OregonFlora: Poa arida vs. Poa

	Poa arida	Poa
		bluegrass
Habit		Plants annual or perennial; cespitose or culms solitary; with or without rhizomes or stolons; usually with bisexual florets but sometimes monoecious or dioecious; basal branching intra- or extravaginal.
Culms		usually unbranched above the base.
Leaves		sheaths mostly closed to mostly open; ligules membranous, truncate to acuminate; blades flat, folded, or involute; the upper side with 2 longitudinal grooves; tips often prow-shaped.
Inflorescences		panicles, occasionally reduced and raceme-like; disarticulation above the glumes and beneath the florets.
Spikelets		usually laterally compressed, occasionally nearly round in cross section, usually lanceolate, sometimes ovate, 2–17 mm long, with (1)2–6(13) florets, occasionally forming bulblets.
Glumes		2, usually shorter than the lowest lemma of the spikelets, usually keeled, 1–3(5)-veined; awnless.
Caryopses		ellipsoidal, 1–4 mm, often shallowly grooved on the ventral side.
Ovaries		glabrous.
Calluses		blunt, glabrous or with a tuft of cobwebby hairs, or with a crown of straight to slightly sinuous hairs.
Lemmas		usually keeled, sometimes rounded over the back, 5(7–11)-veined; margins scarious-hyaline distally; tips truncate or obtuse to acuminate; awnless.
Functional anthers		(1–2)3.

	Poa arida	Poa
Distribution	[WildflowerSearch map] [BONAP county map]	[BONAP county map]
Discussion		Cosmopolitan. Approximately 500 species; 35 species treated in Flora. Poa is the most diverse grass genus in Oregon. Four species have two or more Oregon subspecies, bringing the total to 42 taxa. Bluegrasses are community dominants or important components of dry to moist grasslands, forests, and alpine zones. They are planted for forage as well as for soil stabilization and lawns. Poa annua, P. bulbosa, and some other Poa species may be economically important weeds in crops. Poa pratensis, P. compressa, and P. bulbosa are significant invaders of native ecosystems. The genus Poa exhibits great diversity in breeding systems, though each Poa species is usually consistent in its method of reproduction. Plants may be synoecious (with bisexual florets), gynomonoecious (with both bisexual and pistillate florets on the same plant), sequentially gynomonoecious (with a higher proportion of pistillate florets as the season progresses), gynodioecious (some plants with only pistillate florets and others with bisexual florets), or dioecious (with separate male and female plants). Self-fertilization is common. Some plants set seed apomictically (asexually). One introduced Oregon species produces bulblets asexually in the inflorescence. Many species reproduce vegetatively through spread of rhizomes or stolons. The prevalence of asexual reproduction and selfing in combination with at least occasional outcrossing produces a complex pattern of variation in Poa that can lead to disagreement about species limits. This Poa treatment follows that of Soreng (2007). The most widespread and abundant bluegrasses in Oregon are introduced P. annua, P. bulbosa, and P. pratensis and native P. secunda. Other relatively common species include introduced Poa compressa, P. palustris, and P. trivialis and native P. cusickii and P. wheeleri. All other species are uncommon or rare in Oregon.
Source		Flora of Oregon, volume 1, page 449 Rob Soreng, Barbara Wilson, Richard Brainerd, Nick Otting
Sibling taxa	P. alpina, P. annua, P. bolanderi, P. bulbosa, P. chambersii, P. compressa, P. confinis, P. cusickii, P. fendleriana, P. glauca, P. howellii, P. iconia, P. infirma, P. laxiflora, P. leibergii, P. leptocoma, P. lettermanii, P. macrantha, P. mansfieldii, P. marcida, P. nemoralis, P. nervosa, P. palustris, P. piperi, P. pratensis, P. pringlei, P. reflexa, P. rhizomata, P. secunda, P. stenantha, P. suksdorfii, P. trivialis, P. unilateralis, P. wallowensis, P. wheeleri
Subordinate taxa		P. alpina, P. annua, P. bolanderi, P. bulbosa, P. chambersii, P. compressa, P. confinis, P. cusickii, P. fendleriana, P. glauca, P. howellii, P. iconia, P. infirma, P. laxiflora, P. leibergii, P. leptocoma, P. lettermanii, P. macrantha, P. mansfieldii, P. marcida, P. nemoralis, P. nervosa, P. palustris, P. piperi, P. pratensis, P. pringlei, P. reflexa, P. rhizomata, P. secunda, P. stenantha, P. suksdorfii, P. trivialis, P. unilateralis, P. wallowensis, P. wheeleri
Web links	Local Web sites: Flora NW, PNW Herbaria WildflowerSearch iNaturalist (observations) USDA Plants Database LBJ Wildflower Center SEINet Plants of the World Online Encyclopedia of Life Wikipedia Google Image Search	Local floras: CA, OR, WA Local Web sites: CalFlora, CalPhotos, Flora NW, PNW Herbaria WildflowerSearch iNaturalist (observations) USDA Plants Database LBJ Wildflower Center SEINet Plants of the World Online Encyclopedia of Life Wikipedia Google Image Search

