Basis for Listing

Oenothera rhombipetala was first reported in Minnesota in 1877 and since then less than 20 records have been reported from dry prairie and savanna habitats in southeastern Minnesota. This relative paucity of records and the scarce and sensitive nature of the species' habitat prompted an investigation into whether or not O. rhombipetala warranted endangered or threatened status. This investigation revealed a considerable amount of uncertainty that has yet to be resolved.

To begin with, O. rhombipetala, was said to range "... from Minnesota and Wisconsin to Texas and Indiana; apparently naturalized farther east" (Munz 1965). However, in 1983, W. L. Wagner concluded that the species, as described by Munz, contained two separate entities. One entity was described as being an "autogamous complex heterozygote with about 50% pollen fertility", which he separated out as a new species he named O. clelandii. The new species was described as occurring from southeastern Minnesota and Michigan south to Iowa, Illinois, and scattered localities in Arkansas, Missouri, Kentucky, New Jersey, and New York (Wagner 1983). The plants that Wagner left in O. rhombipetala were described as "larger-flowered, outcrossing, fully fertile, pair-forming populations from South Dakota south to Texas and central Arkansas" (Wagner 1983). It is unclear from this description if Wagner intended to include plants from Minnesota in this species or if he intended to include all Minnesota plants in the new species, O. clelandii (Cleland's evening primrose).

In a later publication, Wagner (as the junior author with W. Dietrich) makes the statement under the heading of Oenothera rhombipetala: "Fields and prairies from southern South Dakota, Nebraska, Kansas, western Missouri, Oklahoma, Texas, scattered areas in central Arkansas, and Lea County, New Mexico; populations in eastern Minnesota, south-western Wisconsin, Illinois, and Michigan probably represent introductions or perhaps relics" (Dietrich and Wagner 1988). From this description it appears that Wagner does indeed consider O. rhombipetala to occur in Minnesota, but does not know if they are native or introduced.

Regardless of whether the entity that occurs in Minnesota is O. rhombipetala or O. clelandii, or both, the plants are distinctive enough and rare enough to merit special concern status. However, before it is possible to determine if a more protective conservation rank is warranted, several issues need to be resolved including whether both O. rhombipetala and O. clelandii occur in the state and if so, whether both or only one of the species is native. If both are present and both are native, the entity found in Minnesota may be even rarer than previously thought and threatened or endangered status may be appropriate for both species.

  Description

Oenothera rhombipetala is described by Dietrich and Wagner (1988) as an erect biennial 30-100 cm (1.0-3.3 ft.) tall, sometimes with lateral branches arising from the base. The stem is densely to sparsely covered with short, stiff hairs and sometimes glandular hairs in the upper parts. The basal leaves are 6-20 cm (2.4-7.9 in.) long and 0.6-2 cm (0.2-0.8 in.) wide, narrowly oblanceolate in shape with toothed or lobed margins, an acutely pointed tip, and a gradually narrowed base. The stem leaves are similar to the basal leaves. The inflorescence is unbranched, with 2 or more flowers per spike opening each day near sunset. The floral tube is 3-4.5 cm (1.2-1.8 in.) long with short, stiff hairs and sometimes glands. The sepals and petals are 1.5-3.5 cm (0.6-1.4 in.) long, yellowish, and hairy. The ovary is 5-8 mm (0.20-0.31 in.) long and 1-1.5 mm (0.04-0.06 in.) in diameter, becoming 1.3-2.5 cm (0.5-1.0 in.) long and 2.5-3 mm (0.10-0.12 in.) wide in fruit.

Oenothera rhombipetala is reported to differ from O. clelandii, most prominently, by having flowers with larger petals: 1.5-3.5 cm (0.6-1.4 in.) long vs. 0.5-1.7 cm (0.2-0.7 in.) long.

  Habitat

It appears that the natural habitat of O. rhombipetala is dry, sandy prairies and dunes in the east-central and southeast portions of Minnesota. There are some indications that the species can tolerate a certain level of human activity within its habitat. This might include light disturbance to the soil surface caused by vehicles or farm machinery, and possibly light grazing by domestic livestock. It may even find temporary habitat on sandy roadsides and in abandoned fields, although it is unlikely that such habitats can sustain a population of O. rhombipetala for very long.

  Biology / Life History

Oenothera rhombipetala is a biennial that lives just two years. The first year it produces only a basal rosette of leaves and a taproot. The second year it produces an erect flowering stem. The flowers open in the evening and are pollinated by a number of night-active insect species including sphinx moths. The flowers are self-incompatible so cross-pollination is required. The dried seed capsules remain on the stem and continue to release seeds for some period of time after the plant has died at the end of its second year. The seeds posses no specialized structure to aid dispersal although they are small enough to be carried short-distances on a strong wind.

The best time to search for O. rhombipetala is when flowers are present between mid-July and September.

  Conservation / Management

The dry prairie habitats of O. rhombipetala are extremely vulnerable to the invasion of non-native plant species, especially Vicia villosa (hairy vetch), Euphorbia virgata (leafy spurge), Centaurea stoebe (spotted knapweed), Bromus inermis (smooth brome), and Lotus corniculatus (birdsfoot trefoil). The usual vectors for invasion include livestock, farm machinery, transportation corridors, and erosion. Soil disturbance is generally cited as a serious threat to native species in dry prairies, but it is usually the invasion of non-native species that follows disturbance that causes the loss of sensitive species. Several populations of O. rhombipetala from the southwestern metropolitan area have been eliminated or threatened by suburban housing developments situated on sand dunes.

Several programs and resources are available to land managers and landowners to help protect and manage remaining prairie parcels including the Native Prairie Bank Program, the Native Prairie Tax Exemption Program, and a prairie restoration handbook.

  Conservation Efforts in Minnesota

Several of the reported O. rhombipetala populations are from public lands, but no known conservation efforts have specifically targeted the species in those areas.

  References and Additional Information

Dietrich, W., and W. L. Wagner. 1988. Systematics of Oenothera section Oenothera subsection Raimannia and subsection Nutantigemma (Onagraceae). Systematic Botany Monographs 24:1-91.

Munz, P. A. 1965. Onagraceae. Page 108 in C. T. Rogerson, editor. North American Flora, Series II, Part 5. New York Botanical Garden Press, New York, New York.

Wagner, W. L. 1983. New species and combinations in the genus Oenothera (Onagraceae). Annals of the Missouri Botanical Garden 70(1):194-196.