This is a diverse family, worldwide in occurrence but especially well-developed in North Temperate regions. Although the sepals are 5, often there are additional large or small bractlets in the calyx, sometimes giving the impression of 10 sepals altogether. Except for Dalibarda, all of our herbaceous species have deeply lobed or compound leaves; many of the woody species have simple leaves.
Several genera (see Amelanchier, Crataegus, and Rubus) are well known for taxonomic difficulty, thanks to hybridization, polyploidy, asexual reproduction, and other problems; the only practical course in a local flora is to recognize species complexes. Many other Rosaceae are cultivated and may occasionally escape; those known as escapes from adjacent states or Ontario include Cydonia, Photinia, and Pyracantha.
1. Plant a woody tree or shrub, or a bristly to thorny erect or trailing bramble.
2. Leaves (at least principal ones) compound; plants of many species armed with numerous prickles or prickles.
3. Stems biennial, ± bristly or prickly (very rarely completely smooth); leaves (at least on flowering stems) trifoliolate or palmately compound (pinnate in the jagged-toothed and cut Rubus laciniatus); fruit an aggregate of juicy druplets; flowers normally white, rarely pale pink.
Rubus (in part)
3. Stems perennial, smooth or armed; leaves pinnately compound (often trifoliolate in the pink-flowered Rosa setigera); fruit various but not juicy nor composed of druplets; flowers white, pink, or yellow.
4. Corolla pink (rarely white or, in escapes from cultivation, yellow), 2 cm or more broad; stems usually ± prickly or bristly; stipules adnate to petiole; fruit a fleshy red to orange, spherical to urn-shaped, floral tube enclosing the achenes (and thus resembling an inferior ovary except for the narrow open throat at the summit).
4. Corolla white or yellow, less than 2 cm broad (to 2.5 cm in Dasiphora), stems unarmed; stipules free; fruit a pome from a truly inferior ovary or a cluster of follicles or hairy achenes.
5. Flowers solitary or few in an inflorescence, the petals yellow and 6–11 (–12) mm long; leaflets 5–7, entire, less than 6 (–8) mm broad; fruit a hairy achene.
5. Flowers numerous in a dense inflorescence, the petals white and less than 5 mm long; leaflets 12–23, toothed, over 10 mm broad; fruit a pome or follicle.
6. Inflorescence an elongate panicle (much longer than broad); ovaries 5, superior, forming 5 follicles; stamens ca. twice as long as petals; leaflets doubly serrate (each primary tooth with several fine teeth); colonial shrub, occasionally escaped from cultivation.
6. Inflorescence a broad flat or dome-shaped corymb (much broader than long); ovary 1, inferior, forming a pome; stamens not longer than the petals; leaflets singly serrate (or a few teeth with a secondary tooth); small erect tree, native (or rarely escaped).
2. Leaves all simple; plants all unarmed or with only strong elongate thorns.
7. Leaves opposite.
7. Leaves clearly alternate on elongate shoots (sometimes appearing whorled on short shoots).
8. Flowers yellow; first and second year twigs bright green.
8. Flowers white or pale pink; at least second year twigs gray or brown.
9. Style and ovary 1; ovary superior, ± surrounded by floral tube, ripening into a drupe; leaves unlobed.
9. Styles more than 1 (solitary in Crataegus monogyna); ovary inferior and ripening into a pome, or if styles more than 1, superior and ripening into a group of druplets or dry dehiscent fruits; leaves lobed or not.
10. Ovaries superior; fruit either dry or a ± fleshy aggregate.
11. Petals 18–25 mm long; leaves mostly at least (7–) 9 cm broad; ovaries numerous, ripening into an aggregate of dryish or juicy red druplets.
Rubus (in part, couplet 3)
11. Petals 1–4.5 mm long; leaves less than 5 (–8) cm broad; ovaries 3–5 (connate in Physocarpus), ripening into dry dehiscent fruits.
12. Leaf blades ± lobed as well as toothed; stipules (or their scars on an elevated ridge) present; fruits ca. 5–9 (–12) mm long (not including persistent style), red (drying light brown), inflated; bark conspicuously peeling into longitudinal shreds.
