What’s a dichotomous key?
Help
Symphyotrichum
See list of 27 species in this genusThough Symphyotrichum is generally regarded as an extremely difficult genus, the vast majority of New England collections are straightforward to assign to species. Once learning the large number of taxa is overcome, infrequent hybridization and rare aberrant forms are some of the few stumbling blocks that botanists will face here in New England. Identification necessitates familiarity with details of the involucral bracts and disk flowers (and the need for magnification to view the details, generally 14 × is sufficient for most people). Many hybrid combinations have been reported from New England, but more work is needed to confirm some of these. It is clear that some collections determined as hybrids are not identified correctly and merely represent atypical forms of species. Symphyotrichum ×gravesii (Burgess) Nesom is a poorly understood taxon that is believed to be a hybrid involving S. dumosum and S. laeve, though S. cordifolium may be a parent (rather than S. dumosum). More work is needed to understand the origin of this plant from CT. References: Semple et al. (2002), Brouillet et al. (2006).
-
1a. Annuals from a short taproot; rays very short and inconspicuous, scarcely or not exceeding the mature pappus, or the rays absent altogether [Fig. 461]
-
2a. Involucral bracts with a chartaceous base and chlorophyllous tip, of several conspicuously different lengths; ray flower corolla longer than the style; saltmarsh plants
-
2b. Involucral bracts, especially the outer, herbaceous, of nearly equal length; ray flower corolla shorter than the style, the style protruding from the corolla; rare introductions in New England
-
3a. Ray flowers numbering 90–110 or more per capitulum, with a short, but evident, strap-like ray 1.5–2 mm long
-
3b. Ray flowers numbering 75–95 (rarely more), lacking rays altogether (note: these flowers are still recognizable as a ray flower due to the lack of lobes at the summit of the corolla tube)
-
-
-
1b. Perennials from a rhizome, caudex, or crown; rays elongate and conspicuous
-
4a. Basal leaves both cordate and borne on a petiole (often merely truncate or broad-rounded at base of the petiolate blade in S. lowrieanum) [Fig. 469]
-
5a. Middle and upper stem leaves sessile and conspicuously clasping the stem; leaves usually with low, crenate teeth
-
5b. Middle and upper stem leaves petiolate or sessile, generally narrowed to the base and not conspicuously clasping; leaves mostly with prominent, sharp teeth
-
6a. Peduncles with few or no bracts, very uneven, some generally exceeding 1 cm; rays 7–15 mm long; involucral bracts usually green at apex, the outer 0.5–1 mm wide
-
6b. Peduncles copiously bracteate, especially when the peduncles approach or exceed 1 cm in length; rays 4–8 mm long; involucral bracts often with the chlorophyllous tip partly or wholly obscured by anthocyanin, the outer mostly 0.2–0.7 mm wide
-
7a. Involucral bracts acuminate, with a slender, elongate chlorophyllous zone; branches of capitulescence often strongly ascending
-
7b. Involucral bracts obtuse to narrow-acute, with an apically dilated chlorophyllous zone [Fig. 464]; branches of capitulescence spreading to ascending
-
8a. Petioles scarcely or not at all winged; lower leaves deeply cordate with prominent basal sinuses; leaf blades usually pubescent, not glaucous on the abaxial surface, smooth to scabrous on the adaxial surface
-
8b. Petioles broadly winged; lower leaves usually rounded or truncate to shallowly cordate; leaf blades glabrous, thinly glaucous on the abaxial surface, very smooth on the adaxial surface
-
-
-
-
-
4b. Basal leaves not both cordate and borne on a petiole
-
9a. Leaf blades auriculate- or cordate-clasping
-
10a. Involucral bracts stipitate-glandular [Fig. 465]
-
11a. Capitula with 45–100 ray flowers, red-purple; stem leaves abundant and crowded
-
11b. Capitula with 15–30 ray flowers, commonly blue; stems with remote to approximate, but not crowded, leaves
-
-
-
12a. Involucral bracts long-acuminate to attenuate at the apex, of nearly equal length; at least the lower portion of the stem usually hispid pubescent with stiff, spreading hairs
-
12b. Involucral bracts mostly obtuse to acute at the apex, often of different lengths; stem glabrous to pubescent, when pubescent the hairs often softer and/or curving toward stem apex
-
13a. Most or all of the stem leaves conspicuously narrowed to a broadly winged and conspicuously clasping petiolar base; stem often notably flexuous, changing angles at successive nodes
-
13b. Stem leaves not conspicuously narrowed nor with a prominently clasping base; stems straight or weakly flexuous
-
14a. Stem and leaves below the capitulescence glabrous and glaucous; involucral bracts with a short, rhombic chlorophyllous zone; plants from a short, stout rhizome or woody caudex
-
14b. Stem and leaves glabrous to pubescent, not glaucous; involucral bracts with an elongate chlorophyllous zone, or the outer involucral bracts entirely green; plants from elongate rhizomes
-
15a. Leaf blades narrow-lanceolate to oblanceolate, narrowing to the base, flat, herbaceous to fleshy; most peduncles shorter than 3 cm; involucral bracts often loose or somewhat spreading apically, commonly green in large part (in part)
-
