Herbs, with alternate or opposite simple leaves and regular flowers; petals more or less united; stamens attached one in front of each petal; ovary 1-celled with 1 style.
| 1a. Leaves all basal; flowers on leafless stalks — 2. | |
| 1b. Stem-leaves present — 5. | |
| 2a. Flowers nodding, the petals reflexed (3-6 dm. high; flowers showy, white or pink, in spring) | Shooting Star, Dodecatheon meadia. |
| 2b. Flowers erect or spreading; petals not reflexed — 3. | |
| 3a. Corolla not longer than the calyx; flowers small and inconspicuous (1 dm. high; flowers white or pink, spring) | Androsace, Androsace occidentalis. |
| 3b. Corolla conspicuous, much longer than the calyx (flowers pink or purple, summer) (Primrose) — 4. | |
| 4a. Leaves white-mealy beneath (1-4 dm. high) | Primrose, Primula farinosa. |
| 4b. Leaves green beneath (2 dm. high or less) | Primrose, Primula mistassinica. |
| 5a. All the stem-leaves in one whorl just below the flower-cluster — 6. | |
| 5b. Stem-leaves several or many, scattered over the stem — 7. | |
| 6a. Stem-leaves about 1 cm. long — 3a. | |
| 6b. Stem-leaves 5-10 cm. long | Star Flower, Trientalis americana. |
| 7a. Flowers red, blue, or white (summer) — 8. | |
| 7b. Flowers yellow (summer) — 9. | |
| 8a. Leaves opposite; flowers axillary (stems spreading, 1-4 dm. long; flowers blue or red) | Pimpernel, Anagallis arvensis. |
| 8b. Leaves alternate; flowers racemose (1-4 dm. high; flowers minute, white) | Water Pimpernel, Samolus floribundus. |
| 9a. Stem creeping | Moneywort, Lysimachia nummularia. |
| 9b. Stem erect (Loosestrife) — 10. | |
| 10a. Flowers in dense spike-like racemes (3-8 dm. high) | Loosestrife, Lysimachia thyrsiflora. |
| 10b. Flowers axillary or racemose (3-9 dm. high) — 11. | |
| 11a. Corolla dotted or streaked with purple or brown; leaves punctate with dark spots — 12. | |
| 11b. Corolla plain yellow; leaves not dark-dotted — 14. | |
| 12a. Flowers in racemes — 13. | |
| 12b. Flowers all axillary | Loosestrife, Lysimachia quadrifolia. |
| 13a. Flowers all in racemes; leaves opposite or some of them alternate | Loosestrife, Lysimachia terrestris. |
| 13b. The lowest flowers axillary; leaves opposite or whorled | Loosestrife, Lysimachia producta. |
| 14a. Leaves ovate, on slender ciliate petioles | Loosestrife, Steironema ciliatum. |
| 14b. Leaves lanceolate, sessile or short-petioled, pinnately veined | Loosestrife, Steironema lanceolatum. |
| 14c. Leaves linear, with one mid-vein | Loosestrife, Steironema quadriflorum. |
Trees or shrubs, with opposite leaves and regular flowers; sepals 4, or calyx none; petals 4, united, or none; stamens usually 2; ovary 2-celled, superior.
| 1a. Leaves simple (shrub 2-5 m. high; flowers blue or white, in showy clusters in spring) | Lilac, Syringa vulgaris. |
| 1b. Leaves compound (trees; flowers greenish, inconspicuous, in spring) (Ash) — 2. | |
| 2a. Lateral leaflets sessile | Black Ash, Fraxinus nigra. |
| 2b. Lateral leaflets stalked — 3. | |
| 3a. Twigs sharply 4-angled | Blue Ash, Fraxinus quadrangulata. |
| 3b. Twigs not distinctly angled — 4. | |
| 4a. Leaves pubescent beneath | Red Ash, Fraxinus pennsylvanica. |
| 4b. Leaves glabrous beneath — 5. | |
| 5a. Leaves pale-green beneath, obscurely serrulate | White Ash, Fraxinus americana. |
| 5b. Leaves bright-green beneath, sharply serrulate | Green Ash, Fraxinus pennsylvanica var. lanceolata. |
Herbs, with opposite or basal, entire, usually simple leaves and regular flowers; sepals, petals, and stamens equal in number, 4-12; ovary superior, 1-celled.
