Hooker's evening primrose (Oenothera elata) is a perennial that blooms copiously and is very showy, with large flowers that vary in color from yellow to orange.  As with other "evening" primroses, the flowers close up during mid-day. The flower stalks arch up 3 to 5 feet high from a basal rosette of leaves and flowers bloom in the spring and summer of the plant’s second year.  The leaf base is usually 1 to 3 feet wide. There are two recognized subspecies, ssp. hookeri and ssp. hirsutissima.

Although this plant is typically found in moist areas in the wild, it  is also surprisingly drought tolerant, tough and easy to grow in the garden. Give it full sun or part shade and well-draining soil of any kind. It tolerates virtually any soil.  It develops a stout taproot and woody stems with age but can be pruned radically in winter to control ranginess and recovers quickly. It is a prolific seeder, and gardeners will need to be prepared to pull up numerous seedlings from areas where it is not wanted. 

A number of insects use this plant, particularly the large Sphinx moths. It hosts a likely total of 15 species of butterfly and moth. Hummingbirds are attracted to the flowers, and smaller birds such as Goldfinches are attracted to the seeds in fall.

Hairy Evening Primrose (Oenothera elata ssp. hirsutissima) is a native perennial herb that grows in northern, southern and central California. It tends to grow in moist places, at elevations from 0-9200 feet.

Hooker's Evening Primrose (Oenothera elata ssp. hookeri) is a native perennial herb in the Onagraceae (Evening Primrose) family that grows primarily along the coast between San Francisco and San Diego and northern Baja. It is fast growing and moderately long-lived. It grows in an upright form to a height of 5 feet, with active growth during the summer. Flowers are yellow and striking, and bloom in the late summer. Leaves are medium green and deciduous. Its form may be either upright or sprawling. It tends to grow in moist places, at elevations from sea level to 7,000 feet where it is a pioneer species. However, it is also somewhat drought-tolerant in garden situations. In the wild it is usually crowded out over time by more woody trees or shrubs. It spreads by prolific seed production, which also makes it attractive to many seed-eating birds.

Oenothera longissima is a species of flowering plant in the evening primrose family known by the common name longstem evening primrose. It is native to the southwestern United States, where it grows in relatively moist areas in scrub and woodland habitat. This is a biennial herb producing a tall, erect stem known to well exceed 2 meters in height. The widely lance-shaped leaves may be 22 centimeters in length and may have slight teeth along the edges. The inflorescence is a spike of showy large flowers. Each has yellow petals which may be more than 6 centimeters in length and fade orange or red with age. The fruit is a lance-shaped capsule up to 5. 5 centimeters long.

Oenothera primiveris is a species of flowering plant in the evening primrose family known by the common name desert evening primrose. It is native to the southwestern United States and northern Mexico, where it grows in many types of desert habitat. This is a hairy annual herb producing a dense rosette of leaves in which the inflorescence occurs; there is generally no true stem. The green or grayish leaves are up to 28 centimeters long and have wavy or crinkled edges, or are cut into deep lobes or teeth. Flowers arise from the axils of the leaves. Each flower has yellow petals up to 4 centimeters in length which fade orange or red with age. The fruit is a straight or curving capsule up to 6 centimeters in length.

Oenothera suffrutescens

Oenothera wolfii is a rare species of flowering plant in the evening primrose family known by the common name Wolf's evening primrose. It is native to the coastline of southern Oregon and northern California, where it grows in coastal prairie, dunes, and coastal forest and woodland habitat. As of 1997 it was known from only about 16 occurrences. The biggest threat to the plant is its easy hybridization with its relative and probable descendent, Oenothera glazioviana. As this rare wild plant crosses with the introduced garden escapee, introgression occurs, causing what is known as genetic pollution; fewer pure individuals of O. wolfii will be seen as they are outnumbered by hybrids. O. wolfii is a hairy biennial herb producing a dense rosette of leaves and an erect stem up to a meter tall. The wavy or toothed leaves are up to 18 centimeters long. In its second year the plant produces an inflorescence, a spike of showy yellow flowers with petals one or two centimeters long. The fruit is a capsule which may be almost 5 centimeters in length.

