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Identification of Small-flowered, White-rayed "Asters"
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Arieh Tal, 2016

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Skip directly to the key below . . .
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The numerous, white-flowered aster species known in the northeastern states have long been considered difficult to identify.  This discussion, and the key below, will hopefully help.

But first, a caveat.  The term "aster" will be used here in the vernacular.  About 10-15 years ago the species that we call asters in the northeastern states and provinces were moved by botanical authorities from the genus Aster, into several other genera.  With one exception (Aster tataricus, a garden escape), our species are all native to North America.  Our native species form an informal grouping, the members of which are not as closely related phylogenetically as you may think.  In fact, one genus, Sericocarpus, is more closely related to goldenrods than to asters.

Most of our white-rayed species have relatively small flower heads.  Most of the asters with larger heads tend to have bluish or purple rays.  Toward the bottom of this page, you will find a chart that provides comparative flower head measurements for the species covered here.  The measurements were made by the author between 2008 and 2013, from numerous populations in New England, mostly in Massachusetts.  The measurements are based on the width of the flower heads, diagonally from the tip of one ray to the tip of a ray diametrically opposite, when the heads were in full flower.  Though this method of measurement is not usually employed in technical publications, it is somewhat more intuitive and convenient for the casual observer, who may not want to keep a hand lens and a scale continuously in hand.  Contact me at the address provided on this website if you need more details of how the measurements were made.

The species included in this analysis were selected based on flower head size.  That is, the included species all have relatively "small" flower heads.  Average (mean) flower head sizes per species, of the white-rayed species, form a continuous range from about 11mm to approximately 26mm.  Species excluded from this analysis have average (mean) flower head sizes greater than 21mm, and include Doellingeria, Eurybia and Oclemena species.  That leaves us with Sericocarpus and Symphyotrichum.

Caveat:  Additionally, the included species all typically have white rays, but forms with pale bluish rays may sometimes be found, though usually they are uncommon or rare.  Some of those may be hybrids.  This information will be noted in the chart below.

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Sericocarpus and Symphotrichum can be distinguished as follows:
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                  Figure 1.  (left) habit of Sericocarpus asteroides.  (right) Habit of Symphyotrichum pilosum.
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SericocarpusCorollas of ray and disc florets are both white.  Flower heads are typically in glomerules (i.e., compactly clustered without peduncles).  Ray florets number 2-7 per head.  Bases of dark green zones are flat (truncated), the green zones appearing "thumbnail-shaped".  Branching occurs at the summit of the stem.  The overall inflorescence is flat- to round-topped.  Same for branch inflorescences.  If you prefer technical terms, branching is corymb-like.
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Symphyotrichum:  Ray floret corollas are white (rarely bluish, or bluish in shade).  Disc floret corollas vary from pale to bright yellow.  Flower heads are borne on evident peduncles.  Ray florets number 8 or more per head.  Bases of dark green zones are tapered, the green zones appearing diamond- or lens-shaped (oblanceolate).  Branching, for species included in this analysis, often occurs from near the base of the stem upwards*.  The overall inflorescence is peaked, usually broadest at base, becoming progressively narrower toward the summit.  Branch inflorescences are usually elongated, with flower heads distributed along most of branch lengths, and sometimes one-sided, or clustered toward branch tips, but not flat-topped.  If you prefer technical terms, branching for these species is raceme-like or panicle-like.
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* This character is especially typical for the species described in the key below, but a few other Symphyotrichum species, not included here, usually produce branches only in the upper 1/4 to 1/3 of the stem.

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         Key to Symphyotrichum Species (with white rays & heads up to 21 mm wide)

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1a.  Basal and lower stem leaves are often present at flowering, borne of evident peduncles; some leaves are
       heart-shaped.  Involucral bracts are loosely appressed to spreading/recurved slightly, not spine tipped.   S. urophyllum.
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1b.  Basal and lower stem leaves are typically not present at flowering (except sometimes for S. tradescantii).
       