Poa arida

Poa

bluegrass

Habit

Plants annual or perennial; cespitose or culms solitary; with or without rhizomes or stolons; usually with bisexual florets but sometimes monoecious or dioecious; basal branching intra- or extravaginal.

Culms

usually unbranched above the base.

Leaves

sheaths mostly closed to mostly open;

ligules membranous, truncate to acuminate;

blades flat, folded, or involute; the upper side with 2 longitudinal grooves;

tips often prow-shaped.

Inflorescences

panicles, occasionally reduced and raceme-like;

disarticulation above the glumes and beneath the florets.

Spikelets

usually laterally compressed, occasionally nearly round in cross section, usually lanceolate, sometimes ovate, 2–17 mm long, with (1)2–6(13) florets, occasionally forming bulblets.

Glumes

2, usually shorter than the lowest lemma of the spikelets, usually keeled, 1–3(5)-veined; awnless.

Caryopses

ellipsoidal, 1–4 mm, often shallowly grooved on the ventral side.

Ovaries

glabrous.

Calluses

blunt, glabrous or with a tuft of cobwebby hairs, or with a crown of straight to slightly sinuous hairs.

Lemmas

usually keeled, sometimes rounded over the back, 5(7–11)-veined;

margins scarious-hyaline distally;

tips truncate or obtuse to acuminate; awnless.

Functional anthers

(1–2)3.

Poa arida

Poa

Distribution

[WildflowerSearch map]

[BONAP county map]

Discussion

Cosmopolitan. Approximately 500 species; 35 species treated in Flora.

Poa is the most diverse grass genus in Oregon. Four species have two or more Oregon subspecies, bringing the total to 42 taxa. Bluegrasses are community dominants or important components of dry to moist grasslands, forests, and alpine zones. They are planted for forage as well as for soil stabilization and lawns. Poa annua, P. bulbosa, and some other Poa species may be economically important weeds in crops. Poa pratensis, P. compressa, and P. bulbosa are significant invaders of native ecosystems. The genus Poa exhibits great diversity in breeding systems, though each Poa species is usually consistent in its method of reproduction. Plants may be synoecious (with bisexual florets), gynomonoecious (with both bisexual and pistillate florets on the same plant), sequentially gynomonoecious (with a higher proportion of pistillate florets as the season progresses), gynodioecious (some plants with only pistillate florets and others with bisexual florets), or dioecious (with separate male and female plants). Self-fertilization is common. Some plants set seed apomictically (asexually). One introduced Oregon species produces bulblets asexually in the inflorescence. Many species reproduce vegetatively through spread of rhizomes or stolons. The prevalence of asexual reproduction and selfing in combination with at least occasional outcrossing produces a complex pattern of variation in Poa that can lead to disagreement about species limits. This Poa treatment follows that of Soreng (2007). The most widespread and abundant bluegrasses in Oregon are introduced P. annua, P. bulbosa, and P. pratensis and native P. secunda. Other relatively common species include introduced Poa compressa, P. palustris, and P. trivialis and native P. cusickii and P. wheeleri. All other species are uncommon or rare in Oregon.