15b. Leaf blades tending to be narrow-lanceolate, scarcely narrowing to the base, folded along the midrib, coriaceous; many peduncles longer than 3 cm; involucral bracts usually appressed and with a chartaceous base and green apex (rarely loose, expanded and foliaceous apically) (in part)
-
-
-
-
-
-
9b. Leaf blades narrowed to the base
-
16a. Involucral bracts with a non-chlorophyllous spine tip, with short, stiff cilia on the margins [Fig. 462]; leaf blades linear to oblanceolate, moderately to densely pubescent on both surfaces and provided with stiff, marginal cilia
-
16b. Involucral bracts lacking a spine tip (though with an involute chlorophyllous tip in S. pilosum that is commonly misinterpreted as a spine), eciliate or softly ciliate on the margins [Fig. 464]; leaf blades not both narrow and pubescent on both surfaces
-
17a. Many of the involucral bracts with a subulate, involute, chlorophyllous tip [Fig. 466]
-
17b. Involucral bracts without an involute tip [Fig. 464]
-
18a. Involucral bracts densely sericeous; leaves densely sericeous on both surfaces (becoming glabrate in age), entire; capitulescence often tall and slender, raceme-like; cypselas densely sericeous, the hairs obscuring the nerves
-
18b. Involucral bracts glabrous to pubescent, but not sericeous; leaves glabrous to pubescent, but not sericeous on both surfaces, entire to toothed; capitulescence variable, though often broad; cypselas usually sparsely strigose, the nerves visible
-
19a. Glabrous, fleshy saltmarsh plants with narrow, linear leaves rarely wider than 10 mm; involucral bracts chartaceous, sometimes green-tinged in the apical portion but often lacking a sharply defined chlorophyllous zone
-
19b. Plants variable in habit, habitat, and leaf morphology, not of saltmarshes except varieties of S. novi-belgii; involucral bracts with a conspicuous chlorophyllous zone in the apical portion or entirely foliaceous
-
20a. Lobes of the disk corolla comprising (45–) 50–75% of the limb, flaring to recurving [Fig. 463]
-
21a. Leaf blades abaxially glabrous or pubescent only on the veins; disk flowers yellow, turning purple post anthesis; lobes of the disk corolla comprising 50–75% of the limb; capitula with 8–15 (–25) ray flowers; plants from a branched caudex or short, stout rhizome
-
21b. Leaf blades abaxially sparsely to densely pubescent on and between the veins (glabrous in rare forms); disk flowers yellow, turning brown post anthesis; lobes of the disk corolla comprising 45–55 (–65)% of the limb; capitula with (10–) 15–26 ray flowers; plants from slender, elongate rhizomes
-
-
20b. Lobes of the disk corolla comprising 15–45% of the limb, erect to ascending [Fig. 468]
-
22a. Branches and peduncles provided with copious, imbricate, short bracts mostly 1–4 mm long that are conspicuously shorter than the leaves of the main stem
-
22b. Branches and peduncles with relatively fewer bracts, the bracts not crowded, mostly longer than 4 mm, and usually gradually decreasing in size from the leaves of the main stem (abruptly reduced in S. racemosum)
-
23a. Lobes of disk corolla comprising 15–30% of the limb [Fig. 468]; ray flowers commonly light blue to purple
-
24a. Leaves with conspicuous veins that connect to form isodiametric aereolae [Fig. 467]; stems glabrous and thinly glaucous
-
24b. Leaves with fainter veins that connect to form aereolae that are longer than wide; stems glabrous to pubescent, not glaucous
-
25a. Leaf blades linear to oblong-linear, often with revolute margins, 2–5 (–9) mm wide; base of stem thinner than 2.5 mm; rhizome thinner than 2 mm; plants mostly of high-pH fens
-
25b. Leaf blades narrow-lanceolate to lanceolate, elliptic, or oblanceolate, with plane margins, 5–25 mm wide; base of stem thicker than 2.5 mm; rhizome thicker than 2 mm; plants mostly of lake and river shores, saltmarshes, and moist fields
-
26a. Leaf blades narrow-lanceolate to oblanceolate, narrowing to the base, flat, herbaceous to fleshy; most peduncles less than 3 cm long; involucral bracts often loose or somewhat spreading apically, commonly green in large part (in part)
-
26b. Leaf blades tending to be narrow-lanceolate, scarcely narrowing to the base, folded along the midrib, coriaceous; many peduncles longer than 3 cm; involucral bracts usually appressed and with a chartaceous base and green apex (rarely loose, expanded and foliaceous apically) (in part)
-
-
-
-
23b. Lobes of disk corolla comprising 30–45% of the limb; ray flowers commonly white
-
27a. Involucre 2.5–3.5 (–4) mm tall; capitula often numerous and secund along the branches
-
28a. Bracteal leaves usually shorter than 1.5 cm, conspicuously reduced in size from the stem leaves
-
28b. Bracteal leaves, at least some of them, usually longer than 1.5 cm, gradually reduced in size from the stem leaves (in part)
-
-
27b. Involucre 4–6 mm tall; capitula usually neither crowded nor notably secund
-
29a. Stems 1–6 dm tall; stems 1–3 mm thick at the base; leaf blades 3–10 ×0.3–1 cm; capitula with 15–30 ray flowers; rays 3–8 mm long; plants of rocky shores
-
29b. Stems 5–15 dm tall; stems 3–6 mm thick at the base; leaf blades 8–15 ×0.3–3.5 cm; capitula with 20–40 ray flowers; rays 4.5–12 mm long; plants mostly of wet-mesic to hydric fields, ditches, and open swamp edges (in part)
-
-
-
-
-
-
-
-
-
-
-
Show photos of:
Each photo represents one species in this genus.