General description: Mentzelia chrysantha is a biennial or monocarpic perennial. In favorable years, it can complete its lifecycle in two years, but it can persist for several years as a rosette awaiting a favorable year. After it bolts and flowers, the plant dies. The plant has thick, erect, mostly unbranched stems, 2 to 6 dm tall. The leaves are 2 to 15 cm long, elongated (ovate-lanceolate to ovate), and sinuous-dentate (Spackman et al. 1997). The leaves, stems, and fruits have a dense covering of hairs. Mentzelia chrysantha produces numerous bright lemon yellow or golden yellow perfect flowers with 10 petals (Spackman et al. 1997). There are 50 to 80 seeds per capsule (Harrington 1954). The seeds are very narrowly winged, with a papillose seed coat (Weber and Wittmann 2001). The characteristics of the seed coat are generally regarded as being of great taxonomic value in Mentzelia (Hill 1976). For identification, the most up-to-date key available is that of Weber and Wittmann (2001), which includes a couplet that can distinguish Mentzelia chrysantha from M. reverchonii. This key and the descriptive information in Spackman et al. (1997) are the two best tools for diagnosing M. chrysantha in the field (Anderson 2006).

Look Alikes: Weber and Wittmann (2001) distinguish N. reverchonii from N. chrysantha using seed characteristics. Nuttallia reverchonii has broadly winged seeds that have few papillae on the seed coat while the seeds of N. chrysantha have narrow wings and are distinctly papillose. However, these characters are subtle and variable, and make it difficult to identify plants in vegetative or flowering stages. In the rosette stage, N. chrysantha, N. reverchonii, N. decapetala, and N. nuda cannot be reliably distinguished (Anderson 2006). Nuttallia nuda is found with N. chrysantha at some locations. Nuttalia nuda has cream-colored flowers and blooms in June while N. chrysantha has golden yellow flowers and blooms from July to early September (Spackman et al. 1997).

Phenology: The seeds of Nuttalia chrysantha germinate in the early spring, or in late summer during a wet monsoon year. Nuttallia chrysantha is in flower through most of the late summer months, during which it bears numerous flowers in a tall inflorescence (Anderson 2006). Plants are in bloom from July to early September, and they are in fruit from late August into September (Spackman et al. 1997). Flowers open at 5-6 p.m, and close just after dark. Seeds are dispersed in the fall and winter. Dead stalks with dehisced fruits remain erect through the fall and into winter (Anderson 2006).

Mentzelia chrysantha is typically found on barren slopes and road cuts of limestone, shale, or alkaline clay. The habitat of M. chrysantha consists of moderately disturbed, wasting slopes such as those above the Arkansas River. Slopes are usually moderately steep in the shale barrens; no particular aspect is favored. Mentzelia chrysantha occupies slopes and road cuts, where it grows prolifically and is often the only plant species growing in large numbers. Mentzelia chrysantha is found on a variety of geologic formations, mainly marine deposits from the upper (late) Cretaceous period. Mentzelia chrysantha is found primarily on the Smoky Hill member of the Niobrara shale, which is widespread throughout the middle Arkansas Valley, especially in the vicinity of Florence. The Smoky Hill member includes seven subunits that vary greatly in texture and color (may be olive black, yellow-brown, olive gray, pale yellow, or yellow gray). Coarse-scale vegetation types in which Mentzelia chrysantha is found include pinyon juniper-woodland and juniper woodland communities. While a few occurrences have actually been documented in pinyon-juniper woodland vegetation, the most commonly associated species are Frankenia jamesii and Atriplex canescens (Anderson 2006). Mentzelia chrysantha is also often associated with other rare plants such as Parthenium tetraneuris and Mirabilis rotundifolia.

Elevation Range: 4,751 - 6,854 feet (1,448 - 2,089 meters)

Colorado endemic: Yes
Global range: Colorado endemic (Fremont and Pueblo counties).  Estimated range is 1,373 square kilometers (530 square miles), calculated in GIS by drawing a minimum convex polygon around the known occurrences (calculated by the Colorado Natural Heritage Program in 2008).

Residential development is considered to be the primary threat to the species at this time. Other threats include commercial development, mining, recreation, right-of-way management, exotic species invasion, grazing, effects of small population size, climate change, and pollution. Fremont County is among the fastest growing counties in the United States, and low-density development is proceeding rapidly throughout the Arkansas Valley. Many of the known occurrences are located in highway right-of-ways where they are at risk from weed invasion and management (Anderson 2006).