The flowers are bilaterally symmetrical, with a spurred lower petal, 2 lateral petals, and 2 upper petals. The petals (especially the spurred one) of all our species have purple lines toward the base: the nectar guides; these are omitted in the statements about flower color in the key. Below the middle of the inside of the lateral petals there may be a tuft of hairs, termed a beard, and in some species there is also a beard on the spurred petal. Unless otherwise indicated, statements about leaf length in the key refer to the entire length of the blade, including the basal lobes if the blade is cordate. The seeds usually have an appendage (caruncle), which is associated with dispersal by ants. Except in V. pedata, cleistogamous flowers are produced later in the season than the familiar spring-blooming petaliferous flowers, and on shorter peduncles (which may be erect or prostrate). Hybrids are occasional, but difficult to recognize except in the “stemmed” blue- and cream-flowered species (see Ballard, 1990).

Especially in the stemless species there are many ambiguities, and exceptions to any character in the key mean that good judgment and intuition are required to base identifications on a judicious balance of characters. Habitats are often quite distinctive, and should be checked in addition to the key. Recent literature (Ballard, 1994; Gil-Ad, 1997; McKinney, 1992) has provided a great deal of useful information, though it has not made identifying violets any easier.

1. Plant leafy-stemmed (i.e., with all petaliferous flowers and at least some leaves borne on above-ground stem).

2. Stipules of cauline leaves well developed, leaf-like, deeply pinnatifid (or bipinnatifid) at the base with a large terminal lobe, the longest basal lobes ca. 30–50% as long as the whole stipule.

3. Petals all ± yellow or cream within (occasionally with purple spot), ± equal in length, shorter than the sepals or exceeding them by at most 2 mm.

V. arvensis

3. Petals (at least the upper, which are longer) deep blue or purple distally, exceeding the sepals by at least 3 mm.

V. tricolor

2. Stipules of cauline leaves entire or fringed with lateral lobes less than 25% as long as the whole stipule.

4. Stipules entire or weakly toothed; petals yellow or white (never blue except outside).

5. Petals white inside (yellow at base), usually ± bluish outside; upper stipules (like the lower) pale and scarious, narrowly tapered or acuminate, scarcely if at all broadened above the base; seeds 1.8–2.2 (–2.4) mm long.

V. canadensis

5. Petals yellow; upper stipules green and herbaceous, acute to obtuse but without prolonged tapered or acuminate tip, often distinctly broader just above the base; seeds mostly 2.4–3.2 (–3.5) mm long.

V. pubescens

4. Stipules prominently fringed along the margins, especially toward the base; petals cream or blue inside (and outside).

6. Petals creamy-white on both surfaces; sepals ciliate (rarely nearly glabrous).

V. striata

6. Petals blue (except in albinos); sepals not ciliate.

7. Spur ca. (5–) 8–13 (–15) mm long (at least as long as the rest of the petal); flowers deeper blue toward the center, the lateral petals glabrous.

V. rostrata

7. Spur less than 5 (–6) mm long (shorter than the rest of the petal); flowers pale blue (or white) toward the center, the lateral petals bearded.

8. Leaf blades usually finely hispidulous on one or both surfaces, subcordate to ± truncate at the base, ovate (usually narrowly so), acute to barely obtuse in outline but blunt at the apex, neither strongly cordate nor reniform and subcordate nor strongly acute; petals deep blue-violet.

V. adunca

8. Leaf blades glabrous (or with small flocculent tufts of knob-shaped hairs or scattered tiny hairs above toward the margins), varying from ovate (definitely cordate at base, acute at apex) to reniform (subcordate, obtuse to rounded in outline at apex); petals pale blue.

V. labradorica

1. Plants with petioles and peduncles all basal (from rhizome or stolon), hence “stem-less.”

9. Ovary and capsule pubescent; seeds ca. 3.5–4 mm long (including prominent appendage ca. 1.5 mm long); style scarcely enlarged but with a recurved conic-tapered tip; escapes from cultivation, spreading by extensive green stolons; leaves finely pubescent, at least above, slightly rugose.

V. odorata

9. Ovary and capsule glabrous; seeds less than 2.3 mm long; style capitate or expanded (and concave or with tiny lateral beak) at tip; native species with stolons none or white (buried in litter).

10. Flowers white, (5–) 7–12 (–14) mm long; rhizome less than 2.5 mm thick (except sometimes at nodes); leaves all cordate to reniform, not otherwise lobed.

11. Leaf blades tapered to the petiole, narrowly elliptic to lanceolate.

V. lanceolata

11. Leaf blades cordate, truncate, or short cuneate to the base, blades lance-ovate to reniform.

12. Leaf blades lance-ovate, the larger ca. 1.8–3 times as long as wide, truncate to short-cuneate at the base.

V. primulifolia

12. Leaf blades ovate to reniform, less than 1.5 times as long as wide, cordate at the base.

13. Leaf blades ovate (longer than broad), glabrous, the crenulations on spring leaves ± straight most of their length with the minute sharp tooth in a notch ± equally deep on both sides; lateral petals glabrous or rarely with scanty short beard; capsules green, speckled with scattered orange dots; seeds ca. 1–1.3 mm long, black at full maturity.

V. macloskeyi

13. Leaf blades ovate to reniform, often sparsely to densely pubescent, especially toward the base and margins or along veins, the crenulations on spring leaves rounded with the tooth usually in an asymmetrical notch deeper on one side than the other (the margin thus ± jagged or scalloped); lateral petals glabrous to heavily bearded; capsules mostly heavily spotted, flecked, or mottled with purple; seeds ca. 1.8–2.2 mm long, brown at full maturity.

