ALL IN THE FAMILY:
THE EVENING PRIMROSES
Onagraceae family
by Richard J. Hull

When thoughts turn to remembrances of warm summer days along a country road or in a quiet meadow, the image of yellow flowers swaying in the breeze often comes to mind. Those yellow flowers are probably Evening-primroses.

The Evening-primrose family, or Onagraceae, is a relatively large (17 genera and 675 species) cosmopolitan plant group that is well represented in North America, especially in the western regions. It exhibits remarkable adaptation, being relatively common along the coast, in inland deserts, and in high mountain meadows. Some species prefer wetlands, while others thrive in exposed, dry locations. Vascular Flora of Rhode Island lists 4 genera and 20 species in the evening-primrose family.

Characteristics
Most evening-primroses are herbaceous, but a few shrub and woody species exist in semitropical areas. They run the gamut from annuals through biennials to perennials, all of which are well represented. Their leaves are mostly simple and narrow with short petioles or none at all (sessile). This family, like many others, is distinguished by its flowers. Their most distinctive feature is an elongated hypanthium (a tubelike fusion of the basal portions of sepals, petals, and stamens) that surrounds the ovary and often extends as a tube well above it. At a variable distance above the inferior ovary, four sepals and petals emerge from the rim of the hypanthium. The sepals often turn downward while the petals extend upward, displaying the white, yellow, red, or pink colors that attract insects and other pollinators. Four to 10 stamens also emerge from the hypanthium rim, the anthers of which produce large triangular-shaped pollen grains that have three distinctive protruding apertures at each point. Several pollen grains are linked together by thin strands of sticky viscin that aid in their dissemination by insects. The single pistil often has an elongated style that raises the normally four-lobed or branched stigma above the flower. The fruit is commonly a four-chambered elongated capsule although, in some genera, berries or nutlets are produced. Evening-primroses are sometimes subdivided into several subfamilies based on their fruit type.

Common Evening-primrose (Oenothera biennis L.) is Rhode Island's most abundant member of this family and the only one listed as ubiquitous. As its name implies, it is a biennial plant that inhabits roadsides, meadows, and most disturbed sites. It self-seeds readily and so can easily be mistaken for a perennial. Like most members of its genus, the flowers of O. biennis are borne in the axils of leaves, but they are concentrated at the top of the plant, forming a loose leafy panicle. Petals are light yellow when they emerge from the bud and become more intensely colored as they age.

Common evening-primrose is included in most weed manuals since it coexists readily with forage crops in pastures and meadows. In gardens, it is a regular visitor to shrub borders and foundation plantings; its 3- to 5-foot height and long flowering season make it a popular back-border plant. The AHS Encyclopedia of Garden Plants lists some 13 species of Oenothera as desirable cultivated plants.

As is characteristic of the family, flowers of many evening-primroses open in early evening and close during late morning, attracting night-flying moths and other nocturnal pollinators. During the day butterflies and even humming-birds can be observed visiting these bright yellow flowers. Since their nectar glands are located near the base of the hypanthium, only pollinators able to reach deeply into the flowers can gain satisfaction from most evening-primroses.

Of the 20 evening-primrose species listed for Rhode Island, 15 are regarded as native; of the remaining 5, only Hairy Willow-herb (Epilobium hirsutum L.) is considered truly naturalized. The others are of uncertain presence or have shown little evidence of spreading from their sites of introduction. Thus, the evening-primroses show few signs of becoming invasive. The Hairy Willow-herb is similar to O. biennis in size and habitat, but it is a hairy plant with opposite leaves and rose-purple flowers containing 4 notched petals and a 4-cleft stigma that forms a small cross in the center of each flower.

Closely related is Fireweed, or great Willow-herb (Epilobium angustifolium L.). It is native to Rhode Island but not at all abundant. It can reach a height of 6 feet and is very prevalent in western states, where it rapidly colonizes wooded sites following a fire. The long and narrow leaves of Fireweed resemble those of the willow. The deep pink flowers are borne in terminal racemes that make a spectacular display when massed. Fireweed petals are not notched and the elongated capsules release numerous seeds with white silky hairs attached (comose) that aid in wind dispersal. The remaining five Epilobium species in Rhode Island are wetland plants that are also not abundant.

Notched petals are most evident in the enchanter's nightshade genus, best represented here by the common form, Circaea lutetiana L. This group deviates from most members of the family in having leaves that are almost as broad as they are long and are arranged oppositely on their stem. Flowers have two recurved sepals and two petals so deeply notched that they appear to be four. Two-petalled flowers are unusual among dicotyledonous plants. The fruit is a bristle-covered nutlet. Even the hypanthium is short in this genus but sufficiently present to place it in the evening-primrose family. The Smaller Enchanter's Nightshade ( Circaea alpina L.) is rare in our state but better distributed in more northern states and Canada. The genus is named for the mythological enchantress, Circe, who presumably used a poisonous member of this plant group in her sorcery.

The Water-primrose or Water-purslane genus (Ludwigia ) is a largely aquatic plant group: most members are emergent but some are submersed. Best represented in Rhode Island by Seedbox (Ludwigia alternifolia L.), which grows in moist locations and is characterized by square, boxlike, capsules containing many seeds. Its flowers have four yellow petals framed by four broad, nonrecurved, green sepals of unequal length. The stigma is an enlarged nonlobed dome in the center of each flower. The other three Ludwigia species native to Rhode Island are relatively rare wetland plants, but they exhibit all the basic characteristics of their family.

Evening-primroses are well represented in Rhode Island, but only one species is really abundant. The remaining plants are worth seeking out when trekking through our state's many beautiful wild places.

Richard Hull, Ph.D., is Professor Emeritus of Plant Science at the University of Rhode Island. He served as Co-President of RIWPS for two years.