Oenothera xylocarpa is a species of flowering plant in the evening primrose family known by the common name woodyfruit evening primrose. It is native to the Sierra Nevada of California, its range extending just into western Nevada. It grows in coniferous forest and meadow habitat, often in soils rich in pumice and other gravel. It is a perennial herb growing from a thick taproot and producing a flat, dense rosette of hairy, gray-green leaves. There is no stem. The showy flowers appear amidst the leaves. Each has four petals which may be nearly 4 centimeters long, bright yellow in color, fading pink to red with age. The fruit is a straight, curving, or twisting capsule which may be up to 9 centimeters long.

Oligomeris linifolia is a species of flowering plant in the small family Resedaceae known by the common name lineleaf whitepuff. It is native to parts of the Middle East and the Indian subcontinent, as well as southern Europe and North Africa, and the southwestern United States and northern Mexico. It grows in many types of habitat, including disturbed areas and saline soils, in deserts, plains, coastline, and other places. It is a fleshy annual plant, producing several erect, ribbed stems 35 to 45 centimeters in maximum height. The linear or widely lance-shaped leaves are narrow, up to 4. 5 centimeters long but only a few millimeters side. The flowers occur in long, erect spikes. Each flower is a rounded, lobed, pocket-like structure with 4 curving sepals and usually two whitish petals which may be partially fused together. The fruit is a capsule a few millimeters long containing tiny shiny black seeds.

Olneya tesota is a perennial flowering tree of the Fabaceae family, legumes (peas, beans, etc), which is commonly known as Ironwood or Desert Ironwood. It is the only species in the monotypic genus Olneya. This tree is part of the western Sonoran Desert complex in the Southwestern United States, which includes flora such as palo verde, saguaro, ocotillo, brittlebush, creosote bush, and mesquite. The Desert Ironwood grows as a bush or tree and reaches heights of about 10 meters (33 feet), and average trunk diameters of about 60 centimeter (24 inches); in exceptional sites in larger protected washes, greater height and more massive.

Olsynium douglasii (syn. Sisyrinchium douglasii, Sisyrinchium grandiflorum) is a flowering plant in the genus Olsynium, native to western North America from southern British Columbia south to northern California, and east to northwest Utah. It is the only species in the genus Olsynium in North America, the remaining 11 species being from South America. It was formerly treated in the related genus Sisyrinchium. It is a perennial herbaceous bulbiferous plant growing to 10-40 centimeter tall. The leaves are slender linear, 10-30 centimeter long and 1.5-3 millimeter broad. The flowers are bell-shaped, 15-25 millimeter long, with six purple petals.

Purple Eyed Grass (Sisyrinchium douglasii var. douglasii) is a native perennial herb that grows in central and northern California. It tends to grow in spring-moist, open places, at elevations from 3000-6000 feet.

Ophioglossum californicum, known by the common name California adder's tongue, is an uncommon species of fern in the family Ophioglossaceae. The fern is native to California, and Baja California in Northwestern Mexico. It is found in the San Joaquin Valley, Sierra Nevada foothills, and along the central and southern coast regions. Habitats include moist areas, such as wet pastures and vernal pools, coastal grasslands, and coastal/montane/interior chaparral micro-habitats. It becomes very rare in dry years. Ophioglossum californicum is a small, fleshy perennial plant growing from a caudex no more than 1. 5 centimeters wide. It produces one leaf per year. The leaf is divided into a thick, green blade-shaped part, which is sterile, and a fertile stalk lined with two rows of sporangia, the reproductive parts.

