Herbs, with simple opposite leaves and slightly irregular flowers in spikes or heads; petals 5, united and bearing the 4 stamens in the corolla-tube; ovary 1, 2-celled or 4-celled, with 1 style.
| 1a. Plants prostrate or spreading — 2. | |
| 1b. Plants erect (flowers in summer) (Vervain) — 3. | |
| 2a. Leaves serrate; flowers in short dense spikes (flowers pale-blue, summer) | Fog Fruit, Lippia lanceolata. |
| 2b. Leaves pinnatifid; flowers in loose bracted spikes (flowers light-purple, summer) | Vervain, Verbena bracteosa. |
| 3a. Spikes dense, continuous (flowers purple or blue, varying to white) — 4. | |
| 3b. Spikes slender, interrupted, the flowers scattered (corolla white or pale-blue) — 6. | |
| 4a. Leaves lanceolate, manifestly petioled (1-2 m. high) | Vervain, Verbena hastata. |
| 4b. Leaves sessile, not lanceolate (5-8 dm. high) — 5. | |
| 5a. Leaves narrowly oblanceolate, tapering at the entire base | Vervain, Verbena angustifolia. |
| 5b. Leaves oblong to obovate, not tapering at the base | Vervain, Verbena stricta. |
| 6a. Leaves incised, tapering to a sessile base (5-10 dm. high) | Vervain, Verbena, officinalis. |
| 6b. Leaves serrate, petioled (1-2 m. high) | Vervain, Verbena urticaefolia. |
Herbs or shrubs, with alternate leaves and regular or slightly irregular flowers; sepals 5, united; corolla of 5 united petals, bearing the 5 stamens attached; ovary 1, 2-5 (usually 2)-celled, with a slender style.
| 1a. Corolla rotate; anthers close together (flowers in summer) — 2. | |
| 1b. Corolla not rotate; anthers separate — 5. | |
| 2a. Stem and leaves prickly (3-8 dm. high) — 3. | |
| 2b. Stem and leaves not prickly — 4. | |
| 3a. Flowers white or bluish | Horse Nettle, Solanum carolinense. |
| 3b. Flowers yellow | Buffalo Bur, Solanum rostratum. |
| 4a. Climbing vine; leaves frequently lobed (flowers blue) | Bittersweet, Solanum dulcamara. |
| 4b. Not climbing; leaves toothed (flowers white) | Nightshade, Solanum nigrum. |
| 5a. Climbing or trailing shrub, with purplish, white, or greenish flowers about 1 cm. wide (frequently thorny; flowers in summer) | Matrimony Vine, Lycium halimifolium. |
| 5b. Herbaceous plants, not climbing — 6. | |
| 6a. Flowers white, red, or blue, 2.5 cm. or more wide (summer) — 7. | |
| 6b. Flowers yellow, yellowish-white, or greenish-yellow (summer) — 12. | |
| 7a. Corolla-tube 10 cm. long or more (5-12 dm. high) — 8. | |
| 7b. Corolla-tube 5 cm. long or less — 10. | |
| 8a. Stem finely pubescent; leaves entire or nearly so | Thorn-apple, Datura metel. |
| 8b. Stem glabrous; leaves coarsely toothed (Jimson Weed) — 9. | |
| 9a. Stem green; corolla white | Jimson Weed, Datura stramonium. |
| 9b. Stem purple; corolla light-blue or purple | Jimson Weed, Datura tatula. |
| 10a. Corolla pale-blue (5-10 dm. high) | Apple of Peru, Nicandra physalodes. |
| 10b. Corolla, red or violet (2-4 dm. high) | Petunia, Petunia violacea. |
| 10c. Corolla white — 11. | |
| 11a. Corolla all white (2-4 dm. high) | Petunia, Petunia axillaris. |
| 11b. Corolla with yellow center | White Ground Cherry, Physalis grandiflora. |
| 12a. Corolla 30 mm. wide or more, somewhat irregular; stamens declined to one side (3-6 dm. high) | Henbane, Hyoscyamus niger. |
| 12b. Corolla smaller, strictly regular — 13. | |
