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Where does
Asclepias
fit in the big picture?
Traditional view
One of 315 genera in the family
Asclepiadaceae
Approximately 2,000 species
Tropical, subtropical, warm temperate regions
In the Midwest,
Asclepias, Cynanchum
,
Matelea
Current view
One of 400 genera in the family
Apocynaceae
Approximately 4,555 species
Tropical to warm temperate regions
In the Midwest,
Asclepias, Cynanchum
,
Matelea,
Apocynum, Amsonia, Vinca
Characteristics common to the old “
Asclepiadaceae
” and Apocynaceae
* milky latex; leaves simple, opposite/whorled, entire; flowers perfect, actinomorphic
* 2 carpels, ovaries distinct, stigmas fused to each other (connate); 5 stamens; seeds comose
*
5 sepals mostly connate
vs.
5 sepals distinct or connate
*
5 petals, mostly fused, lobes reflexed
vs. 5 petals mostly fused, scale-like appendages inside
*
stamen filaments fused to each other and to the corolla tube, anthers adnate to stigma
vs.
stamen filaments distinct, fused to (inserted on) the corolla tube, anthers adherent to stigma
*
pollen in pollinia
vs. pollen in monads, sometimes tetrads
*
fruit is a follicle
vs. fruit is a follicle, capsule, berry, drupe
Distinctive differences:
Asclepias
has - corona, hoods, horns
- gynostegium
Asclepiadaceae
Gynoecium
- carpels
stigmatic disk
Perianth
- sepals
- petals
Androecium
- stamens
filaments
tube
anthers
corona
hood
horn
Gynostegium
styles
stamens filaments (tube)
anthers
pollinium
corpusculum
translator
stigmatic slit
Iowa
Asclepias
Reference Table. Data compiled by Dr. Thomas Rosburg from Eilers and Roosa 1994, Kartesz 2015, Voss and Reznicek 2012,
Yatskievych 2006, Kaul et al. 2006, Christiansen and Muller 1999
Flora of North America Eilers and Roosa 1994 Habitat Iowa Biogeography (IPP) BONAP Biogeography
Status: native Iowa CC: 4 to 6 H
Asclepias amplexicaulis
sand milkweed
sand prairies, sandy soil, dry to dry-mesic prairies; savannas, glades; dry, upland oak woodland and open forest
Status: not present Iowa CC:
Asclepias engelmanniana
eared milkweed
rocky to sandy mid-grass
and shortgrass prairies not present
Status: native Iowa CC: 6 to 6 H
Asclepias exaltata
poke milkweed
mesic, upland forest, forest edges and mesic woodland
BONAP Status: native Iowa CC: 5 to 5 M Asclepias hirtella prairie milkweed tall greed milkweed
BONAP:
Asclepias longifolia var. hirtella
dry-mesic to mesic prairie,
sand prairies, rocky glades.
Status: native Iowa CC: 4 to 4 H
Asclepias incarnata swamp milkweed
temporarily flooded marshes, wetland edges, sedge or wet meadows and wet-mesic prairies, prairie swales, wet ditches
Status: State Threatened Iowa CC: 9 to 9 L
Asclepias lanuginosa = A. nuttalliana
= A. otarioides
wooly milkweed
dry, sandy or gravelly prairie; mid-grass and sand prairies; sandy to rocky, often calcareous soils
INAI
red = none after 1960 purple = needs verification
Status: State Endangered Federally Threatened
Iowa CC: 10 to 10 H
Asclepias meadii Mead’s milkweed
dry-mesic to mesic prairies,
upland prairies and igneous glades
INAI
red = none after 1960 Status: native
Iowa CC: 6 to 8 L
Asclepias ovalifolia oval-leaf milkweed
dry prairies, hill and sand prairies; loam and sandy savannas; upland, open oak woodlands
Status: native Iowa CC: 7 to 7 H
Asclepias purpurascens purple milkweed