Ophioglossum pusillum is a species of fernlike plant in the family Ophioglossaceae known by the common name northern adder's tongue. It is native to northern North America, where it is widespread in moist areas such as marshes, fens, and meadows. It is found from northern California through Alaska on the west, and from central Appalachia through the northern Great Plains and the Great Lakes regions, across the Northeastern United States and Eastern Canada. Ophioglossum pusillum is a small, inconspicuous, fleshy perennial plant growing from a caudex no more than 3 centimeters wide. It produces one leaf per year. The leaf is divided into a thin, pale green blade-shaped part, which is sterile, and a fertile stalk lined with two rows of sporangia.

Opuntia hybrid.

Named for its broad, flat leaf pads, Beavertail Pricklypear (Opuntia basilaris) is a medium sized cactus that's known for its vibrant magenta flowers. While the pads do not have the long spines of other cactus species, they do have small barbs that can be painful when touched. Beavertail Pricklypear blooms profusely in spring and early summer. The bright flowers attract pollinators and provide visual interest in a xeric landscape.

This succulent is native to the Southwestern desert regions of the US. It also grows in Mexico where it is called Nopal. Indigenous peoples consumed the fruit, leaf pad buds, and seeds. Beavertail Pricklypear requires minimal supplemental irrigation. Plant in full sun in sandy, well-draining soil. This cactus grows in clusters that can spread up to 6 ft wide. The pads can shrivel in the summer but will plump up during the winter rainy season. Dead pads can be removed, otherwise little maintenance is needed.

Beavertail Pricklypear (Opuntia basilaris var. basilaris) is a native shrub that grows in southern and central California. It tends to grow in desert areas, at elevations from 500-7200 feet.

Bakersfield cactus (Opuntia basilaris var. treleasei) is a succulent in the Opuntia or prickly-pear cactus family. It has beavertail-like pads and produces large, showy magenta flowers.

Bakersfield cactus differs from other varieties of beavertail cactus in that it has rigid spines.  The eye-spots on leaf pads contain spines in addition to bristles. Though some species of prickly-pear cacti have fleshy fruits, the fruit of Bakersfield cactus is dry. 

It is native to Kern County in the southern San Joaquin Valley but is now listed as federally and state endangered because its habitat continues to be lost to agricultural and urban uses. It occurs on sandy soil on floodplains, ridges, bluffs and rolling hills in saltbush scrub plant communities, and occasionally in blue oak woodland or riparian woodland at elevations from 460 to 1,800 feet. 

This plant may work well in a rock garden or container. A single plant may, in time, consist of hundreds of pads, which originate both at ground level and from the tips of other pads.

Opuntia chlorotica is a species of plant in the Cactaceae family. It is a species of prickly pear native to the southwestern United States and northern Mexico. Its common names include pancake prickly pear, flapjack prickly pear and dollarjoint prickly pear. This cactus is bluish-green and manifests a bushy to erect form that may attain a height of two meters or more. Some analysis of the prehistoric range of O. chlorotica has been conducted by studies in the Waterman Mountains of Arizona; these pollen core analyses indicate that this cactus species was present in northern Arizona in late Wisconsinan glaciation period.

Opuntia engelmannii is a prickly pear common across the southwestern United States and northern Mexico. It goes by a variety of common names, including "cow's tongue cactus", "cow tongue prickly pear", "desert prickly pear", "discus prickly pear", "Engelmann's prickly pear", and "Texas prickly pear" in the US, and "nopal", "abrojo", "joconostle", and "vela de coyote" in Mexico. Its overall form is generally shrubby, with dense clumps up to 3.5 meter high, usually with no apparent trunk. The pads are green (rarely blue-green), obovate to round, about 15-30 centimeter long and 12-20 centimeter wide. The glochids are yellow initially, then brown with age. Spines are extremely variable, with anywhere from 1-8 per areole, and often absent from lower areoles; they are yellow to white, slightly flattened, and 1-6 centimeter long. The flowers are yellow, occasionally reddish, 5-8 centimeter in diameter and about as long. The purple fleshy fruits are 3-7 centimeter long.

