100 MISC. PUBLICATION 200, U. S. DEPT. OF AGRICULTURE 



in open or contracted panicles, the relatively narrow blades flat, folded, or 

 involute, ending in a boat-shaped tip. Standard species, Poa pratensis. Name 

 from Greek, poa, grass. 



There are several groups of Poa that present many taxonomic difficulties. 

 In the groups containing, for example, P. nervosa, P. arctica, P. scabrella, 

 and P. nevadensis many species have been proposed which are not here recog- 

 nized as valid, because they were based upon trivial or variable characters. 

 The keys are based upon average specimens, but the student may find oc- 

 casional intermediates between the valid species. 



The bluegrasses are of great importance because of their forage value, 

 some species being cultivated for pasture and others forming a large part of 

 the forage on the mountain meadows of the West. The most important is 

 Poa pratensis, commonly known as bluegrass or Kentucky bluegrass. In 

 the cooler parts of the United States it is cultivated for lawns and is the 

 standard pasture grass in the humid regions where the soil contains plenty 

 of lime. It has been extensively used in the improvement of badly depleted 

 western mountain ranges. P. compressa, Canada bluegrass, is cultivated for 

 pasture in the Northeastern States and Canada, especially on poor soils. 

 P. trivialis and P. palustris are occasionally grown in meadow mixtures, but 

 are of little agricultural importance. P. arachnifera, Texas bluegrass, has been 

 used in some parts of the South for winter pasture and as a lawn grass. P. 

 annua is a common weed in lawns and gardens. P. bulbosa is cultivated about 

 Medford, Oreg., and elsewhere. 



With very few exceptions the bluegrasses are palatable and nutritious and 

 are often the most important grasses in many parts of the West. At high 

 altitudes, P. alpina, P. arctica, P. epilis, and P. rupicola are important. In 

 the mountains mostly below timber line are found P. fendleriana (mutton 

 grass), P. longiligula, P. nervosa, P. secunda (Sandberg bluegrass), P. canbyi, 

 and P. juncifolia, all of wide distribution. P. interior is most abundant in the 

 Rocky Mountains; P. scabrella is probably the most important forage grass 

 of the lower elevations in California; P. gracillima and P. ampla are mostly 

 in the Northwestern States; P. arida is the most valuable bluegrass of the 

 Plains. P. bigelovii, an annual, is important in the Southwestern States. 

 P. macrantha and P. confinis are native sandbinders of the sand dunes on the 

 coast of Washington and Oregon, but are not cultivated. 



Spikelets little compressed, narrow, much longer than wide, the lemmas convex on the back, 



the keels obscure, the marginal and intermediate nerves usually faint. All bunchgrasses. 



Lemmas crisp-puberulent on the back toward the base (the pubescence sometimes obscure 



or only at the very base 7. Scabrellae. 



Lemmas glabrous or minutely scabrous, but not crisp-puberulent 8. Nevadenses. 



Spikelets distinctly compressed, the glumes and lemmas keeled. 



Plants annual 1. Annuae. 



Plants perennial. 



Creeping rhizomes present 2. Pratenses. 



Creeping rhizomes wanting. 



Lemmas webbed at base (web sometimes scant or obscure in P. interior). 



3. Palustres. 

 Lemmas not webbed at base (sometimes sparsely webbed in P. fernaldiana and P. 

 pattersoni). 

 Lemmas pubescent on the keel or marginal nerves or both, sometimes pubescent 



also on the internerves — 5. Alpinae. 



Lemmas glabrous (minutely pubescent at base in P. unilateralis and sometimes in 

 P. curta). 



Blades narrow, usually involute - 6. Epiles. 



Blades flat, 4 to 8 mm. wide, bright green, often splitting at the apex. Panicles 

 about 15 cm. long with slender spreading branches 4. Homalopoae. 