woodland, savanna, glades, upland prairies, blufftops, mesic to dry-mesic open upland forests; meadows in wooded areas, woodland borders BONAP Status: native Iowa CC: 8 to 8 H Asclepias quadrifolia four-leaf milkweed
dry to mesic forests; rich or dry rocky open woodlands, on upland slopes or ridges
BONAP Status: State Threatened
Iowa CC: 6 to 8 L
Asclepias speciosa showy milkweed
wet-mesic, mesic to dry-mesic prairie, pasture, open woodland; in western areas in pastures, meadows, forest clearings, fields, roadsides, sometimes described as weedy
INAI Status: State Endangered
Iowa CC: 7 to 9 M
Asclepias stenophylla narrow-leaf milkweed
dry, deep loess or gravel prairie; sandy or rocky limestone prairies; savannas, glades, exposed ledges and tops of bluffs
Status: native Iowa CC: 7 to 8 H
Asclepias sullivantii Sullivant’s milkweed smooth milkweed
mesic, wet-mesic prairie;
shorelines, marsh borders, streambanks
Status: native Iowa CC: 0 to 1 H
Asclepias syriaca common milkweed
disturbed mesic to dry-mesic soils Status: native Iowa CC: 6 to 7 H Asclepias tuberosa subsp. interior butterfly milkweed
dry to mesic prairie, pastures, often associated with perturbation or disturbance Status: native Iowa CC: 0 to 1 H Asclepias verticillata whorled milkweed
mesic to dry prairies, pastures, roadsides
Status: native Iowa CC: 6 to 7 H
Asclepias viridiflora
green milkweed
dry to mesic prairies, sand prairies
Status: native Iowa CC: pending
Asclepias viridis
spider milkweed
dry prairies, pastures, disturbed ground, on sandy or rocky limestone soils
INAI Fields in the Asclepias Reference Table
1-Currently accepted scientific name in Flora of North America. Iowa status (if listed) and data concerning occurrence in Iowa. Iowa Coefficient of Conservatism. 2-Nomenclature in Eilers and Roosa 1994, synonyms. Common names.
3-General habitat description
4-Iowa distribution and expected native range, based on Christiansen and Muller 1999 for some species, based on the Iowa Natural Areas Inventory or BONAP for other species
5-Biogeographical range in the United States according to BONAPand Kartesz 2015 Hybrid species
A. syriaca and A. purpurascens is reported in Missouri
A. syriaca and A. speciosa is widely reported
A. sullivantii and A. syriaca is reported in Ohio
A. exaltata and A. syriaca is reported in Virginia
Asclepias species unknown for Iowa, but which occur in adjacent states
Asclepias arenaria(NE, SD)
Asclepias asperula (NE)
Asclepias latifolia (NE)
Asclepias perennis (MO, IL)
Asclepias subverticillata (MO)
References
Christiansen, P. and M. Muller. 1999. An Illustrated Guide to Iowa Prairie Plants. University of Iowa Press, Iowa City, IA
Eilers, L.J. and D.M. Roosa. 1994. The Vascular Plants of Iowa: An Annotated Checklist and Natural History. University of Iowa Press, Iowa City, IA
Kartesz, J.T. 2015. The Biota of North America Program (BONAP). North American Plant Atlas. http://bonap.net/napa. Chapel Hill, NC.
Kaul, R.B., D.M. Sutherland, and S.B. Rolfsmeier. 2006. The Flora of Nebraska. School of Natural Resources, University of Nebraska-Lioncoln, Lincoln,
NE, 966 pages
Voss, E.G. and A.A. Reznicek. 2012. Field Manual of Michigan Flora. University of Michigan Press, Ann Arbor, MI.
Yatskievych, G. 2006. Steyermark’s Flora of Missouri Volume 2. Missouri Botanical Garden Press, St. Louis MO and Missouri Department of
Conservation, Jefferson City, MO.