Opuntia fragilis is a prickly pear cactus known by the common names brittle prickly pear and little prickly pear native to much of North America. It occurs in several Canadian provinces. It is known from farther north than any other cactus, occurring near the Arctic Circle in Alberta. There is an isolated and possibly genetically unique population in Eastern Ontario known as the "Kaladar Cactus". Brittle Prickly Pear is a small decumbent cactus that grows to a maximum height of 10 centimeters. Both the common and scientific names refer to the easily detached stem segments. This is known to be a means of plant dispersal. Opuntia fragilis, Haw. A small, prostrate plant, rarely more than 4 inches high: joints tumid, fragile, easily detached, oval, elliptical, or subglobose, 1-2 inches long and nearly as thick as broad, bright green: areoles - inches apart, with whitish wool and a few white to yellow bristles, which are much longer and more abundant on older joints; spines 1-4, occasionally a few small additional ones, weak, dark brown, the upper one usually longer and stronger than the others, rarely an inch in length: flowers greenish yellow, 1-1 inches wide: fruit ovate to subglobose. with few spines or bristles, mostly sterile, an inch or less long; seeds few and large. Rocky Mountain region from Canada to New Mexico.

A species of prickly pear cactus known by the common name coastal prickly pear. Commonly confused with Opuntia semispinosa, which has longer, sometimes spineless pads. It is native to southern California and Baja California, where it grows in coastal sage scrub and chaparral habitat. It generally occurs in dense clumps spreading several meters wide and up to a meter tall. It seldom hybridizes with Opuntia oricola. The branches are made up of orbicular flat segments up to 22 centimeters long. It is covered in clusters of yellowish spines 2 to 4 centimeters long. The flowers are yellow. The fruit is purplish red, up to 5 centimeters long, and less spiny than Opuntia oricola fruit. An important wildlife plant.

Opuntia oricola is a species of prickly pear cactus known by the common name chaparral pricklypear. It is native to southern California and Baja California, where it grows in coastal sage scrub and chaparral habitats. Opuntia oricola is a large treelike cactus often exceeding 2 meters in height. The branches are made up of rounded flat segments up to 25 centimeters long. It is covered in clusters of curved, yellowish spines usually around 2 centimeters long. Opuntia oricola is a large treelike cactus often exceeding 2 meters in height. The branches are made up of rounded flat segments up to 25 centimeters long. It is covered in clusters of curved, yellowish spines usually around 2 centimeters long. The flowers are yellow, sometimes orange-tinged. The spherical fruit is purplish red on the outside, whitish and juicy inside, and up to 6 centimeters long.

Opuntia phaeacantha is a species of prickly pear cactus known by the common names tulip prickly pear and desert prickly pear. It is found across the southwestern United States and northern Mexico. It has a characteristic prickly pear appearance, with flattened green pads bearing wool and spines. The spines are brown, reddish-brown, or gray, and often over 3 centimeters in length. The flowers are usually bright yellow with reddish centers, and sometimes peach, pinkish or reddish overall. The edible fruits are red or purple with green flesh. The pads and fruit bear tiny glochids which can lodge in the skin and cause irritation. The plant forms dense but localized thickets. There are several varieties of this particular species, and it also hybridizes easily with other prickly pears, making identification sometimes tricky.