Stem leaves are neither heart-shaped nor borne on evident peduncles.  Involucral bracts are
        appressed or spreading, some also recurved and with spine-like tips.
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         2a.  Involucral bracts are spreading and/or recurved, spine tipped, or with a tip that appears spine-like.
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                3a.  Peduncles are relatively short, of more or less uniform length, 5-10(-20) mm.  Disc florets
                       6-12(-20) per head.  Branch leaves are narrowly elliptic, terminated by a minute spine tip.  Involucral
                       bracts are spreading or reflexed, spine-tipped.  Heads are extremely crowded .              S. ericoides
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                3b.  Peduncles are relatively longer, of highly variable length 5-30(-50) mm.  Disc florets (13-)17-39(-67)
                        per head.  Branch leaves are linear to lanceolate, or subulate, mostly ascending, narrowed to a
                        long, slender, spine-like tip.   Heads are not highly crowded.              S. pilosum
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         2b.  Involucral bracts are appressed or spreading, but not spine tipped (the tips may be short-pointed).
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                 4a.  Corollas of disc florets are pale yellow, cream color or translucent, turning pink/magenta with age.
                        Lobes of disc corollas are spreading or reflexed.
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                         5a.  Lobes of disc corollas are ascending to spreading, but not reflexed, comprising less than
                                45% of corolla limbs.  Basal and lowermost stem leaves are sometimes persistent at flowering.
                                Stem and branch leaves are ascending.  Heads tend to be borne at ends of branches in
                                small numbers.               S. tradescantii
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                         5b.  Lobes of disc corollas are typically reflexed, comprising 50% or more of corolla limbs.
                                 Basal and lowermost stem leaves are not usually persistent at flowering.  Stem and branch
                                 leaves are spreading to reflexed.  Heads are often borne along much of branch lengths.
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                                     6a.  Ray florets are 8-15(-23) per head.  Lobes of the disc corollas are strongly reflexed.
                                            Leaves are typically pubescent primarily along the main vein of leaf lower surface. 
                                            S. lateriflorum
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                                     6b.  Ray florets are (10-)15-26 per head.  Lobes of the disc corollas are slightly reflexed.
                                            Leaves are glabrous (var. glabratum) or uniformly pubescent on the lower surface
                                            (var. ontarionis).             S. ontarionis
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                 4b.  Corollas of disc florets are bright yellow, turning reddish with age.  Lobes of disc corollas are
                         spreading to ascending, but not very recurved.
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                         7a.  Flower heads are typically crowded in one-sided order along most of branch lengths.   Flower
                                head diameters (across rays) 7-14 mm.              S. racemosum
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                         7b.  Flower heads are not highly crowded on branches, nor conspicuously in one-sided order.
                                 Flower head diameters (across rays) 11-30 mm.
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                                        8a.  Peduncles are of conspicuously different lengths (some greater than 50 mm).  Branch leaflets
                                          and peduncle bracts are very small, widely spaced along branches and peduncles; oblong
                                          or elliptic.  Involucral bracts are well appressed, not or scarcely spreading.  Flower head
                                          diameters (across rays) 10-20 mm.   Mostly coastal or coastal plain.            S. dumosum
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                                        8b.  Peduncles are more nearly uniform in length.  Branch leaflets are variable in size, but
                                          not widely spaced along branches and peduncles; lanceolate or oblanceolate.  Involucral
                                          bracts are loosely appressed to sometimes spreading slightly. 
Flower head diameters
                                          (across rays) 11-30 mm.  Widespread.           S. lanceolatum
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Table 1.  Comparative head sizes of small-flowered, white-rayed asters of the northeastern U.S. states and Canadian provinces.  Measurements made across the flower heads, from tip of a ray to the tip of a ray diametrically opposite.
Smallest Largest
Num. Obs. Measurement Measurement Mean   Ray
(Populations) (mm) (mm) (mm)   Color
 
Sericocarpus asteroides 2 10 15 13   white
Symphyotrichum dumosum 2 10 20 16   white - rarely bluish
Symphyotrichum ericoides 6 8 16 12   white
Symphyotrichum lanceolatum 14 11 30 21   white - bluish
Symphyotrichum lateriflorum 10 8 15 12   white - bluish
Symphyotrichum ontarionis n/a n/a n/a n/a   white - rarely bluish
Symphyotrichum pilosum 9 10 22 17   white
Symphyotrichum racemosum 12 7 14 11   white - rarely bluish
Symphyotrichum tradescantii 1 12 14 13   white
Symphyotrichum urophyllum n/a n/a n/a n/a   white - rarely bluish

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Copyright Arieh Tal and  botphoto.com, 2016.  All rights reserved.  ( Terms of use )

Page updated 2016.11.05

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