| 13a. Flowers in terminal panicles; corolla tubular, with slightly spreading lobes (5-10 dm. high) | Wild Tobacco, Nicotiana rustica. |
| 13b. Flowers solitary in the axils; corolla short, widely spreading (3-8 dm. high) (Ground Cherry) — 14. | |
| 14a. Annuals with branching slender roots — 15. | |
| 14b. Perennials with thickened roots and rootstocks — 16. | |
| 15a. Plants pubescent | Ground Cherry, Physalis pubescens. |
| 15b. Plants smooth, or with a few scattered hairs | Ground Cherry, Physalis ixocarpa. |
| 16a. Stem viscid-pubescent | Ground Cherry, Physalis heterophylla. |
| 16b. Stem glabrous or slightly pubescent, not viscid — 17. | |
| 17a. Leaves and stem distinctly pubescent | Ground Cherry, Physalis virginiana. |
| 17b. Leaves and stem almost glabrous | Ground Cherry, Physalis subglabrata. |
Herbs with opposite or alternate leaves and usually irregular flowers; corolla of united petals, bearing the 2 or 4 (or rarely 5) stamens attached; petals actually 5, but sometimes apparently only 2 or 4; a sterile fifth stamen sometimes present; ovary superior, 2-celled.
| 1a. Anther-bearing stamens 5 (6-15 dm. high; flowers in summer) (Mullein) — 2. | |
| 1b. Anther-bearing stamens 4; a sterile fifth stamen may or may not be present — 3. | |
| 1c. Anther-bearing stamens 2 — 37. | |
| 2a. Leaves densely white-woolly; flowers yellow, in dense spikes | Mullein, Verbascum thapsus. |
| 2b. Leaves smooth or nearly so; flowers yellow or white, in loose racemes | Moth Mullein, Verbascum blattaria. |
| 3a. Flowers (not the bracts) greenish-yellow, yellow, or orange — 4. | |
| 3b. Flowers blue, purple, brown, red, pink, or white, never yellow — 16. | |
| 4a. Flowers in dense terminal leafy-bracted spikes — 5. | |
| 4b. Flowers in loose racemes or axillary — 9. | |
| 5a. Corolla 7 mm. long or less, or none — 6. | |
| 5b. Corolla 12 mm. long or more — 7. | |
| 6a. Leaves alternate (3-6 dm. high; early summer) | Synthyris, Synthyris bullii. |
| 6b. Leaves opposite (1-2 dm. high; summer) | Eyebright, Euphrasia arctica. |
| 7a. Stem-leaves entire (2-6 dm. high; summer) | Painted Cup, Castilleja pallida var. septentrionalis. |
| 7b. Stem-leaves palmately lobed, bracteal leaves scarlet (3-6 dm. high; early summer) | Painted Cup, Castilleja coccinea. |
| 7c. Stem-leaves pinnately lobed or incised (Lousewort) — 8. | |
| 8a. Flowers in spring (2-4 dm. high) | Lousewort, Pedicularis canadensis. |
| 8b. Flowers in late summer (3-8 dm. high) | Lousewort, Pedicularis lanceolata. |
| 9a. Upper lip of the corolla very different in size and shape from the lower lip — 10. | |
| 9b. Upper lip of the corolla resembling the lower lip in shape, and not very different in size (5-12 dm. high; summer) (False Foxglove) — 13. | |
| 10a. Leaves alternate (2-5 dm. high; summer) | Butter-and-eggs, Linaria vulgaris. |
| 10b. Leaves opposite — 11. | |
| 11a. Stem erect; leaves narrowed at the base — 30b. | |
| 11b. Stem creeping or spreading (summer) — 12. | |
| 12a. Leaves pinnately veined, ovate | Musk Flower, Mimulus moschatus. |
| 12b. Leaves palmately veined, circular or nearly so | Yellow Monkey Flower, Mimulus glabratus var. jamesii. |
| 13a. Stem glabrous — 14. | |
| 13b. Stem pubescent — 15. | |