13
Asclepias
species
GROUP A
Most of the leaves alternate
GROUPS B, C, D, E, F
Leaves opposite or whorled
* corollas orange, reddish orange,
yellowish orange; sap clear
* corollas greenish, pale green to
yellowish green; sap milky
butterfly
flowers 9.5 to 12.5 mm tall; corona
hoods 4.0 to 5.0 mm long; corolla
lobes 5.2 to 7.0 mm long
flowers 7.5 to 9.5 mm tall; corona
hoods 1.5 to 2.5 mm long; corolla
lobes 4.0 to 6.0 mm long
green
prairie
1-butterfly
2-butterfly
4-butterfly
5-butterfly
6-green
7-green
9-green
8-green
10-green
3-butterfly
GROUP A
Most of the leaves alternate
GROUPS B, C, D, E, F
Leaves opposite or whorled
* leaves all or nearly all in whorls; leaves linear,
0.5 to 2.5 mm wide
* leaves all opposite, or if whorls present then
blades > 5 mm wide GROUPS C, D, E, F
GROUP B -- whorled
1-prairie
2-prairie
3-prairie
4-prairie
5-prairie
6-whorled
7-whorled
8-whorled
9-whorled
GROUP C
Corona hoods lacking horns
(see GROUP A)
GROUPS C, D, E, F
GROUPS D, E, F
Corona hoods each with a horn
green
Corona hoods shorter than or at most
slightly longer than the gynostegium
head; the horns extended well beyond
the tips of the hoods GROUP D
Corona hoods clearly longer than or at most slightly longer
than the gynostegium head; the horns arched over the
gynostegium head, but not extended beyond the tips of
the hoods GROUPS E, F
* leaves sessile; leaf bases cordate to
truncate, ± clasping so that the
bases of leaves at the same node
overlap; leaf margins wavy
* leaves petiolate, a petiole at least
3 mm long present; leaf bases
narrowed (wedge-shaped) or
rounded, sometimes slightly cordate
but not clasping; leaf margins flat
sand
corollas white or greenish, 7-10 mm
long; flowers on pedicels 2.5 to 4.5 cm
long; umbels nodding; woodland
habitats
corollas pink to rose-purple, 4-6 mm
long; flowers on pedicels 0.8 to 1.6 cm
long; umbels erect; wetland
habitats
poke
swamp
1-sand
2-sand
3-sand
4-sand
5-sand
GROUP D
1-poke
2-poke
3-poke
4-poke
5-poke
6-poke
7-swamp
8-swamp
9-swamp
10-swamp
11-swamp
GROUPS E, F
GROUP E
Ventral surface of leaves moderately to
densely pubescent (villous, tomentulose)
GROUP F
Ventral surface of leaves glabrous to
glabrate (sometimes sparsely pubescent
on leaf margin or midvein)
* corona hoods 9.0 to 15.0 mm long;
narrowly lanceolate, strongly narrowed
to an acute tip
* corona hoods 3.3 to 8.0 mm long;
broadly lanceolate to oblong or ovate,
gradually narrowed to a rounded
somewhat acute tip, more broadly
rounded obtuse tip, or truncate tip
showy
margins of hoods without
prominent lobes; corollas reddish
purple, deep red to dark purple
margins of hoods with a pair of
sharp, triangular lobes at or near the
midpoint; corollas white,
greenish-white, whitish lavender, rose or pink
purple
●
corolla lobes 5.0 to 6.0 mm long,
greenish-white sometimes tinged with
lavender or rose; follicles 6.0 to 8.0 mm
long
●
corolla lobes 6.5 to 10.0 mm long,
light rose to purple (lilac, pink,
lavender), whitish; follicles 7.0 to 11.0
mm long
oval-leaf
common
1-showy
2-showy
3-showy
4-showy
5-purple
6-purple
7-purple
8-purple
GROUP E
1-oval-leaf
2-oval-leaf
3-oval-leaf
4-oval-leaf
5-oval-leaf
6-common
7-common
8-common
9-common
10-common
11-common
12-common
GROUP F
* leaves opposite, sessile or subsessile; leaf bases
rounded to slightly cordate; corolla lobes 9.0-12.0
mm long, light pink to purplish-pink
* leaves opposite, but 1 or 2 mid-cauline internodes
very reduced and appearing as 1 or 2 whorls of 4
leaves each, distinctly petiolate; leaf bases
narrowed or tapered; corolla lobes 4.5-6.0 mm long,
light or pale pink to white
four-leaf
Sullivant’s
1-Sullivant’s
2-Sullivant’s
3-Sullivant’s
4-Sullivant’s
5-Sullivant’s
6-four-leaf
7-four-leaf
8-four-leaf
9-four-leaf
10-four-leaf
11-four-leaf
Iowa
Asclepias
Thomas R. Rosburg (January 2021)
Adapted from keys in Yatskievych 2006 (Steyermark’s Flora of Missouri Volume 2), Voss and Reznicek 2012
(Michigan Flora) and Hartman 1986 (Flora of the Great Plains)
1a. Most of the leaves alternate
2a. Corollas orange, reddish orange, yellowish orange; sap clear ………….
A. tuberosa
(butterfly milkweed)
2b. Corollas greenish, pale green to yellowish green; sap milky
3a. Calyx lobes spreading; corolla lobes 10-17 mm long, ascending to spreading……….…..