12. Leaf blades entire to toothed but not lobed (occasionally slightly 3-lobed in Spiraea ×vanhouttei); stipules none; fruits ca. 2.5–4 mm long, yellow-brown to blackish, scarcely inflated; bark at most slightly shredding.
13. Petals ca. 2-5 mm long; flowers in umbels, umbel-like racemes, or pyramidal or flat-topped panicles, usually more than 10 except when in sessile umbels; fruits ca. 2.5-4 mm long.
13. Petals ca. 15-25 mm long; flowers up to 10 in a simple raceme; fruits ca. 8-11 mm long.
10. Ovary inferior; fruit fleshy (a pome).
14. Leaves with elongate red ± appressed glands on the midrib above, at least basally, not at all lobed; plants without thorns or spines; petals (less than 7 mm long) less than twice as long as wide.
14. Leaves without glands on midrib (or leaves not yet open at flowering time), lobed or not; plants armed or not; petals various.
15. Petals more than twice as long as broad; leaves (if open) regularly toothed but not lobed.
Amelanchier (in part)
15. Petals less than twice as long as broad (or plants with only leaves and fruit); leaves various.
16. Leaves regularly to coarsely toothed and usually also at least slightly lobed or notched; branches usually armed with distinct sturdy thorns ca. 1.5–11.5 cm long; inflorescence usually a compound (branched) corymb (simply and obscurely corymbose in Malus).
17. Seeds enclosed in very hard bony nutlets; inflorescence (at least the larger ones) usually a compound corymb; styles separate to the base and glabrous; petals white; stamens ca. 20 or fewer; thorns usually shiny; bud scales glabrous.
17. Seeds only in papery-cartilaginous carpels; inflorescence a simple cluster; styles connate and very pubescent at the base; petals pink; stamens ca. 20; thorns dull; bud scales pubescent.
Malus (in part)
16. Leaves regularly toothed or entire but not at all lobed; branches without definite thorns except on elongated shoots of Chaenomeles; inflorescence unbranched.
18. Leaves with entire margins; petals ca. 2–3 mm long; small to medium sized bushy introduced shrubs.
18. Leaves with toothed margins; petals much larger; trees or shrubs.
19. Fruit berry-like, less than 15 mm thick, apparently 10-locular with 1 seed per locule; flowers and fruit usually in racemes (flowers 1–3 in A. bartramiana); common native shrubs.
Amelanchier (in part)
19. Fruit pear- or apple-like (pome), with a “core,” 5-locular with usually 2 seeds in each locule (more in Chaenomeles); flowers crowded with the leaves on short spur-branches; introduced trees and shrubs of disturbed areas.
20. Flowers and fruits essentially sessile; shrubs with many twiggy, dense, branches from the ground, with the elongated shoots thorny.
20. Flowers and fruits clearly pediceled; small to medium sized trees, usually with a single trunk and lacking thorns.
21. Young branchlets densely pubescent; petals pink, at least outside when young; styles densely pubescent basally and connate; fruit an apple, without grit cells.
Malus (in part)
21. Young branchlets glabrous or nearly so; petals white; styles glabrous and separate to the base; fruit a pear, with grit cells.
1. Plant herbaceous, unarmed (stem dying to the ground each year, or slightly woody at the base).
22. Leaves all simple, cordate and crenulate.
22. Leaves all or mostly compound or pinnatifid.
23. Leaves all trifoliolate or palmately compound.
24. Style with a prominent joint near or above the middle, the lower portion hooked at apex and persistent as a long beak on the achene, the terminal portion hooked at its base and deciduous; plant with an erect leafy stem.
Geum (in part)
24. Style not jointed, entirely deciduous from fruit; plant erect or with only basal leaves (stemless or stoloniferous).
25. Calyx without bractlets between the sepals (or rarely an incomplete set of minute ones).
26. Largest leaves sessile or nearly so (petiole, if any, less than a fourth as long as leaflets); carpels 5, ripening into follicles.