Opuntia polyacantha is a common species of cactus known by the common names plains pricklypear, hairspine cactus, panhandle pricklypear, and starvation pricklypear. It is native to North America, where it is widespread in Western Canada, the Great Plains, the central and Western United States, and Chihuahua in northern Mexico. This cactus grows in a wide variety of habitat types, including sagebrush, Ponderosa pine forest, prairie, savanna, shrublands, shrubsteppe, chaparral, pinyon-juniper woodland, and scrub. Opuntia polyacantha grows up to 40 centimetres (16 in) tall. It forms low mats of pads which may be 2-3 metres (6. 6-9. 8 ft) wide. Its succulent green pads are oval or circular and reach 27 by 18 centimetres (10. 6 by 7. 1 in) wide. Its areoles are tipped with woolly brown fibers and glochids. Many of the areoles have spines which are quite variable in size and shape. They may be 0. 4 to 18. 5 centimetres (0. 16 to 7. 28 in) in length, stout or thin, straight or curling, and any of a variety of colors. The flowers are 2. 5 to 4 centimetres (0. 98 to 1. 57 in) long and may be yellow or magenta in color. The fruit is cylindrical, brownish, dry and spiny. The cactus reproduces by seed, by layering, and by resprouting from detached segments. Uses. Native Americans used it as a medicinal plant, with different parts treating various symptoms. This pricklypear provides food for many types of animals. It provides over half the winter food for the black-tailed prairie dog in one area. Pronghorn deer eat it, especially after the spines are burned off in wildfires. Ranchers intentionally burn stands of the plant to make it palatable for livestock when little other food is available. It will also grow in waste areas where good forage will not take hold. In fact, an abundance of the cactus indicates land that is poor in quality. Several insects attack the cactus, including the cactus moth Melitara dentata, the blue cactus borer Olycella subumbrella, and the cactus bug Chelinidea vittiger.

Orcuttia californica is a rare species of grass known by the common name California Orcutt grass. It is native to southern California and northern Baja California, where it grows in scattered locations in vernal pool habitat. In 1993 it was known from fewer than 20 occurrences, including those at the Santa Rosa Plateau, a creek drainage near Hemet, Otay Mesa in San Diego County, and one spot in Woodland Hills. Orcuttia californica is a federally listed endangered species, and its existence is still threatened by the disappearance of vernal pools in the region, a naturally rare habitat type that has been reduced further by urban development. Orcuttia californica is a small, hairy annual grass with prostrate stems sometimes forming small tufts or mats, rarely exceeding 15 centimeters tall. It is bright green, aromatic, and glandular, secreting sticky, bitter-tasting exudate. The leaves are 1 or 2 centimeters long, the first set produced when the pool is wet, another set growing during the dry season. The inflorescence is up to 6 centimeters long with spikelets of pinkish flowers.

Orcuttia inaequalis is a rare species of grass known by the common name San Joaquin Valley Orcutt grass. It is endemic to the Central Valley of California, where it grows only in vernal pools, a rare and declining type of habitat. Many known occurrences of the plant have been extirpated as land in the heavily agricultural Central Valley has been altered, and it was federally listed as a threatened species in 1997. Orcuttia inaequalis is a small, hairy, gray-green annual bunchgrass forming tufts or mats up to about 15 centimeters tall. The fluffy, clustered inflorescence is a dense, headlike mass of spikelets, the characteristic that separates this Orcutt grass from the others, which have more spreading inflorescences.

Orcuttia pilosa is a rare species of grass known by the common name hairy Orcutt grass. It is endemic to the Central Valley of California, where it grows only in vernal pools, a rare and declining type of habitat. Many known occurrences of the plant have been extirpated as land in the heavily agricultural Central Valley has been altered, and it was federally listed as an endangered species in 1997. In that year there were 25 known remaining populations of the grass, only 12 of which were considered stable. Orcuttia pilosa is a small, densely hairy annual bunchgrass forming tufts up to about 20 centimeters tall when growing erect. The inflorescence is a crowded cluster of overlapping hairy spikelets.

Orcuttia tenuis (slender Orcutt grass) is a species of grass which is endemic to northern California. It grows in vernal pool habitat in the western and northern foothills surrounding the Sacramento Valley.

Orcuttia viscida is a rare species of grass known by the common name Sacramento Orcutt grass. It is endemic to Sacramento County, California, where it grows only in vernal pools, a rare and declining type of habitat. As of 1997, two of the nine known populations had been extirpated as habitat has been consumed for urban development, and it was federally listed as an endangered species. Since its listing, one additional occurrence of the plant has been discovered, for a total of eight extant populations. Orcuttia viscida is a small, hairy, aromatic annual grass forming sticky, glandular tufts up to 10 or 15 centimeters in maximum height. The inflorescence is a small, crowded cluster of spikelets with awned tips that curve outward at maturity, giving the spikes a bristly appearance.

