freezing. Precipitation peaks from June through September, during monsoon season. The plant grows on hills and escarpments with well-drained, calcium-rich, loam or sandy loam soils derived from gypsum, limestone, and/or dolomite, characterized by sparse vegetation and full sun.

Marginal habitat for Tharp’s bluestar occurs near suitable habitats and includes cliffs, mineral drainages, and loamy bottomlands (a short distance into the alluvial deposits at the base of slopes) as well as terraces, ridges, and hilltops (a short distance into upland vegetation).

Tharp’s bluestar is a suffrutescent herb, meaning it has a woody base and herbaceous stems that die back each year. Clusters of stems grow from its woody rootstocks and are assumed to be individual plants. However, a single rootstock may give rise to several clumps, so “individuals” aren’t always genetically unique plants. Genetically unique plants cannot be determined without harming existing plants.

Tharp’s bluestar grows up to about 24 inches tall and has multiple green to purple stems that grow from a woody rootstock. Its leaves grow alternately along the sometimes-branching stems. Although its leaf blades are reported to be “distinctly dimorphic” or have different leaf forms, the varying leaf blades could also be described as gradually reduced as they ascend the stems. The plant’s leaves are smooth, with few hairs, and have pointed tips.

Tharp’s bluestar has multiple, pale-blue or greenish-white star-shaped flowers that are about half an inch long. 

Tharp’s bluestar is a root-sprouting, long-lived perennial. Its growing season varies, influenced by drought conditions and freezing temperatures. In March, plants start to emerge from dormancy. However, spring droughts may delay emergence, and plants may not sprout until the onset of monsoon precipitation in late summer. Tharp’s bluestar typically blooms from April to May, but this species may not flower at all during and following spring drought, or it may bloom during monsoon season. Seeds from spring flowers ripen in July, and seeds from monsoon-season flowers ripen in October. Plants typically go dormant for the winter following the onset of freezing temperatures. However, plants may go dormant earlier, during their growing season, in response to drought

Tharp’s bluestar’s lifespan has not been established. It is considered long-lived and, based on its natural history and habitat, may live up to about 90 years. Plants grown from seed in the early 1990s are now at least 30 years old.

Tharp’s bluestar reproduces by seed. Its flowers are self-incompatible, so seed production requires pollinators. Its pollinators are unknown, but native beetles, bees, hawkmoths, and hummingbirds have been observed visiting Tharp’s bluestar flowers. Plants may, alternately, invest their resources into producing additional clumps of stems from existing roots. Both seed and clump production increase with increased precipitation and decrease with greater drought severity. While Tharp’s bluestar plants can be multiplied by root division in horticulture, clonal reproduction by root division in the wild is unlikely. Therefore, while clump production results on more “individuals,” it does not necessarily equate to true clonal reproduction.

Tharp’s bluestar looks similar to Fugate’s bluestar (Amsonia fugatei). Although these plants are closely related, they are genetically distinct species. These species can only reliably be distinguished by their location. These species are separated by multiple mountain ranges and by over 155 miles (250 kilometers). Tharp’s bluestar grows at a lower elevation than Fugate’s bluestar, on the east side of central New Mexico’s mountain ranges, whereas Fugate’s bluestar grows on the west side.