| 1a. Leaves reduced to small scales (1-4 dm. high; flowers small, greenish-yellow, in summer) | Bartonia, Bartonia virginica. |
| 1b. Leaves rounded, floating (flowers white, summer) | Floating Heart, Nymphoides lacunosum. |
| 1c. Leaves compound (2-4 dm. high; flowers white or bluish, early summer) | Buckbean, Menyanthes trifoliata. |
| 1d. Leaves simple, whorled (1-2 m. high; flowers yellowish-white, summer) | American Columbo, Frasera caroliniensis. |
| 1e. Leaves simple, opposite — 2. | |
| 2a. Corolla rotate, with spreading lobes, 2-4 cm. broad, pink (5-8 dm. high; summer) | Rose Pink, Sabbatia angularis. |
| 2b. Corolla bell-shape, each petal with a spur at the base, purplish or white, and not over 1 cm. long (1-4 dm. high; summer) | Spurred Gentian, Halenia deflexa. |
| 2c. Corolla bell-shape, tubular, funnel-form, or salver-form, not spurred — 3. | |
| 3a. Corolla-lobes fringed (flowers bright-blue) (Fringed Gentian) — 4. | |
| 3b. Corolla-lobes entire — 5. | |
| 4a. Leaves lanceolate (2-8 dm. high; autumn) | Fringed Gentian, Gentiana crinita. |
| 4b. Leaves linear (1-4 dm. high; late summer) | Fringed Gentian, Gentiana procera. |
| 5a. Corolla 2 cm. long or a little less — 6. | |
| 5b. Corolla 2.5-5 cm. long (late summer and autumn) (Gentian) — 7. | |
| 6a. Upper leaves linear or narrowly lanceolate (2-4 dm. high; flowers pink-purple, late summer) | Centaury, Centaurium umbellatum. |
| 6b. Upper leaves ovate, with several principal veins (1-5 dm. high; flowers blue, late summer and autumn) | Gentian, Gentiana quinquefolia. |
| 7a. Calyx-lobes rough or ciliate at the margin (flowers blue, or rarely white) — 8. | |
| 7b. Calyx-lobes smooth (2-8 dm. high) — 10. | |
| 8a. Corolla-lobes spreading; leaves narrowly lanceolate, indistinctly veined (2-5 dm. high) | Gentian, Gentiana puberula. |
| 8b. Corolla-lobes erect or incurved; leaves ovate to ovate-lanceolate, with 3-7 principal veins (3-6 dm. high) — 9. | |
| 9a. Calyx-lobes equaling or exceeding the calyx-tube | Gentian, Gentiana saponaria. |
| 9b. Calyx-lobes shorter than the calyx-tube | Gentian, Gentiana andrewsii. |
| 10a. Leaves ovate or ovate-lanceolate, somewhat cordate at base (flowers greenish-white or yellowish-white) | Gentian, Gentiana flavida. |
| 10b. Leaves narrowly lanceolate or nearly linear, not cordate (flowers blue) | Gentian, Gentiana linearis. |
Herbs, with opposite simple entire leaves and regular flowers; sepals, petals, and stamens each 5; petals united; stamens attached to the corolla; ovaries 2, with a single style or stigma.
| 1a. Plant creeping or trailing; flowers blue, axillary, 2-3 cm. broad (spring) | Periwinkle, Vinca minor. |
| 1b. Plant erect or essentially so; flowers 1 cm. broad or less (4-12 dm. high) — 2. | |
| 2a. Corolla pinkish, about 8 mm. long by 6-8 mm. broad (early summer) | Dogbane, Apocynum androsaemifolium. |
| 2b. Corolla white or greenish, about 6 mm. long by 4 mm. broad (summer) — 3. | |
| 3a. Leaves petioled, acute at the base | Indian Hemp, Apocynum cannabinum. |
| 3b. Leaves sessile, rounded or truncate at the base | Indian Hemp, Apocynum cannabinum var. hypericifolium. |
Herbs, with simple entire leaves and regular flowers; juice usually milky; except in the first species, which is a twining vine. The flowers have an unusual structure: calyx of 5 sepals; petals 5, united with each other, and spreading or reflexed so that they conceal the calyx; stamens 5, united with each other and with the stigma to form a complex organ in the center of the flower; ovaries 2; on the back of each stamen is a colored projecting hood, which is frequently the most conspicuous part of the flower, and may be mistaken for the corolla.