A. viridis
(spider milkweed)
3b. Calyx lobes reflexed; corolla lobes 4-8 mm long, reflexed
4a. Flowers 9.5 to 12.5 mm tall; corona hoods 4.0 to 5.0 mm long; corolla lobes 5.2 to 7.0 mm long
……….…
A. viridiflora
(green milkweed)
4b. Flowers 7.5 to 9.5 mm tall; corona hoods 1.5 to 4.0 mm long; corolla lobes 3.8 to 6.0 mm long
5b. Horns present; leaf margins usually revolute, leaf blades 1 to 6 mm wide…...
A. stenophylla
(narrow-leaved milkweed)
5a. Horns absent; leaf margins flat, leaf blades 3 to 20 mm wide
6a. Gynostegium “stalked”, i.e., lower portion visible between the corona and corolla; hoods
overlapping the lower ⅓ or less of the gynostegium head
…..
A. hirtella
(prairie milkweed)
6b. Gynostegium “sessile”, i.e., lower portion not easily visible, the corona base touching, or
nearly so, the upper corolla; hoods overlapping ⅔ or more of the gynostegium head
……….…….
A. lanuginosa
(wooly milkweed)
1b. Leaves opposite or whorled
7a. Leaves all or nearly all in whorls; leaves linear, 0.5 to 2.5 mm wide ………..…..
A. verticillata
whorled milkweed
7b. Leaves all opposite, or if whorls present then blades > 5 mm wide
8a. Corona hoods lacking horns
9a. Corolla lobes reflexed, 6-8 mm long………..…..……
A. viridiflora
(green milkweed)
9b. Corolla lobes spreading to ascending, 10-17 mm long ……..…
A. viridis
(spider milkweed)
8b. Corona hoods each with a horn
10a. Corona hoods shorter than or at most slightly longer than the gynostegium head; the horns
extended well beyond the tips of the hoods
11a. Leaves sessile; leaf bases cordate to truncate, ± clasping so that the bases of leaves at
the same node overlap; leaf margins wavy………….
A. amplexicaulis
(sand milkweed)
11b. Leaves petiolate, a petiole at least 3 mm long present; leaf bases narrowed (wedge-
shaped) or rounded, sometimes slightly cordate but not clasping; leaf margins flat
12a. Corollas white or greenish, 7-10 mm long; flowers on pedicels 2.5 to 4.5 cm long;
umbels nodding; woodland habitats……….….
A. exaltata
(poke milkweed)
12b. Corollas pink to rose-purple, 4-6 mm long; flowers on pedicels 0.8 to 1.6 cm long;
umbels erect; wetland habitats ……….……..
A. incarnata
(swamp milkweed)
10b. Corona hoods clearly longer than or at most slightly longer than the gynostegium head; the
horns arched over the gynostegium head, but not extended beyond the tips of the hoods
13a. Ventral surface of leaves moderately to densely pubescent (villous, tomentulose)
Iowa
Asclepias
14a. Corona hoods 9.0 to 15.0 mm long; narrowly lanceolate, strongly narrowed to an acute tip
.……….……….
A. speciosa
(showy milkweed)
14b. Corona hoods 3.3 to 8.0 mm long; broadly lanceolate to oblong or ovate, gradually narrowed to a
rounded somewhat acute tip, more broadly rounded obtuse tip, or truncate tip
15a. Margins of hoods without prominent lobes; corollas reddish purple, deep red to dark purple
……….………
A. purpurascens
(purple milkweed)
15b. Margins of hoods with a pair of sharp, triangular lobes at or near the midpoint; corollas white,
greenish-white, whitish lavender, rose or pink
16a. Corolla lobes 5.0 to 6.0 mm long, greenish-white sometimes tinged with lavender or rose;
follicles 6.0 to 8.0 mm long, without tubercles but densely puberulent……...
A. ovalifolia
(oval-leaf milkweed)
16b. Corolla lobes 6.5 to 10.0 mm long, light rose to purple (lilac, pink, lavender), whitish;
follicles 7.0 to 11.0 mm long, smooth or densely covered with soft, subulate prickles and
densely tomentose……….………
A. syriaca
(
common milkweed)
13b. Ventral surface of leaves glabrous to glabrate (sometimes sparsely pubescent on leaf margin or midvein)
17a. Leaves sessile or subsessile; leaf bases rounded to slightly cordate
18a. Corollas white to pale cream to greenish-cream, sometimes tinged with purple; leaves mostly in
2 to 5 pairs per stem, leaf margins with minute rough hairs……….