| 14a. Principal stem-leaves pinnatifid | False Foxglove, Gerardia virginica. |
| 14b. Principal stem-leaves entire | False Foxglove, Gerardia laevigata. |
| 15a. Corolla hairy on the outside | False Foxglove, Gerardia pedicularia. |
| 15b. Corolla smooth on the outside | False Foxglove, Gerardia flava. |
| 16a. Leaves all basal; flowers on leafless stalks (1 dm. high or less; flowers pink or white, summer) | Mudwort, Limosella aquatica var. tenuifolia. |
| 16b. Leaves opposite (those subtending the flowers may be alternate) — 20. | |
| 16c. Leaves alternate or irregularly scattered — 17. | |
| 17a. Leaves entire — 18. | |
| 17b. Leaves pinnately lobed or incised — 8a. | |
| 17c. Leaves palmately veined and lobed; stem trailing (flowers blue, summer) | Kenilworth Ivy, Linaria cymbalaria. |
| 18a. Corolla-tube less than 1 cm. long, spurred — 19. | |
| 18b. Corolla more than 2 cm. long, not spurred (3-8 dm. high; flowers red-purple, summer) | Snapdragon, Antirrhinum majus. |
| 19a. Stem and foliage pubescent (1-3 dm. high; flowers blue, summer) | Small Snapdragon, Linaria minor. |
| 19b. Stem and foliage glabrous (2-6 dm. high; flowers blue, summer) | Toad-flax, Linaria canadensis. |
| 20a. Leaves with 1 or 2 lobes near the base (3-5 dm. high; flowers purple, summer) | Gerardia, Gerardia auriculata. |
| 20b. Leaves linear (2-6 dm. high; flowers rose-purple, summer and autumn) (Gerardia) — 21. | |
| 20c. Leaves lanceolate or broader, not lobed — 26. | |
| 21a. Pedicels equaling or but little longer than the calyx, and conspicuously shorter than the subtending leaf — 22. | |
| 21b. Pedicels much longer than the calyx, and generally equaling or exceeding the subtending leaf — 24. | |
| 22a. Plants of moist ground, bogs, and shores — 23. | |
| 22b. Plants of dry uplands | Gerardia, Gerardia aspera. |
| 23a. Corolla about 25 mm. long | Gerardia, Gerardia purpurea. |
| 23b. Corolla less than 20 mm. long | Gerardia, Gerardia paupercula. |
| 24a. Stem rough on the angles — 25. | |
| 24b. Stem glabrous | Gerardia, Gerardia tenuifolia. |
| 25a. Leaves 2-5 mm. wide | Gerardia, Gerardia tenuifolia var. macrophylla. |
| 25b. Leaves thread-like, 1 mm. wide or less | Gerardia, Gerardia skinneriana. |
| 26a. Corolla 16 mm. long, or shorter — 27. | |
| 26b. Corolla 20 mm. long, or longer — 32. | |
| 27a. Corolla dull-purple, brown, or greenish; one sterile stamen present (1-2.5 m. high; flowers in summer) (Figwort) — 28. | |
| 27b. Corolla blue or white (1-4 dm. high) — 29. | |
| 28a. Sterile stamen purple | Figwort, Scrophularia marilandica. |
| 28b. Sterile stamen yellow | Figwort, Scrophularia leporella. |
| 29a. Flowers nearly or quite sessile (summer) — 30. | |
| 29b. Flowers on pedicels 10 mm. long or more (spring) — 31. | |
| 30a. Foliage-leaves prominently toothed — 6b. | |
| 30b. Foliage-leaves entire, or with 1 or 2 small teeth at the base | Cow Wheat, Melampyrum lineare. |
| 31a. Corolla more than 10 mm. long, blue and white | Blue-eyed Mary, Collinsia verna. |
| 31b. Corolla 5-8 mm. long, blue and white | Collinsia, Collinsia parviflora. |
| 32a. Flowers solitary in the axils of the upper foliage-leaves (4-8 dm. high; flowers blue, in summer) (Monkey Flower) — 33. | |