| 1a. Twining vine, with dark purple flowers (summer) | Black Swallow-wort, Cynanchum nigrum. |
| 1b. Stems not twining — 2. | |
| 2a. Leaves whorled (3-6 dm. high; summer) (Milkweed) — 3. | |
| 2b. Leaves opposite or alternate — 4. | |
| 3a. Leaves in whorls of 4, lanceolate (flowers pink) | Milkweed, Asclepias quadrifolia. |
| 3b. Leaves in whorls of 4-7, linear (flowers greenish-white) | Milkweed, Asclepias verticillata. |
| 4a. Umbel sessile (4-8 dm. high; flowers green, summer) | Green Milkweed, Acerates viridiflora. |
| 4b. Umbel peduncled — 5. | |
| 5a. Leaves linear or narrowly linear-lanceolate (4-8 dm. high; flowers greenish-white, summer) | Green Milkweed, Acerates floridana. |
| 5b. Leaves lanceolate or broader (flowers in summer) — 6. | |
| 6a. Leaves pubescent beneath — 7. | |
| 6b. Leaves glabrous or nearly so — 10. | |
| 7a. Flowers brilliant orange (3-6 dm. high) | Butterfly Weed, Asclepias tuberosa. |
| 7b. Flowers red or purple — 8. | |
| 8a. Reflexed lobes of corolla merely purple-tinged (1-2 m. high) | Milkweed, Asclepias syriaca. |
| 8b. Reflexed lobes of corolla bright-red or purple — 9. | |
| 9a. The erect hoods of each flower about 5 mm. long (7-12 dm. high) | Milkweed, Asclepias purpurascens. |
| 9b. The erect hoods of each flower about 3 mm. long (6-10 dm. high) | Swamp Milkweed, Asclepias incarnata var. pulchra. |
| 10a. Leaves broadly rounded and almost sessile at base (flowers purplish) — 11. | |
| 10b. Leaves narrowed at the base, distinctly petioled (8-15 dm. high) — 12. | |
| 11a. Umbel solitary, terminal and erect on a long peduncle (4-8 dm. high) | Milkweed, Asclepias amplexicaulis. |
| 11b. Umbels terminal or lateral, bent toward one side (7-12 dm. high) | Milkweed, Asclepias sullivantii. |
| 12a. Corolla (not hoods) red (1-2 m. high) | Swamp Milkweed, Asclepias incarnata. |
| 12b. Corolla (not hoods) greenish (8-15 dm. high) | Milkweed, Asclepias exaltata. |
Twining or trailing herbs (except one species), with regular flowers; sepals 5; corolla 5-angled or 5-lobed; stamens 5, attached to the corolla; ovary superior, 2-3-celled.
