Otay Tarplant [Hemizonia conjugens Keck]

Otay Tarplant [Hemizonia conjugens Keck]

Listing CNPS List 1B R-E-D Code 3-3-2

State/Federal. Status -- CE/C2 ASTERACEAE May-Jun.

Global Rank G1 State Rank S1.1

Distribution: Southern San Diego County; Baja California, Mexico

Habitat: Fractured clay soils in grasslands or lightly vegetated Diegan Sage Scrub are the preferred habitat of the Otay Tarplant. Most of the sites near Sweetwater Reservoir are mapped as Diablo clay. Usually there is little competition from woody shrubs where this annual grows.

Known Sites: Most U.S. sites for this state endangered plant occur in the Chula Vista region. The population along the west side of Otay Lake Road, south of Bonita Road, and a second fragmented population in Poggi Canyon are both substantial sites. More than ten thousand plants occurred at each location in the late 1980s. The Otay Lakes Road site was heavily impacted by a "tract home" entrance road in 1989, cutting right through the population. In Poggi Canyon the population will be difficult to protect from nearby development even if placed into dedicated open space owing to its open grassland habitat. A small population just north of Otay Valley Road and west of Rock Mountain is potentially threatened by a planned road widening; another small population occurs on the lower north-facing slopes of Otay Valley, south of the river and Maxwell Road. During spring 1990 a number of new sites were found. Extensive populations were seen scattered on slopes east of Horseshoe Bend near Proctor Valley Road. A number of other small but dense colonies noted along Proctor Valley Road included a site around an isolated utilities structure at Gobbler's Knob, a second locale west of Upper Otay Reservoir in the northeastern quarter of Section 26, and near Elevator Road south of Dictionary Hill. Also, new reports are from the southeastern flank of San Miguel Mountain. An older biological survey report where possibly no longer extant is from the southern terminus of El Rancho Grande Road in Bonita. Data Base reports are all from locales near the above sites, from between Otay Lakes Road and Corral Canyon Road in the Long Canyon drainage, east of Otay Mesa on the western edge of Otay Mountain about 5 miles east southeast of Brown Field, east of Dennery Canyon on Otay Mesa, west of the junction of Siempre Vista Road and La Media Road on Otay Mesa, just east of the Chester Grade on northern Otay Mesa, both in Wolf Canyon and near Rock Mountain near Otay Valley, 0.7 mile south of the Lower Otay Reservoir, the southern end of Salt Creek 0.5 mile west of Lower Otay Reservoir, 0.5 mile north and slightly west of Upper Otay Reservoir, and Wildman's Canyon near Mother Miguel Mountain.

Two collections for Baja California are found at the herbarium for the San Diego Natural History Museum; south to 32 26' North where collected by Moran (SD 97795) on a barren north slope 3 km south of La Presa. It has also been collected on the south side of Guadalupe Valley.

Status: Otay Tarplant is substantially declining; most sites are endangered by residential development. It is strongly recommended the State of California take stronger action to protect this species from urban pressures. This species should be given strong consideration for Federal Endangered status; the current status as State Endangered has not adequately protected this species. Sympatric presence of the closely related Hemizonia paniculata within the very limited range of Hemizonia conjugens is considered questionable despite old reports which place the latter at nearby locales (Paradise Valley, 2 miles east of San Ysidro, Spring Valley, and Telegraph Canyon). More taxonomic work is needed. H. paniculata is abundant in western Riverside County where it grows in various soil types in xeric sage scrub; it is not restricted to clays. It is uncommon in northern San Diego County south to near Barham Road in San Marcos. H. conjugens is restricted to cracking clay soils generally devoid of woody shrubs. It often grows interdigitated but not sympatrically with Hemizonia fasciculata, the common tarweed of the region, at locales where "fingers" of clay intrude into loams. Taking into account that population numbers at single sites may be very high even within relatively confined areas, it is recommended that substantial portions of all populations of Otay Tarplant should be protected and placed into dedicated biological open space.


Copyright May 1994 Craig H. Reiser.

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