This is a tall, glandular-aromatic annual that appears along dry, sun-baked roadsides during the hot months of summer and fall. Although it is often called a weed, it is actually a native species that competes very well with naturalized weeds in disturbed areas.

This is a tall annual with a large, thick taproot. It is native to chaparral areas of San Diego County. During late spring and summer, it appears along roadsides and dry, disturbed slopes. In the hottest and driest months it grows very rapidly into a tall, upright axis with numerous ascending branches, each with solitary or clustered heads at the nodes. The plant in left photograph (black arrow) was photographed in December, 2003 along a dirt road in the Merriam Mountains of northern San Diego County. It was about 8 feet (2.4 m) in height and still had many heads with flowers in bloom.

The heads of this species are ligulate (strap-shaped) and without tubular disk flowers. They are sessile or on short peduncles. The phyllaries are in two distinct sets, the inner (longer) and outer (shorter). The outer, shorter phyllaries are erect and appressed, not reflexed. In addition, the phyllaries are not glandular as in other species. Each head contains up to 12 achenes representing 12 separate flowers. The achenes are longitudinally ribbed with rows of minute tubercles, and with a narrow groove (crack) between the tuberculate ribs. In the Jepson Manual of California Plants, if one interprets the groove as only a valley between the tuberculate ribs, this species would key out to S. virgata ssp. pleurocarpa. The two species are very similar overall, and many specimens of S. diegensis have been misclassified as S. virgata.

Left: Stephanomeria diegensis. The minute one-seeded fruits (achenes) are about 3 mm in length. Each achene has a narrow longitudinal groove (crack) down the center of one or more sides. Right: Achenes from three species of Stephanomeria in southern California. A. S. diegensis. B. S. virgata ssp. pleurocarpa. The achenes are tuberculate-ribbed without a groove between the ribs. C. S. exigua ssp. deanei. The 5-sided (angular) achenes have a distinct longitudinal groove on each side. They also have a persistent pappus base at the apical end.

Left: A seed head composed of 12 achenes, each with a pappus of hairs resembling a parachute. Individual hairs are plumose at their upper ends with numerous branched hairs. Right: Like miniature parachutes the achenes become airborne with the slightest gust of wind. Airborne achenes such as these have enabled species in this remarkable family to cross prominent barriers of land and sea to colonize new habitats. With 12 seeds per head, and many hundreds of heads during its long blooming season, there is a good possibility that some airborne achenes of this annual will land in a suitable spot for the next season's crop.