| 1a. Plants with green foliage and conspicuous flowers (summer) — 2. | |
| 1b. Leafless brown or yellow plants, with very small flowers (Dodder) — 7. | |
| 2a. Style divided at the top into linear or oblong stigmas (flowers white or pink) (Bindweed) — 3. | |
| 2b. Style not divided at the top; stigmas sessile, capitate (Morning Glory) — 6. | |
| 3a. Stem erect; leaves rounded or somewhat cordate at base, not hastate or sagittate (1-3 dm. high) | Bindweed, Convolvulus spithamaeus. |
| 3b. Stem trailing or twining; leaves sagittate or hastate — 4. | |
| 4a. Calyx almost concealed by two large heart-shape bracts — 5. | |
| 4b. Bracts at base of calyx none | Bindweed, Convolvulus arvensis. |
| 5a. Leaves triangular-hastate, with sharp basal lobes | Bindweed, Convolvulus sepium. |
| 5b. Leaves oblong-ovate, the basal lobes obtuse | Bindweed, Convolvulus sepium var. pubescens. |
| 6a. Stem smooth or nearly so; ovary 2-celled (flowers white) | Wild Potato Vine, Ipomoea pandurata. |
| 6b. Stem with reflexed hairs; ovary 3-celled (flowers of various colors) | Morning Glory, Ipomoea purpurea. |
| 7a. Introduced weed, growing as a parasite on clover | Dodder, Cuscuta epithymum. |
| 7b. Native species, on various shrubs and herbs — 8. | |
| 8a. Flowers sessile — 9. | |
| 8b. Flowers distinctly pedicelled — 12. | |
| 9a. Sepals united below into a gamosepalous calyx — 10. | |
| 9b. Sepals separate from each other — 11. | |
| 10a. Calyx-lobes obtuse | Dodder, Cuscuta arvensis. |
| 10b. Calyx-lobes acute | Dodder, Cuscuta obtusiflora. |
| 11a. Flowers in dense rope-like twists on various species of herbs | Dodder, Cuscuta paradoxa. |
| 11b. Flowers in dense clusters on various species of shrubs | Dodder, Cuscuta compacta. |
| 12a. Tips of the petals inflexed | Dodder, Cuscuta coryli. |
| 12b. Tips of the petals erect or spreading — 13. | |
| 13a. Capsule depressed at the summit | Dodder, Cuscuta cephalanthi. |
| 13b. Capsule pointed at the summit | Dodder, Cuscuta gronovii. |
Herbs with alternate or opposite leaves and conspicuous regular flowers; sepals 5, united; petals 5, united and bearing the 5 stamens in the corolla-tube; ovary superior, 3-celled.
| 1a. Leaves pinnately compound and alternate (2-4 dm. high; flowers blue, in spring) | Greek Valerian, Polemonium reptans. |
| 1b. Leaves fascicled, narrowly linear (about 1 dm. high; flowers pink-purple, in spring) | Moss Pink, Phlox subulata. |
| 1c. Leaves simple, strictly opposite — 2. | |
| 2a. Corolla-lobes deeply 2-cleft to the middle (1-2 dm. high; flowers pink, in spring) | Cleft Phlox, Phlox bifida. |
| 2b. Corolla-lobes entire and rounded, or somewhat notched at the apex — 3. | |
| 3a. Flowers in summer (8-15 dm. high; flowers purple) | Garden Phlox, Phlox paniculata. |
| 3b. Flowers in spring (3-6 dm. high) — 4. | |
| 4a. Corolla blue-purple; stems ascending | Sweet William, Phlox divaricata. |
| 4b. Corolla pink or red-purple; stems erect | Sweet William, Phlox pilosa. |
Herbs with alternate lobed or divided leaves and regular flowers; sepals 5; petals 5, united; stamens 5, attached to the corolla-tube and projecting beyond it; ovary 1-celled.
| 1a. Leaves palmately veined and lobed (4-8 dm. high; flowers purple, early summer) | Water-leaf, Hydrophyllum canadense. |
| 1b. Leaves pinnately veined and lobed (2-6 dm. high; flowers blue or purple, varying to white) — 2. | |
| 2a. Corolla-lobes much shorter than the corolla-tube (summer) | Phacelia, Phacelia franklinii. |
| 2b. Corolla-lobes much longer than the corolla-tube (late spring and summer) (Water-leaf) — 3. | |
| 3a. Calyx with a small reflexed appendage between each pair of sepals | Water-leaf, Hydrophyllum appendiculatum. |
| 3b. Calyx without appendages | Water-leaf, Hydrophyllum virginicum. |
Herbs with alternate entire leaves; sepals 5; petals 5, united, corolla generally regular; stamens 5, attached to the corolla-tube; ovary deeply 4-lobed with a single style.