14. Plants strongly stoloniferous (with surficial or shallow runners), even at flowering time; largest leaf blades (not necessarily the first of the season) ± ovate, the midrib ca. (70–) 75–95 (–115) % as long as blade is broad; pubescence on upper surface of blade if only one surface is pubescent (otherwise, blades glabrous or pubescent on both surfaces).

V. blanda

14. Plants not at all stoloniferous (but with short ± vertical, sometimes twisted rhizomes); largest leaf blades ± reniform, usually with midrib ca. 50–77 (–82) % as long as blade is broad; pubescence on lower surface of blade if only one surface is pubescent (otherwise, blades glabrous or pubescent on both surfaces).

V. renifolia

10. Flowers blue to violet, (12–) 13–22 (–25) mm long; rhizome various, at least 2 mm thick (except in V. selkirkii and V. epipsila), in most species much thicker; leaves various, in some species ± deeply lobed.

15. Leaves basically ternate, the blade divided nearly to the base into 3 parts of which at least the lateral 2 are further deeply divided, all the segments linear (central portion scarcely if at all broader than the linear segments); rhizome erect, usually short.

16. Lateral (and all other) petals glabrous; rhizome (except in juvenile plants) ca. 5–12 mm in diameter; stamens conspicuously protruding beyond throat of corolla; cleistogamous flowers never produced.

V. pedata

16. Lateral petals (and usually the spurred one) bearded; rhizome ca. 5 mm in diameter or less; stamens not protruding beyond throat of corolla; cleistogamous flowers produced.

V. pedatifida

15. Leaves more shallowly or broadly lobed (with central portion broader than the lobes) or not lobed at all (except for cordate base); rhizome usually elongate and ± angled or horizontal.

17. Lateral petals (like the others) beardless; spur 4–6 mm long; rhizomes very slender (less than 2 mm); leaf blades with narrow sinus (the basal lobes often nearly or quite touching each other), glabrous beneath but pubescent with tiny ± erect hairs above (especially on veins and toward lobes and margins) and with glabrous petioles.

V. selkirkii

17. Lateral (and often spurred) petal bearded; spur less than 4 mm long; rhizomes stouter (except in V. epipsila); leaf blades with broader sinus (lobes not touching), or truncate to subcordate, and pubescence not usually distributed as above.

18. Leaf blades all or mostly lobed or at least with basal teeth larger and coarser than the rest of the margin.

19. Blades of leaves (including lobes) scarcely if at all longer than broad; sepals ± oblong but acute to rounded at tip, ciliate, their auricles in fruit less than 1.5 mm long; spurred petal glabrous within or nearly so; capsules of cleistogamous flowers heavily flecked with purple.

V. palmata

19. Blades of leaves averaging at least 1.5 times as long as broad; sepals narrowly acute, ciliate or not, their auricles in fruit ca. 1.5–4 mm or more long; spurred petal densely bearded within; capsules of cleistogamous flowers green.

V. sagittata (in part)

18. Leaf blades all ± evenly toothed, without lobes (other than at a cordate base) or unusually large teeth.

20. Leaf blades ovate-triangular (or sagittate), most of them distinctly longer than broad and strongly acute in outline at the apex; sepals usually narrowly acute or tapered (though they may be blunt at the tip).

21. Leaf blades parallel-sided to narrowly elliptic-ovate, truncate to subcordate at base, teeth often somewhat enlarged near the base of the blade; leaves ± uniformly pubescent on blades and petioles.

V. sagittata (in part)

21. Leaf blades ovate-triangular, clearly widest near the strongly cordate base, teeth not conspicuously enlarged near the base of the blade; leaves often glabrous or glabrate on blades or petioles or both (except in V. novae-angliae).

22. Peduncles pubescent; leaves and petioles clearly pubescent; sepals often ciliate.

V. novae-angliae

22. Peduncles glabrous; leaves and petioles glabrous to sparsely pubescent; sepals not ciliate.

23. Spurred petal with at least sparse beard; sepal auricles poorly developed, less than 0.8 mm long; beard of at most slightly clavate hairs; flowers slightly if at all overtopping leaves, which are glabrous to occasionally somewhat pubescent.

V. affinis

23. Spurred petal beardless; sepal auricles well developed, becoming 1–2.5 mm long (or 5 mm on fruit of cleistogamous flowers); beard of lateral petals often of short strongly clavate hairs; flowers usually much overtopping the leaves, which are completely glabrous except (usually) for a few tiny hairs only on upper surface of basal lobes and toward margins.

V. cucullata

20. Leaf blades usually broadly ovate-cordate to reniform, most of them little if any longer than broad and nearly or quite obtuse in outline at the apex; sepals ± oblong (or occasionally ovate), rather abruptly rounded at the tip.

24. Rhizome at most scarcely 2 mm thick; foliage and spurred petal completely glabrous; capsules green; leaf blades ± crenulate.

V. epipsila

24. Rhizome much stouter; foliage or spurred petal (or both) usually at least sparsely pubescent; capsules green or marked with purple; leaf blades serrate.

25. Foliage glabrous (at most a few tiny hairs on petioles or upper surface of basal lobes of the blades); spurred petal at least sparsely bearded (very rarely glabrous); capsules of cleistogamous flowers green; sepals glabrous.

V. nephrophylla

25. Foliage usually pubescent, at least on base of the blade beneath and summit of petiole (also on peduncle), though sometimes completely glabrous; spurred petal glabrous (rarely sparsely bearded); capsule of cleistogamous flowers flecked or mottled with purple; sepals often ciliate.

V. sororia