clovers

genus Trifolium

Member of legume family (family Fabaceae)
dicots (class Magnoliopsida)
flowering plants (subphylum Angiospermae)

Except for historical records that I'm ignoring, there are no other wild species of this genus in the bay area.

Much depends on the bracts (involucre), so always photograph the clover head from beneath, and measure the length of the bracts. The involucre is always snuggled up against the inflorescence, and depending on its size it may or may not hide the pedicels at the base of the inflorescence.

There are generally 5 sepals, generally fused into a calyx for each flower. There are also generally 5 petals, which may be free, fused, or only the lower 2 ± fused into a keel. It may be useful to pluck a calyx/corolla combination to get a closer look at the calyx.

Camera angles:

close look at top/side of inflorescence with scale
close look at bottom of inflorescence with scale
leaflets with scale (not the reduced leaflets directly below the inflorescence)
peduncle with scale

Key features:

bracts shape (especially if fused) under inflorescence
calyx lobes plumose or not
corolla color
lengths of bracts, sepals (calyx), petals (corolla), leaflets, leaf stipules, peduncle, and consider pulling off a corolla for the pedicel length
leaflets palmate or pinnate
leaflet shape
various parts hairy or glabrous

strawberry clover
Trifolium fragiferum

involucre bracts ± 2 mm, ± free, green
inflorescence not sessile
calyx hairy, inflated in fruit
banner not inflated in fruit

Involucre bracts generally free, generally > 1 mm
Inflorescence not sessile above 1–2 leaves
Banner soon inflated in fruit

bull clover
Trifolium fucatum

glabrous
Leaflets: 3, palmate, 1–2.5 cm, elliptic to ± round, ± entire to toothed
Bracts: ± free to ½ fused
Inflorescence: head-like, 5–many-flowered; flowers quickly spreading or reflexed
Calyx: 4–7 mm, glabrous, longer lobes bristle-like, generally branched
Corolla: 10–20 mm, dull white or ± yellow, purple-tipped (or not?), banner inflated in fruit
see also pale sack clover (Trifolium depauperatum var. amplectens)

involucre

saline clover
Trifolium hydrophilum

corolla 4–9 mm
involucre bracts 2–2.5 mm, ± free to ½ fused
inflorescence generally 1–1.5 cm wide
uncommon, generally found in the east bay

cowbag clover
Trifolium depauperatum

glabrous
Leaflets: 3, palmate, 0.5–2 cm, narrowly oblong to obovate, entire or toothed, occasionally lobed, tip generally truncate
Bracts: fused, vestigial, ring-like in ssp. depauperatum; ± free in variants
involucre bracts < 1 mm, fused at base
Inflorescence: head-like, 0.5–1 cm wide, 3–many-flowered
Corolla: 4.5–9 mm, pink-purple (but CalPhotos also shows cream w/ purple above), white-tipped, banner inflated in fruit

clovers

Involucre bracts generally free, generally > 1 mm Inflorescence not sessile above 1–2 leaves Banner soon inflated in fruit

Involucre bracts generally fused, generally > 1 mm, generally forming cup or bowl about base of inflorescence Inflorescence not sessile above 1–2 leaves Calyx and corolla not inflated in fruit

Involucre cup- or bowl-shaped, when pressed generally partly hiding flowers

Involucre wheel-shaped, when pressed hiding only bases of flowers

Involucre bracts 0 or occasionally < 1 mm or forming a vestigial ring Inflorescence occasionally ± sessile above 1–2 ± reduced leaves (stipules of which occasionally involucre-like)

pedicels 1–6 mm, ± equal flowers generally becoming reflexed

pedicels < 2 mm flowers becoming reflexed or not

inflorescence ± sessile above a pair of reduced leaves, stipules, or both

inflorescence peduncled, peduncle generally > 10 mm, generally > leaves

Involucre bracts generally free, generally > 1 mm
Inflorescence not sessile above 1–2 leaves
Banner soon inflated in fruit

Involucre bracts generally fused, generally > 1 mm, generally forming cup or bowl about base of inflorescence
Inflorescence not sessile above 1–2 leaves
Calyx and corolla not inflated in fruit

Involucre bracts 0 or occasionally < 1 mm or forming a vestigial ring
Inflorescence occasionally ± sessile above 1–2 ± reduced leaves (stipules of which occasionally involucre-like)

pedicels 1–6 mm, ± equal
flowers generally becoming reflexed

pedicels < 2 mm
flowers becoming reflexed or not