Mann's Bluegrass

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Mann's Bluegrass

Poa mannii

ListedNovember 10, 1994
FamilyPoaceae (Grass)
DescriptionA perennial grass with tufted stems and reddish brown grain-like fruit.
HabitatCliffs and rock faces in lowland and montane tropical mesic forest.
ThreatsFeeding by introduced herbivorous mammals, competition with invasive alien plants, erosion of its habitat.


Poa mannii (Mann's bluegrass) is a perennial grass with short underground stems (rhizomes) and erect, tufted and bunched stems (culms) that are 0.02 in (.05 cm) long; a tooth about 0.08-0.2 in. (0.2-0.5 cm) long with a fringed margin occurs at the junction of the leaf blade and sheath. The leaf blade, up to 6 in (15 cm) long and 0.08-0.2 in (0.2-0.5 cm) wide, has a rough upper surface and a hairless lower surface. The branched flower clusters (panicles) are usually less than 2 in (5 cm) long and have primary branches 0.2-0.8 in (0.5-2 cm) long. The 0.2-0.3 in. (0.5-0.7 cm) long, flattened ultimate flower clusters (spikelets) are pale greenish or yellowish brown and usually are comprised of four or five flowers. The small pair of bracts at the base of each spikelet (glumes) are about 0.1 in (0.3 cm) long. The outer bract at the base of a floret (lemma) is 0.1-0.2 in. (0.3-0.5 cm) long and has cobwebby hairs at its base. The inner bract at the base of a floret (palea) is about 0.1 in (0.3 cm) long and has a sharp, longitudinal ridge. The reddish brown grain-like fruit is elliptical to spindle-shaped and about 0.06 in (0.15 cm) long. All three native species of Poa in the Hawaiian Islands are endemic to the island of Kauai. Poa mannii is distinguished from both Poa siphonoglossa and Poa sandvicensis by its fringed ligule and from Poa sandvicensis by its shorter panicle branches.


Mann's bluegrass typically grows on cliffs and rock faces at elevations between 1,510-3,770 ft (460-1,150 m) in lowland and montane mesic forests. Associated species include: Chamaesyce sp. ('akoko), Exocarpos luteolus (heau), Labordia helleri (kamakahala), and Nototrichium sp. in Kalalau Valley; Cyrtandra wawrae (ha'iwale) in Makaha Valley; Acacia koa (koa), Alectryon macrococcus (mahoe), and Anti-desma platyphyllum (hame) in Koaie Valley; and Bidens cosmoides (po'ola nui), Carex meyenii, Dodonaea viscosa ('a'ali'i), and Schiedea amplexicaulis in Waialae Valley.


Mann's bluegrass is found only on the northwestern and west-central portions of the island of Kauai. The four known populations, extending over a distance of about 6.5 by 5.3 mi (10.5 by 8.5 km), occur on state lands at Kalalau, Makaha, Koaie, and Waialae Valleys. The species was formerly found in Olokele Gulch.

Approximately 125 individuals have been observed in these four extant populations.


Mann's bluegrass is threatened by habitat damage caused by the browsing and trampling activities of introduced feral goats, competition with invasive alien plants, erosion and landslides in its steep habitat, fire, and lack of legal protection or difficulty in enforcing laws that become effective with Federal listing. The small population size and limited distribution of this species make it especially vulnerable to reduced reproductive vigor and extinction caused by stochastic events, such as a hurricane.

The area of Kauai in which Mann's bluegrass survives has undergone extreme alteration because of past and present land management practices, including grazing, introductions of alien plant and animal species, water diversion, and recreational development. Feral mammals, particularly goats, have had the greatest overall impact on the ecosystem, altering and degrading the vegetation and habitats. Feral goats have inhabited the drier and more rugged areas of Kauai since the 1820s. They over graze native vegetation, trample roots and seedlings, cause erosion, and promote the invasion of alien plants.

Feral goats on Kauai are managed as a game species with a limited hunting season, but their numbers are still large enough to cause much habitat damage. Mann's bluegrass survives only in steep places that are inaccessible to goats, suggesting that this herbivore has eliminated this endangered plant from more accessible habitats, as has been the case for other rare plants from northwestern Kauai. Some populations of Mann's bluegrass have been damaged by erosion and landslides that were partly initiated by goat activities.

Competition by alien plant species is also a major threat to Mann's bluegrass. The daisy fleabane, brought to Hawaii as a cultivated plant, has become naturalized in wetter regions of four islands. The daisy fleabane (Erigeron karvinskianus ) has invaded Kalalau, Koaie, and Waialae Valleys, three of the four areas where the Mann's bluegrass still occurs. Lantana (Lantana camara ), brought to Hawaii as an ornamental plant, is another aggressive, thicket-forming shrub that can now be found on all of the main islands in mesic forests, dry shrublands, and other disturbed habitats that are not watered. Lantana threatens all known populations of Mann's bluegrass. The prickly Florida blackberry (Rubus argutus ), an aggressive alien species in disturbed mesic to wet forests and subalpine grasslands on four islands, threatens the Kalalau and Waialae Valley populations of Mann's bluegrass.

Fire is considered an immediate threat to the rare plants of the cliff faces and valleys of the Na Pali Coast, where the largest known population of the Mann's bluegrass occurs. Under dry conditions, human-set fires would spread rapidly due to the strong prevailing winds and dry fuel load on cliff ledges. Any plants in the path of these fires, even those on steep cliffs, would be incinerated; dormant seeds could also be destroyed by the intense heat.

The potential for extinction of the Mann's blue-grass caused by stochastic events is greatly increased because the species only has about 125 remaining individuals in four populations.

Conservation and Recovery

All four populations of the Mann's bluegrass are located on State conservation district land, which is managed for multiple-use. The state Division of Forestry and Wildlife has fenced a plant sanctuary in the Kalalau Rim area. To properly conserve the rare plant, all of its critical habitats should be strictly protected. The habitat must also be managed to enhance the rare grass, which requires the elimination of goats and reduction or eradication of alien plants. The populations of the Mann's bluegrass should be monitored, and research undertaken into its biology and habitat needs, including methods of beneficial management. The rare plant should be propagated in captivity, with the aim of providing stock for out-planting to establish new populations in places with suitable habitat. The National Tropical Botanical Garden holds seeds of Mann's bluegrass in storage.


U. S. Fish and Wildlife Service
Pacific Islands Ecoregion
300 Ala Moana Boulevard, Room 3-122
P.O. Box 50088
Honolulu, Hawaii 96850
Telephone: (808)541-3441
Fax: (808) 541-3470

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
Eastside Federal Building
911 N. E. 11th Ave.
Portland, Oregon 97232-4181
Telephone: (503) 231-6121


U.S. Fish and Wildlife Service. 10 November 1994. "Endangered Status for the Plant Poa mannii (Mann's Bluegrass)." Federal Register.