| 1a. Corolla reddish-purple, about 8 mm. wide (4-10 dm. high; spring) | Hound's Tongue, Cynoglossum officinale. |
| 1b. Corolla blue with a yellow center, 4-8 mm. wide (1-5 dm. high; spring and early summer) (Forget-me-not) — 13. | |
| 1c. Corolla deep orange, salver-form (2-6 dm. high; spring) (Puccoon) — 15. | |
| 1d. Corolla white or blue, or lightly tinged with yellow or red — 2. | |
| 2a. Corolla rotate, with a very short tube, bright-blue, about 20 mm. broad (3-8 dm. high; summer) | Borage, Borago officinalis. |
| 2b. Corolla tubular, funnel-form, or salver-form — 3. | |
| 3a. Corolla 10 mm. long or more; its tube distinctly longer than the calyx — 4. | |
| 3b. Corolla less than 10 mm. long; its tube equaling or shorter than the calyx — 8. | |
| 4a. Flowers yellowish-white, or somewhat tinged with pink or greenish — 5. | |
| 4b. Flowers blue or purple — 6. | |
| 5a. Corolla-lobes erect; leaves sessile (3-8 dm. high; early summer) | False Gromwell; Onosmodium occidentale. |
| 5b. Corolla-lobes spreading; leaves decurrent (6-10 dm. high; summer) | Comfrey, Symphytum officinale. |
| 6a. Stem and leaves glabrous (3-6 dm. high; spring) | Bluebell, Mertensia virginica. |
| 6b. Stem and leaves pubescent (4-8 dm. high) — 7. | |
| 7a. Corolla regular; leaves ovate or ovate-lanceolate (summer) | Bluebell, Mertensia paniculata. |
| 7b. Corolla irregular; leaves linear-oblong (summer) | Blueweed, Echium vulgare. |
| 8a. Ovary and fruit covered with hooked prickles — 9. | |
| 8b. Ovary and fruit not prickly — 12. | |
| 9a. Principal leaves 2.5 cm. wide or more — 10. | |
| 9b. Principal leaves 2 cm. wide or less (3-8 dm. high; flowers blue or white, summer) (Stickseed) — 11. | |
| 10a. Leaves chiefly basal, the racemes on long leafless peduncles (4-8 dm. high; flowers pale blue, early summer) | Wild Comfrey, Cynoglossum boreale. |
| 10b. Stems leafy (8-12 dm. high; flowers white, summer) | Beggar Lice, Lappula virginiana. |
| 11a. A bract at the base of each flower | Stickseed, Lappula echinata. |
| 11b. Racemes without bracts at the base of each flower | Stickseed, Lappula deflexa var. americana. |
| 12a. Racemes bractless, or bracted only at the base (1-4 dm. high) — 14. | |
| 12b. Raceme with a bract at the base of each flower (flowers white or yellowish) — 17. | |
| 13a. Corolla 4 mm. wide | Forget-me-not, Myosotis laxa. |
| 13b. Corolla 6-8 mm. wide | Forget-me-not, Myosotis scorpioides. |
| 14a. Calyx-lobes all of equal length (summer) | Scorpion Grass, Myosotis arvensis. |
| 14b. Calyx-lobes distinctly unequal in length (spring) | Scorpion Grass, Myosotis virginica. |
| 15a. Corolla-lobes denticulate | Puccoon, Lithospermum angustifolium. |
| 15b. Corolla-lobes entire — 16. | |
| 16a. Flowers sessile; stem softly pubescent | Puccoon, Lithospermum canescens. |
| 16b. Flowers on pedicels 2-5 mm. long; stem hispid or bristly | Puccoon, Lithospermum gmelini. |
| 17a. Corolla white; fruit brown and wrinkled (weed 2-4 dm. high; spring and summer) | Corn Gromwell, Lithospermum arvense. |
| 17b. Corolla yellowish-white; fruit white and smooth (5-10 dm. high) — 18. | |
| 18a. Corolla distinctly surpassing the calyx in length (spring and summer) | Corn Gromwell, Lithospermum officinale. |
| 18b. Corolla equaling or shorter than the calyx (spring) | Wild Gromwell, Lithospermum latifolium. |
Herbs with opposite leaves, square stems, and usually aromatic odor; flowers irregular, with united petals, or almost regular; stamens 2 or 4, attached to the tube of the corolla; ovary deeply 4-lobed, with a single style.
