It can grow 5–60 cm tall, with alternately branching glaucous blue-green foliage. The leaves are ternately divided into round, lobed segments. The flowers are solitary on long stems, silky-textured, with four petals, each petal 2-6 cm long and broad; their color ranges from yellow to orange, and flowering is from February to September. The fruit is a slender dehiscent capsule 3-9 cm long, which splits in two to release the numerous small black or dark brown seeds. It is perennial in mild parts of its native range, and annual in colder climates; growth is best in full sun and sandy, well-drained, poor soil.
It grows well in disturbed areas and often recolonizes after fires. In addition to being planted for horticulture, revegetation, and highway beautification, it often colonizes along roadsides and other disturbed areas. It is drought-tolerant, self-seeding, and easy to grow in gardens. It is also pictured in welcome signs while entering California.
The California poppy is the California state flower. It was selected as the state flower by the California State Floral Society in December 1890, winning out over the Mariposa lily (genus Calochortus) and the Matilija poppy (Romneya coulteri) by a landslide, but the state legislature did not make the selection official until 1903. Its golden blooms were deemed a fitting symbol for the Golden State. April 6 of each year is designated "California Poppy Day."
A common misconception associated with the plant, because of its status as a state flower, is that the cutting or damaging of the California poppy is illegal. There is no such law in California, outside of state law that makes it a misdemeanor to cut or remove any plant growing on state or county highways or public lands except by authorized government employees and contractors; it is also against the law to remove plants on private property without the permission of the owner (Cal. Penal Code Section 384a).
Extract from the California poppy acts as a mild sedative when smoked. The effect is far milder than that of opium, which contains a different class of alkaloids.
The Antelope Valley California Poppy Reserve is located in northern Los Angeles County, California. At the peak of the blooming season, orange petals seem to cover all 1,745 acres (7 km²) of the reserve.
Interestingly, the introduced Chilean populations of California poppy appear to be larger and more fecund in their introduced range than in their native range (Leger and Rice, 2003). Introduced populations have been noted to be larger and more reproductively successful than native ones (Elton, 1958), and there has been much speculation as to why. Increase in resource availability, decreased competition, and release from enemy pressure have all been proposed as explanations.
One hypothesis is that the resources devoted in the native range to a defense strategy, can in the absence of enemies be devoted to increased growth and reproduction (the EICA Hypothesis, Blossey & Nötzold, 1995). However, this is not the case with introduced populations of E. californica in Chile: the Chilean populations were actually more resistant to Californian caterpillars than the native populations (Leger and Forister, 2005).
Within the USA, it is also recognized as a potentially invasive species, being classified in Tennessee as a Rank 3 (Lesser Threat) species, i.e. an exotic plant species that spreads in or near disturbed areas, and is not presently considered a threat to native plant communities (Tennessee Exotic Pest Plant Council). Also, no indications of ill effects have been reported for this plant where it has been introduced outside of California. Ironically, it has been displaced in large areas of its original habitat, such as Southern California, by more invasive exotic species, such as mustard or annual grasses.
It is not known whether efforts are being undertaken anywhere in its introduced range to control or prevent further spread, nor what methods would be best suited to do so.