| 32b. Flowers in dense terminal or subterminal spikes (3-9 dm. high; summer) — 34. | |
| 32c. Flowers in loose terminal panicles (flowers white or pale-violet) (Beard-tongue) — 35. | |
| 33a. Leaves clasping at the base | Monkey Flower, Mimulus ringens. |
| 33b. Leaves petioled, not clasping | Monkey Flower, Mimulus alatus. |
| 34a. Stem and foliage glabrous (flowers white) | Turtlehead, Chelone glabra. |
| 34b. Stem and foliage pubescent (flowers purple) | Blue Hearts, Buchnera americana. |
| 35a. Stem finely pubescent (3-6 dm. high; flowers pale-violet, late spring) | Beard-tongue, Pentstemon hirsutus. |
| 35b. Stem glabrous below the inflorescence (6-12 dm. high) — 36. | |
| 36a. Corolla-tube gradually enlarged from base to tip (flowers pale-violet, in early summer) | Beard-tongue, Pentstemon laevigatus. |
| 36b. Corolla-tube abruptly enlarged just beyond the calyx (flowers white, early summer) | Beard-tongue, Pentstemon laevigatus var. digitalis. |
| 37a. Corolla distinctly irregular, 2-lipped (1-4 dm. high; flowers yellowish or white, summer) — 38. | |
| 37b. Corolla regular or nearly so and 2-lobed, or none — 6a. | |
| 37c. Corolla regular or nearly so, 4-lobed — 40. | |
| 38a. Leaves narrowed at the base, with mid-vein | Hedge Hyssop, Gratiola virginiana. |
| 38b. Leaves rounded or somewhat clasping at the base, with 3-5 principal veins (False Pimpernel) — 39. | |
| 39a. Peduncles longer than the subtending leaves | False Pimpernel, Ilysanthes anagallidea. |
| 39b. Peduncles shorter than the subtending leaves | False Pimpernel, Ilysanthes dubia. |
| 40a. Leaves whorled (8-20 dm. high; flowers white or pale-blue, in spikes, summer) | Culver's Root, Veronica virginica. |
| 40b. Leaves alternate or opposite (Speedwell) — 41. | |
| 41a. Flowers in racemes, which arise from the axils of the opposite leaves (flowers pale-blue to nearly white, late spring and summer) — 42. | |
| 41b. Flowers solitary in the axils of leaf-like bracts, or in terminal bracted racemes (1-4 dm. high; spring and summer) — 46. | |
| 42a. Stem and foliage glabrous; swamp plants 2-7 dm. high — 43. | |
| 42b. Stem and foliage pubescent; plants of dry ground, 1-3 dm. high — 45. | |
| 43a. Leaves linear or narrowly lanceolate | Marsh Speedwell, Veronica scutellata. |
| 43b. Leaves ovate or ovate-lanceolate — 44. | |
| 44a. Stem-leaves sessile and somewhat clasping | Water Speedwell, Veronica anagallis-aquatica. |
| 44b. Stem-leaves on short petioles | Brooklime, Veronica americana. |
| 45a. Leaves narrowed at base into a petiole | Speedwell, Veronica officinalis. |
| 45b. Leaves rounded or heart-shape at the base | Speedwell, Veronica chamaedrys. |
| 46a. Bracteal leaves entire; stem glabrous or minutely pubescent — 47. | |
| 46b. All leaves serrate; foliage pubescent (flowers blue) — 48. | |
| 47a. Flowers white, about 2 mm. wide | Speedwell, Veronica peregrina. |
| 47b. Flowers pale-blue with darker stripes, 3-4 mm. wide | Speedwell, Veronica serpyllifolia. |
| 48a. Flowers nearly sessile, about 2 mm. wide | Speedwell, Veronica arvensis. |
| 48b. Flowers on slender pedicels, 5-8 mm. wide | Speedwell, Veronica tournefortii. |
Small herbs, growing on rocks, in mud, or in water; calyx and corolla both 2-lipped; stamens 2, attached to the corolla; ovary 1-celled.