A. meadii
(Mead’s milkweed)
18b. Corollas light pink, purplish-pink, pinkish-rose; leaves in 7 to 15 pairs per stem, leaf margins
glabrous………..…..
A. sullivantii
(Sullivant’s milkweed)
17b. Leaves distinctly petiolate; leaf bases narrowed or tapered…….
A. quadrifolia
(four-leaf milkweed)
Plant Glossary
Reproductive Terms
Achene – a single seeded indehiscent dry fruit with the seed free from the pericarp except at the funicule (the stalk of an ovule attaching it to the placenta of the ovary)
Calyx – collective term for the sepals of a flower, the lower and outermost (or first) whorl of flower parts
Corolla – collective term for the petals of a flower, the second whorl of flower parts
Corona – an apparent third sterile whorl of a perianth derived from appendages of either petals or filaments
Cypsella – a certain type of achene characteristic of the Asteraceae, developed from an inferior ovary and usually bearing a pappus; dry, one-seeded, indehiscent
Follicle – a dry, dehiscent fruit derived from a single carpel that dehisces either along the suture or along the midrib but not both (1 line of dehiscence)
Gynostegium – a structure in Apocynaceae and some Aristolochiaceae formed from adnation of the stamens to the stigma; fusion of the androecium and gynoecium
Inflorescence – the part of a shoot above the uppermost node with foliage leaves that bears flowers, also, the groupings or arrangements in which these flowers are borne
Involucre – one or more whorls of bracts immediately subtending a flower or inflorescence, often forming a cup-like structure
Pappus – specialized and modified calyx consisting of scales, bristles, or awns characteristic of the Asteraceae
Perianth – the collective term for all of the outer (lower) sterile parts of a flower, comprising the calyx and corolla when both are present
Phyllary – one of the involucral bracts present in the involucre of a head (or capitulum) inflorescence in Asteraceae
Pollinium – a coherent mass of pollen shed as a unit in some Apocynaceae and Orchidaceae
Primary inflorescence – the arrangement of individual flowers or florets
Radial head – inflorescence in the Asteraceae bearing disk flowers in the center and ray florets around the periphery
Secondary inflorescence
– the arrangement of the primary inflorescences
Vegetative Terms
Areole – the non-vascularized spaces or tissue between the veins and veinlets of a net-veined leaf
Cauline – describing leaves borne on an aerial stem, usually separated by elongated internodes
Caulescent – possessing a stem visible above the ground
Clasping – a sessile leaf with lobes of blade tissue projecting around either side of the stem
Crenate – margin with regular rounded teeth making a scalloped margin
Crenulate – minutely crenate, with very small rounded teeth
Entire – margin that is smooth or of unbroken outline, without teeth
Glabrous – surface smooth or lacking trichomes (plant hairs, or epidermal outgrowths)
Glaucous – a bluish-green, pale gray/whitish waxy surface covering
Hispid – pubescent with stiff bristle-like hairs
Involute – the margins of a flat surface rolled inward toward the upper surface
Node – the joint (or transverse plane) of a stem at which one or more leaves and associated axillary buds arise
Petiolate – a leaf possessing a stalk or petiole, attached by a leaf stalk
Puberulent – pubescent with very short hairs, minutely pubescent
Pubescent
– surface with trichomes present
Scabrous
– pubescent with short, stout hairs making the surface feel like sandpaper
Serrate
– sawtooth margin with sharp teeth bent toward the leaf apex
Serrulate – minutely serrate, with very small teeth bent toward the leaf apex
Sessile – a leaf blade attached directly to a node, lacking a petiole
Sheathing – a modified petiole that is prolonged into a tube that partially or completely surrounds the stem above the node to which the leaf is attached
Striate – with several parallel longitudinal lines or ridges, often rather fine and close, usually separated by grooves
Strigose – pubescent with short hairs that lie flat against the surface
Subentire – nearly or almost entire
Adnate – fusion of unlike parts (e.g., stamens adnate to petals)
Connate – fusion of two or more structures of the same kind (e.g., a sympetalous corolla results from the fusion of petals to one another)
Distal – remote from the point of origin or attachment (e.g., in regard to leaves, near the top of the stem)
Distinct – not fused to parts of the same type or whorl
Free – not fused to other kinds of structures
