Hall's bent grass, Agrostis hallii, June-July, native

The question of Agrostis hallii or A. pallens (the latter including A. diegoensis)

These rhizomatous, leafy grasses are usually easy to pick out in their in their vegetative states in appropriate habitat by their relatively long cauline leaf blades and short stem internodes. Determining species even when in bloom can be difficult because critical measurements of local material frequently overlap.

In The Jepson Manual 2 (adapted):
plants from rhizomes
4. Floret callus hairs 1.5–2 mm, gen > 1/2 lemma; ligule 4–7 mm ..... A. hallii (3)
4' Floret callus hairs gen minute, sparse, or 0; ligule gen < 3 mm .... A. pallens

rhizomes or stolons 0
31. Floret callus hairs 1.5–2 mm, gen slightly > 1/2 lemma; anthers >= 1.5 mm ..... A. hallii
31' Floret callus glabrous or hairs minute, << lemma; anthers gen < 1.5 mm .... A. pallens

Barbara Ertter writes in the Diablo Flora, 2nd ed (2002) about A. hallii:

The very similar A. pallens Trin. (= A. diegoensis Vasey) differs primarily in that the hairs at the base of the floret are lacking or nearly so, and florets and ligules are smaller in general. The placement of most specimens from the East Bay in A. hallii, including all those from Mt. Diablo, is based on previously existing identifications, even though they are not compatible with the key break given by Harvey (JepMan) (i.e., the hairs are mostly in the 0.5-1.5 mm range.) Harvey notes, however, that the geographic and ecological variation of A. pallens is in need of further study. (p. 348)

In the addenda (p. 404) a collection of A. pallens is cited (Ertter & Morosco 16433)

In the Marin Flora (Howell 1970), A. diegoensis is common and A. hallii uncommon, and Howell writes “this species [>A. diegoensis] and the preceeding [A. hallii] are nearly confluent and at times difficult to distinguish, although A. diegoensis is generally more delicate in foliage and inflorescence.” p. 82.

Howell’s leads:
h. spikelets 3.5-4.5 mm l.; lemma 3 mm. l; hairs at base of lemma more than
1 mm long .... A. hallii

hh. spikelets 2.5-3.5 mm l; lemma 2-2.5 mm l; hairs at base of lemma 1 mm
or less .... A. diegoensis

J. Thomas' leads in his Flora of the Santa Cruz Mountains:

spikelets 3.5-4.5 mm l; lemmas about 3 mm l; the hairs at the base 1 mm l or
longer; . . . hallii
spikelets usu less than 3.5 mm l; lemmas 2-2.5 mm l; the hairs less than 1
mm l; . . . diegoensis

A. hallii panicle illustration from Manual of the Grasses of the United States, 2 ed. PDF: http://standish.stanford.edu/bin/object?00003911

A. diegoensis panicle illustration from Manual of the Grasses of the United States, 2 ed. PDF: http://standish.stanford.edu/bin/object?00003911

CalPhotos images of Agrostis hallii herbarium vouchers by Steve Matson.
CalPhotos images of Agrostis pallens herbarium vouchers by Steve Matson.