| 1a. Corolla purple — 2. | |
| 1b. Corolla yellow (flowers in summer) (Bladderwort) — 4. | |
| 2a. Leaves oval to elliptical, entire (about 1 dm. high, on rocks; flowers in summer) | Butterwort, Pinguicula vulgaris. |
| 2b. Leaves dissected or none, submerged (flowers in summer) (Bladderwort) — 3. | |
| 3a. Flower-stalk with a single bract near the middle | Bladderwort, Utricularia resupinata. |
| 3b. Flower-stalk without bracts, except at the base of the pedicels | Bladderwort, Utricularia purpurea. |
| 4a. Stem and numerous dissected leaves floating in water | Bladderwort, Utricularia vulgaris var. americana. |
| 4b. Stem and minute leaves creeping on the bottom of ponds or in mud, while the flowers are borne on erect stalks, easily detached from the delicate stems — 5. | |
| 5a. Upper lip of corolla conspicuous, as long or nearly as long as the lower lip; lower lip with a prominent raised palate — 6. | |
| 5b. Upper lip of corolla half as long as the lower lip, or less — 7. | |
| 6a. Spur of corolla very short and blunt | Bladderwort, Utricularia gibba. |
| 6b. Spur of corolla very long and slender | Bladderwort, Utricularia cornuta. |
| 7a. Spur of corolla very short and blunt, or almost none | Bladderwort, Utricularia minor. |
| 7b. Spur of corolla long and slender | Bladderwort, Utricularia intermedia. |
Parasitic plants without green color and with scales in place of leaves; corolla 2-lipped, of united petals; stamens 4, attached to the corolla.
| 1a. Flowers in a widely branching panicle, numerous; growing under beech trees (1-5 dm. high; flowers white and purple, late summer) | Beech Drops, Epifagus virginiana. |
| 1b. Flowers sessile in a dense bracted spike (1-2 dm. high; flowers pale-yellow, early summer) | Squaw-root, Conopholis americana. |
| 1c. Flowers 1-15, each on a long erect naked peduncle (1-2 dm. high; flowers yellowish or pale-violet, spring and summer) (Cancer-root) — 2. | |
| 2a. Stem erect and scaly, 5-10 cm. high | Cancer-root, Orobanche fasciculata. |
| 2b. Stem very short, almost below the surface of the ground, with long erect peduncles | Cancer-root, Orobanche uniflora. |
Herbs with opposite simple leaves; corolla of united petals, 2-lipped or almost regular; stamens 2 or 4, attached to the corolla; ovary 2-celled.
| 1a. Corolla about 10 mm. long; flowers in dense heads (4-10 dm. high; flowers blue or white, summer) | Water Willow, Dianthera americana. |
| 1b. Corolla about 30 mm. long; flowers axillary (3-8 dm. high; flowers blue, in summer) (Ruellia) — 2. | |
| 2a. Foliage glabrous or slightly pubescent | Ruellia, Ruellia strepens. |
| 2b. Foliage densely hirsute | Ruellia, Ruellia ciliosa. |
Herb with opposite leaves and irregular flowers in long slender spikes; petals united, corolla 2-lipped; stamens 4, attached to the corolla; ovary 1-celled.
| One species only, 5-10 dm. high; flowers purple, in summer | Lopseed, Phryma leptostachya. |
Herbs with basal leaves and small white flowers in spikes; sepals 4; petals 4, united; stamens 4; ovary 2-celled.