| 1a. Stamens 2 — 2. | |
| 1b. Stamens 4 — 15. | |
| 2a. Corolla regular or nearly so; flowers white, in dense axillary clusters; plants usually of wet grounds (2-8 dm. high; summer and autumn) — 3. | |
| 2b. Corolla distinctly irregular and more or less 2-lipped — 7. | |
| 3a. Calyx-teeth short, triangular, acute or obtuse (Bugle Weed) — 4. | |
| 3b. Calyx-teeth narrow, acuminate or cuspidate (Water Hoarhound) — 5. | |
| 4a. Stems and stolons bearing tubers | Bugle Weed, Lycopus uniflorus. |
| 4b. Stems and stolons not bearing tubers | Bugle Weed, Lycopus virginicus. |
| 5a. Leaves serrate; calyx-teeth sharp-pointed — 6. | |
| 5b. Leaves coarsely incised; calyx-teeth awn-tipped | Water Hoarhound, Lycopus americanus. |
| 6a. Corolla twice as long as the calyx; leaves narrowed at the base | Water Hoarhound, Lycopus rubellus. |
| 6b. Corolla barely longer than the calyx; leaves sessile or nearly so | Water Hoarhound, Lycopus lucidus var. americanus. |
| 7a. Corolla blue, 3-4 mm. long; flowers in loose axillary clusters (1-4 dm. high; summer) (Pennyroyal) — 8. | |
| 7b. Corolla 8-40 mm. long — 9. | |
| 8a. Leaves serrate | Pennyroyal, Hedeoma pulegioides. |
| 8b. Leaves linear, entire | Pennyroyal, Hedeoma hispida. |
| 9a. Lower lobe of the corolla fringed, very much longer than the upper (5-15 dm. high; corolla pale-yellow; late summer) | Horse Balm, Collinsonia canadensis. |
| 9b. Lower lobe of the corolla nearly or quite as long as the upper and not fringed — 10. | |
| 10a. Calyx narrowly tubular; its teeth about equal in size (5-10 dm. high; flowers in dense terminal heads, in summer) — 11. | |
| 10b. Calyx campanulate, 2 of its teeth different in size from the other 3 (4-8 dm. high; flowers pink-purple, in terminal clusters in summer) — 14. | |
| 11a. Corolla scarlet | Oswego Tea, Monarda didyma. |
| 11b. Corolla bright crimson or rose-red | Wild Bergamot, Monarda fistulosa var. rubra. |
| 11c. Corolla white, pink, pale-purple, or yellowish — 12. | |
| 12a. Flower-clusters all terminal — 13. | |
| 12b. Flower-clusters both terminal and axillary | Horse Mint, Monarda punctata. |
| 13a. Leaves and stem with soft spreading pubescence | Wild Bergamot, Monarda fistulosa. |
| 13b. Leaves and stem grayish with fine appressed pubescence | Wild Bergamot, Monarda mollis. |
| 14a. Upper calyx-teeth about 3 times as long as the lower | Blephilia, Blephilia hirsuta. |
| 14b. Upper calyx-teeth but little longer than the lower | Blephilia, Blephilia ciliata. |
| 15a. Calyx with a distinct protuberance on the back of the upper side (Skullcap) — 16. | |
| 15b. Calyx without a distinct protuberance — 20. | |
| 16a. Corolla 5-8 mm. long; flowers in axillary racemes (3-8 dm. high; flowers blue, in summer) | Mad-dog Skullcap, Scutellaria lateriflora. |
| 16b. Corolla 6-10 mm. long; flowers axillary, solitary (1-3 dm. high; flowers violet, early summer) | Skullcap, Scutellaria parvula. |
| 16c. Corolla 12-30 mm. long; flowers axillary or in terminal racemes (4-8 dm. high; flowers blue, summer) — 17. | |
| 17a. Stem-leaves cordate | Skullcap, Scutellaria versicolor. |
| 17b. Stem-leaves not distinctly cordate — 18. | |
| 18a. Stem-leaves sessile or nearly so; plant of swamps and river-banks | Skullcap, Scutellaria galericulata. |