Oreocarya confertiflora is a species of wildflower in the borage family known by the common names basin yellow catseye and Mojave popcorn flower. This is a common desert plant native to the southwestern United States. It is an erect perennial herb approaching half a meter in height. The stems grow from a woody caudex and form a rough clump of hairy, bristly gray-green leaves in dry, rocky areas. Out of the clump grow erect stems topped with dense inflorescences of hairy mustard-yellow flowers. Each flower is tubular with sepals wrapped around the tube below a flat-faced or curled-back corolla of five lobes. The fruit is a nutlet 3 to 4 millimeters wide.

Oreocarya crymophila is a perennial plant in the Borage Family (Boraginaceae). It is commonly called Subalpine Cryptantha.

Oreocarya flavoculata is a species of flowering plant in the borage family known by the common name roughseed cryptantha. It is native to the western United States from California to Montana, where it is common in many types of habitat. It is a perennial herb growing an unbranching stem up to about 35 centimeters tall from a woody caudex. It is coated in soft bristly hairs. The densely hairy to bristly leaves vary in shape and may reach 11 centimeters long. The inflorescence is a cylindrical cluster or rounded head of flowers which elongates as the fruits develop from the bottom up. Each tubular flower is about a centimeter long topped with a five-lobed white corolla with yellow appendages at the center.

Oreocarya humilis ssp. humilis is a perennial plant in the Borage Family (Boraginaceae). It is commonly called Low Cryptantha.

Sierra Cryptantha (Oreocarya nubigena) is a common native perennial herb that grows in northern, southern and central California. It tends to grow in volcanic gravel, at elevations from 8000-12000 feet.

Oreocarya roosiorum is a species of flowering plant in the borage family known by the common name bristlecone cryptantha. It is endemic to Inyo County, California, where it is known from only a few occurrences in the northern Inyo Mountains.

Oreocarya virginensis is a species of wildflower in the borage family known by the common name Virgin River cryptantha. This is a small plant native to the southwestern United States (Arizona, California, Nevada and Utah) where it is a common plant in scrub and woodland. It is named for the Virgin River, a tributary of the Colorado River which runs through the region. This cryptantha is an annual or occasionally a perennial up to 40 centimeters in height. It is coated densely in long white hairs and bristles. The inflorescence is cylindrical or club-shaped, packed with tubular flowers with flat-faced corollas. The flower is usually bright white with yellow throat parts at the tube opening. The fruit is a rough, ridged nutlet.

Oreonana clementis, the pygmy mountainparsley, is a perennial plant in the carrot family (Apiaceae) that grows in the United States Sierra Nevada mountain range. :18, 115It can be found in the Sierra Nevada from 5,500 to 13,000 feet (1,700 to 4,000 m), in dry, granitic gravels. :18, 115A tuft of 1 inch (2. 5 cm), gray, hairy leaves are attached directly to the root, with no stems. :18, 115Balls of whitish flower clusters on short stems occur from May to August. :18, 115

Oreostemma alpigenum (formerly Aster alpigenus) is a perennial plant to subshrub in the Sunflower Family (Asteraceae) of the mountains of western United States, commonly known as tundra aster or Anderson's mountain crown.

Alpine Aster (Oreostemma alpigenum var. andersonii) is a perennial in the Asteracae or Aster family.  As the name “alpine” suggests, its purple daisy-like flowers star the ground at high elevations, blooming June to September.  It grows in meadows, lake edges, marshes, pine woods and alpine tundra at an elevation of 3,000 to 9,000 feet, primarily in the Sierra Nevada within California (where it is common) and also in Southern Oregon and Nevada.

This plant looks good among grasses in a garden version of a mountain meadow. It will need summer water.