| 1a. Leaves linear (1-4 dm. high; summer) — 2. | |
| 1b. Leaves broader, lanceolate to broadly ovate or cordate (summer) — 3. | |
| 2a. Spikes mixed with bracts several times longer than the flowers | Buckhorn, Plantago aristata. |
| 2b. Bracts about as long as the flowers | Plantain, Plantago purshii. |
| 3a. Leaves cordate, pinnately veined; plant of wet ground and marshes (4-8 dm. tall) | Plantain, Plantago cordata. |
| 3b. Leaves with 3 to many longitudinal ribs or veins — 4. | |
| 4a. Leaves densely pubescent with grayish hairs — 5. | |
| 4b. Leaves smooth or slightly pubescent — 6. | |
| 5a. Flower-stalks 3-6 dm. high | Plantain, Plantago media. |
| 5b. Flower-stalks less than 3 dm. high | Plantain, Plantago virginica. |
| 6a. Flower-stalks 3-6 dm. high; spikes not over 10 cm. long | English Plantain, Plantago lanceolata. |
| 6b. Scapes 1-4 dm. high; spikes long and slender, usually equaling or longer than the peduncle; dooryard plantains — 7. | |
| 7a. Leaves green at the base | Plantain, Plantago major. |
| 7b. Leaves reddish at the base | Plantain, Plantago rugelii. |
Herbs or shrubs, with opposite or whorled leaves and regular flowers; sepals 4, or minute or almost wanting; petals 4, united; stamens 4; ovary inferior.
| 1a. Shrub (1-3 m. tall; flowers white, in spherical heads, summer) | Button Bush, Cephalanthus occidentalis. |
| 1b. Herbaceous — 2. | |
| 2a. Leaves opposite — 3. | |
| 2b. Leaves whorled; flowers white, green, or purple (Bedstraw) — 6. | |
| 2c. Leaves whorled; flowers yellow | Bedstraw, Galium verum. |
| 3a. Leaves about as long as wide (trailing; flowers paired, white, in spring) | Partridge Berry, Mitchella repens. |
| 3b. Leaves at least twice as long as wide — 4. | |
| 4a. Corolla salver-form, about 1 cm. wide; peduncles 1-flowered (about 1 dm. high; flowers blue or white, in spring) | Bluets, Houstonia coerulea. |
| 4b. Corolla funnel-form, about 5 mm. wide; flowers in clusters (1-2 dm. high; flowers white or pale-purple, summer) (Houstonia) — 5. | |
| 5a. Basal leaves strongly ciliate | Houstonia, Houstonia ciliolata. |
| 5b. Basal leaves smooth | Houstonia, Houstonia longifolia. |
| 6a. Leaves in whorls of 4-7. | |
| 6b. Leaves in whorls of 6-8 — 16. | |
| 7a. Ovary and fruit hispid with hooked bristles (3-7 dm. high; summer) — 8. | |
| 7b. Ovary and fruit not bristly (early summer) — 11. | |
| 8a. Leaves with 1 principal vein (flowers dull purple) | Bedstraw, Galium pilosum. |
| 8b. Leaves with 3 principal veins — 9. | |
| 9a. Flowers bright-white | Bedstraw, Galium boreale. |
| 9b. Flowers greenish, yellowish, or purplish — 10. | |
| 10a. Leaves acuminate | Bedstraw, Galium lanceolatum. |
| 10b. Leaves acute or obtuse | Bedstraw, Galium circaezans. |
| 11a. Corolla-lobes 3 (2-6 dm. high; flowers white or greenish) — 12. | |
| 11b. Corolla-lobes 4 — 13. | |
| 12a. Flowers in clusters of 2 or 3 | Bedstraw, Galium claytoni. |
| 12b. Flowers solitary in the axils, on long hair-like pedicels | Bedstraw, Galium trifidum. |
| 13a. Corolla brownish or purple (3-6 dm. high) | Bedstraw, Galium latifolium. |