| 18b. Stem-leaves with petioles 1 cm. or more long; plants of dry or moist woods — 19. | |
| 19a. Stem glandular-pubescent toward the summit; corolla 16 mm. long or less | Skullcap, Scutellaria pilosa. |
| 19b. Stem not glandular; corolla 20 mm. long | Skullcap, Scutellaria incana. |
| 20a. Calyx-teeth 5, all equal or nearly so at the time of flowering — 28. | |
| 20b. Calyx-teeth 5, one of them different in size and shape from the other four (2-6 dm. high; flowers light blue, summer) | Dragon Head, Dracocephalum parviflorum. |
| 20c. Calyx-teeth 5, two of them different in size and shape from the other three — 21. | |
| 20d. Calyx-teeth 10, subulate (woolly plant 4-10 dm. high, with whitish flowers in axillary clusters in summer) | Hoarhound, Marrubium vulgare. |
| 21a. Corolla deeply split on the upper side and the stamens protruding; upper lip of the calyx much shorter than the lower (5-10 dm. high; flowers pink-purple, in terminal spikes, summer) (Wood Sage) — 22. | |
| 21b. Corolla not deeply split on the upper side — 23. | |
| 22a. Calyx canescent | Wood Sage, Teucrium canadense. |
| 22b. Calyx villous | Wood Sage, Teucrium occidentale. |
| 23a. Flowers in dense terminal head-like spikes, none axillary (1-5 dm. high; flowers pink-purple or blue, in summer) | Self-heal, Prunella vulgaris. |
| 23b. Some or all of the flowers in axillary clusters — 24. | |
| 24a. Leaves linear, entire (1-4 dm. high; flowers purple, summer) — 25. | |
| 24b. Leaves oblong to ovate (summer) — 26. | |
| 25a. Pedicels shorter than the calyx | Summer Savory, Satureja hortensis. |
| 25b. Pedicels much longer than the calyx | Calamint, Satureja glabra. |
| 26a. Leaves 1 cm. long or less, entire (stems growing in mats, 1-3 dm. long; flowers purple, in summer) | Wild Thyme, Thymus serpyllum. |
| 26b. Leaves dentate (flowers purple, summer) — 27. | |
| 27a. Flowers subtended by bracts as long as the calyx (2-5 dm. high) | Basil, Satureja vulgaris. |
| 27b. Flowers with minute bracts or none (1-3 dm. high) | Basil-thyme, Satureja acinos. |
| 28a. Corolla 2-lipped or nearly regular, the upper lip flattened, not conspicuously arched over the stamens — 29. | |
| 28b. Corolla conspicuously 2-lipped, the stamens ascending under the concave upper lip — 42. | |
| 29a. Flowers in dense terminal spikes; corolla 2-lipped (8-15 dm. high; summer) — 30. | |
| 29b. Flowers peduncled, 1-4 in the axils of linear leaves — 25b. | |
| 29c. Flowers in many-flowered whorls, which are axillary or terminal, or aggregated into terminal spikes or racemes — 31. | |
| 30a. Corolla yellowish | Giant Hyssop, Agastache nepetoides. |
| 30b. Corolla purplish | Giant Hyssop, Agastache scrophulariaefolius. |
| 31a. Corolla distinctly irregular, the lower lip longer than the upper — 32. | |
| 31b. Corolla almost regular, the lobes nearly uniform in size — 35. | |
| 32a. Stem-leaves sessile or very nearly so (flowers blue) — 33. | |
| 32b. Stem-leaves long-petioled — 34. | |
| 33a. Leaves linear-oblong, acute at both ends (3-8 dm. high; summer) | Hyssop, Hyssopus officinalis. |
| 33b. Leaves oblong to ovate, rounded at the ends (2-4 dm. high; late spring) | Bugle, Ajuga reptans. |
| 34a. Leaves ovate to oblong, acute; flowers pink, white, or pale purple (6-15 dm. high; summer) | Catnip, Nepeta cataria. |