26. Largest leaves long-petioled; carpels 2–many, ripening into achenes or fleshy druplets.
27. Petals yellow; carpels 2–6, ripening into achenes, ± concealed within the floral tube (long styles protruding); leaves all basal (at least at flowering time).
Geum (in part)
27. Petals white or pink; carpels normally more than 6, ripening into fleshy druplets conspicuous on receptacle; leaves cauline.
Rubus (in part, couplet 4)
25. Calyx with bractlets nearly or quite as large as the sepals, hence appearing 10-lobed.
28. Bractlets of calyx with 3 (–5) teeth or lobes at summit.
28. Bractlets entire (very rarely 2-toothed at tip).
29. Receptacle greatly enlarging in fruit, becoming fleshy and red; plants with elongate stolons flat on the ground; petals white, the flowers several on a peduncle; leaflets 3.
29. Receptacle not enlarging, dry; plants without conspicuous stolons (or these ± arching and the flowers solitary); petals yellow or white; leaflets 3, 5, or 7.
30. Flowers yellow, leaflets 3, 5, or 7, regularly toothed, deciduous.
Potentilla (in part)
30. Flowers white; leaflets 3, entire except for a 3 (–5)-toothed apex, evergreen.
23. Leaves mostly pinnately compound or divided.
31. Leaflets dissected into linear segments; stamens 5, opposite the white petals.
31. Leaflets definite, with broad flat blades; stamens and petals various but not as above (stamens 4 and petals absent, or numerous, or petals yellow).
32. Leaves twice (or thrice) compound, estipulate; flowers unisexual (plants dioecious or with polygamous tendency); inflorescence a large (ca. 1–4 dm long) panicle of numerous spike-like racemes; carpels 3 (–4), ripening into strongly reflexed follicles.
32. Leaves once-pinnate, stipulate; flowers bisexual (except in Poterium); inflorescence various; carpels 2-many, ripening into achenes (or indehiscent 1-seeded “follicles”).
33. Carpels numerous on a hemispherical or conical receptacle; calyx with bractlets alternating with the true sepals, thus appearing 10-parted (bractlets absent in Geum vernum).
34. Styles elongating and much exceeding the achene at maturity, and at least partly persistent on it, in most species jointed near or above the middle and usually at least partly pilose on lower and/or terminal segment.
Geum (in part)
34. Styles short, slightly if at all exceeding the achene and deciduous from it, neither jointed nor pilose nor greatly elongating in fruit.
35. Petals deep maroon to purple; sepals red tinged; stem usually decumbent, the lower portion in water or wet ground, rooting at nodes.
35. Petals yellow or white; sepals green; stem usually erect, or with slender stolons; mostly upland.
36. Pubescence not glandular; petals deep yellow.
Potentilla (in part)
36. Pubescence glandular-viscid; petals white to pale yellow.
33. Carpels ca. 5 or fewer, on a flat to concave receptacle; calyx 5-lobed without additional bractlets.
37. Petals pink or white; carpels ca. 5, evident on a flattish receptacle, ripening into fruits resembling follicles (but indehiscent); inflorescence a panicle; leaflets deeply lobed or cleft.
37. Petals none or yellow; carpels 1–2, set in a deeply concave floral tube, the ovaries and achenes largely or entirely covered but the styles protruding; inflorescence a compact short to narrow and elongate spike-like raceme; leaflets not lobed or cleft.
38. Floral tube with hooked bristles at its summit but not winged; petals yellow; sepals 5, green; inflorescence an elongate short-pediceled raceme.
38. Floral tube angled or winged but not bristly; petals none; sepals 4, white to greenish or brown; inflorescence crowded and compact.
39. Inflorescence cylindrical, ca. 3–15 (or more) times as long as thick; leaflets mostly ca. 3–7 cm long; flowers bisexual, the carpel 1 and the stamens 4.
39. Inflorescence ovoid, less than twice as long as thick; leaflets less than 2 cm long; flowers unisexual or bisexual, the carpels 2 and the stamens numerous.