| 13b. Corolla white (1-4 dm. high) — 14. | |
| 14a. Flowers rather numerous in small cymes | Bedstraw, Galium palustre. |
| 14b. Flowers in clusters of 2 or 3, or solitary — 15. | |
| 15a. Principal leaves spreading or ascending | Bedstraw, Galium tinctorium. |
| 15b. Principal leaves recurved or reflexed | Bedstraw, Galium labradoricum. |
| 16a. Ovary and fruit bristly or hispid — 17. | |
| 16b. Ovary and fruit not bristly (summer) — 18. | |
| 17a. Leaves narrowly lanceolate to linear, mostly 6-8 in a whorl (stem 5-15 dm. long; flowers white; spring and summer) | Bedstraw, Galium aparine. |
| 17b. Leaves narrowly oval or elliptical, mostly in whorls of 6; flowers in clusters of 3 (1-5 dm. high; flowers greenish, summer) | Bedstraw, Galium triflorum. |
| 18a. Leaves cuspidate or mucronate at the apex (flowers white) — 19. | |
| 18b. Leaves obtuse at the apex (flowers white or greenish; 2-6 dm. high) — 20. | |
| 19a. Flowers very numerous in terminal panicles (stem 3-8 dm. long) | Bedstraw, Galium mollugo. |
| 19b. Flowers in axillary clusters (1-3 dm. high) | Bedstraw, Galium tricorne. |
| 19c. Flowers few, in small loose terminal cymes — 20. | |
| 20a. Stem smooth or nearly so (2-4 dm. high) | Bedstraw, Galium concinnum. |
| 20b. Stem hispid with reflexed bristles (5-15 dm. long) | Bedstraw, Galium asprellum. |
Shrubs or herbs, with opposite leaves; corolla regular or irregular, petals 4 or 5, united; stamens 4 or 5; ovary inferior, 1-5-celled.
| 1a. Leaves compound (shrubs 1-4 m. high; flowers white, in large clusters in early summer) (Elder) — 2. | |
| 1b. Leaves simple — 3. | |
| 2a. Pith of the twigs white; inflorescence flattened or convex | Elder, Sambucus canadensis. |
| 2b. Pith of the twigs brown; inflorescence pyramidal | Elder, Sambucus racemosa. |
| 3a. Plant trailing; flowers nodding, in pairs (1 dm. high; flowers pink, summer) | Twin Flower, Linnaea borealis var. americana. |
| 3b. Erect herbs (6-12 dm. high; flowers dull-red, axillary, early summer) (Feverwort) — 4. | |
| 3c. Shrubs, small trees, or woody vines — 5. | |
| 4a. Leaf-bases broadly connate and 2-5 cm. wide | Feverwort, Triosteum perfoliatum. |
| 4b. Leaf-bases narrowly connate, not over 1 cm. wide | Feverwort, Triosteum aurantiacum. |
| 5a. Climbing vines (spring and early summer) (Honeysuckle) — 6. | |
| 5b. Erect or spreading shrubs or small trees — 11. | |
| 6a. Flowers in 2-flowered axillary clusters (flowers white or pink) | Honeysuckle, Lonicera japonica. |
| 6b. Flowers in terminal clusters — 7. | |
| 7a. Leaves distinctly pubescent beneath (flowers yellow) — 8. | |
| 7b. Leaves glabrous beneath, or very minutely puberulent — 9. | |
| 8a. Leaves pubescent above | Honeysuckle, Lonicera hirsuta. |
| 8b. Leaves glabrous above | Honeysuckle, Lonicera glaucescens. |
| 9a. Corolla purple on the outside, glabrous within | Honeysuckle, Lonicera caprifolium. |
| 9b. Corolla yellow on the outside (or slightly tinged with purple), pubescent within — 10. | |
| 10a. Corolla-tube 6-8 mm. long | Honeysuckle, Lonicera dioica. |
| 10b. Corolla-tube 11-14 mm. long | Honeysuckle, Lonicera sullivantii. |