| 34b. Leaves nearly circular or kidney-shape; flowers blue (creeping; flowers in spring and summer) | Ground Ivy, Nepeta hederacea. |
| 35a. Flowers in terminal spikes, or the lower axillary (3-8 dm. high; flowers pink-purple or white, summer) (Mint) — 36. | |
| 35b. Flowers all in axillary whorls (flowers pink-purple or white, summer) (Mint) — 38. | |
| 35c. Flowers in terminal capitate corymbed clusters (4-8 dm. high; flowers white or dotted with purple, summer) (Mountain Mint) — 41. | |
| 36a. Leaves sessile or with very short petiole | Spearmint, Mentha spicata. |
| 36b. Leaves with manifest petioles — 37. | |
| 37a. Principal leaves less than half as broad as long | Peppermint, Mentha piperita. |
| 37b. Principal leaves more than half as broad as long | Bergamot Mint, Mentha citrata. |
| 38a. Stem glabrous; leaves ovate to obovate (4-8 dm. high) | Downy Mint, Mentha gentilis. |
| 38b. Stem pubescent, at least on the angles (1-6 dm. high) — 39. | |
| 39a. Principal leaves distinctly petioled and somewhat rounded at base | Wild Mint, Mentha arvensis. |
| 39b. Leaves tapering to the base — 40. | |
| 40a. Leaves and stem pubescent | Wild Mint, Mentha arvensis var. canadensis. |
| 40b. Leaves glabrous; stem pubescent on the angles only | Wild Mint, Mentha arvensis var. glabrata. |
| 41a. Leaves linear; calyx-teeth awl-shape | Mountain Mint, Pycnanthemum flexuosum. |
| 41b. Leaves narrowly lanceolate; calyx-teeth triangular-ovate | Mountain Mint, Pycnanthemum virginianum. |
| 42a. Stems decumbent to diffuse; leaves cordate to nearly circular (stems 2-5 dm. long or high; flowers in spring and summer) (Dead Nettle) — 43. | |
| 42b. Stem erect; leaves palmately cleft; calyx-teeth spiny (6-15 dm. tall; flowers pink, in summer) | Motherwort, Leonurus cardiaca. |
| 42c. Stems erect or ascending; leaves ovate-lanceolate to linear (summer) — 45. | |
| 43a. Upper leaves closely sessile (flowers red-purple) | Dead Nettle, Lamium amplexicaule. |
| 43b. Leaves all petioled — 44. | |
| 44a. Flowers red or purple | Dead Nettle, Lamium maculatum. |
| 44b. Flowers white | Dead Nettle, Lamium album. |
| 45a. Flowers 2-2.5 cm. long, in loose terminal spikes (5-10 dm. tall; flowers rose-color) | False Dragon Head, Physostegia virginiana. |
| 45b. Flowers 1-2 cm. long, in axillary and terminal spiked whorls — 46. | |
| 46a. Calyx-teeth spiny pointed (flowers pink or pale-purple) (Hemp Nettle) — 47. | |
| 46b. Calyx-teeth acute to awl-shape, but not spiny (3-10 dm. high; flowers pale-purple) (Hedge Nettle) — 48. | |
| 47a. Leaves ovate (3-8 dm. high) | Hemp Nettle, Galeopsis tetrahit. |
| 47b. Leaves linear to lanceolate (1-4 dm. high) | Hemp Nettle, Galeopsis ladanum. |
| 48a. Leaves glabrous — 49. | |
| 48b. Leaves distinctly pubescent — 50. | |
| 49a. Leaves ovate-lanceolate, serrate | Hedge Nettle, Stachys tenuifolia. |
| 49b. Leaves linear-oblong, entire or nearly so | Hedge Nettle, Stachys hyssopifolia. |
| 50a. Stem pubescent on the angles alone; leaves petioled | Hedge Nettle, Stachys tenuifolia var. aspera. |
| 50b. Stem pubescent on both sides and angles; leaves nearly sessile — 51. | |
| 51a. Leaves oblong or oblong-lanceolate, more than 1 cm. wide | Hedge Nettle, Stachys palustris. |
| 51b. Leaves linear-lanceolate, 1 cm. wide or less | Hedge Nettle, Stachys arenicola. |