| 11a. Corolla tubular at base; style long and slender — 12. | |
| 11b. Corolla rotate or somewhat bell-shape, style very short (flowers white, late spring or early summer) — 22. | |
| 12a. Flowers yellow or yellowish (spring and early summer) — 13. | |
| 12b. Flowers white, pink, or red — 18. | |
| 13a. Leaves serrate (5-10 dm. tall) | Bush Honeysuckle, Diervilla lonicera. |
| 13b. Leaves entire; flowers in pairs (Honeysuckle) — 14. | |
| 14a. Each pair of flowers subtended by 2 broad leaf-like bracts (1-3 m. high) | Honeysuckle, Lonicera involucrata. |
| 14b. Bracts at the base of each pair of flowers linear or narrowly lanceolate — 15. | |
| 15a. Native species of woods and bogs — 16. | |
| 15b. Introduced species, growing mostly near dwellings; leaves very pubescent beneath | Honeysuckle, Lonicera xylosteum. |
| 16a. Peduncles 15 mm. long or more (1-4 m. high) — 17. | |
| 16b. Peduncles about 5 mm. long (1 m. high, or less) | Honeysuckle, Lonicera coerulea var. villosa. |
| 17a. Leaves ciliate | Honeysuckle, Lonicera canadensis. |
| 17b. Leaves not ciliate | Honeysuckle, Lonicera oblongifolia. |
| 18a. Corolla irregular, over 1 cm. long (1-4 m. high; spring) | Honeysuckle, Lonicera tatarica. |
| 18b. Corolla regular, less than 1 cm. long (5-15 dm. high; flowers white or pink, in axillary clusters, early summer) — 19. | |
| 19a. Flowers in axillary spikes | Wolfberry, Symphoricarpos occidentalis. |
| 19b. Flowers almost sessile in the axils — 20. | |
| 20a. Flowers numerous in each axil | Indian Currant, Symphoricarpos orbiculatus. |
| 20b. Flowers 1 or 2 in each axil (Snowberry) — 21. | |
| 21a. Leaves green beneath | Snowberry, Symphoricarpos racemosus. |
| 21b. Leaves whitened beneath | Snowberry, Symphoricarpos racemosus var. pauciflorus. |
| 22a. Leaves palmately lobed — 23. | |
| 22b. Leaves not lobed — 25. | |
| 23a. Outermost flowers of the cluster enlarged and imperfect (1-4 m. high) | Cranberry Tree, Viburnum opulus var. americanum. |
| 23b. All flowers of the cluster alike — 24. | |
| 24a. Flower-clusters 4-10 cm. broad (1-2 m. high) | Arrow Wood, Viburnum acerifolium. |
| 24b. Flower-clusters 2-3 cm. broad | Squashberry, Viburnum pauciflorum. |
| 25a. Outer flowers of the cluster enlarged and imperfect (1-3 m. high) | Hobble-bush, Viburnum alnifolium. |
| 25b. All flowers of the cluster alike — 26. | |
| 26a. Leaves finely serrate; the veins not prominent — 27. | |
| 26b. Leaves coarsely serrate, all or most of the teeth terminating in a prominent vein — 29. | |
| 27a. Peduncle of the flower-cluster, below its branches, at least 2 cm. long (1-3 m. high) | Withe-rod, Viburnum cassinoides. |
| 27b. Peduncle of the cluster 1 cm. long, or even shorter (3-8 m. high) — 28. | |
| 28a. Leaves distinctly acuminate | Sheep-berry, Viburnum lentago. |
| 28b. Leaves obtuse or barely acute | Black Haw, Viburnum prunifolium. |
| 29a. Leaves densely pubescent beneath (6-15 dm. high) | Arrow-wood, Viburnum pubescens. |
| 29b. Leaves glabrous beneath, or with tufts of hairs in the forks of the veins (1-4 m. high) | Arrow-wood, Viburnum dentatum. |