Posted by A.L. Gibson Tuesday, June 25, 2013 http://floraofohio.blogspot.com/2013/06/a-guide-to-milkweeds-of-ohio.html A Guide to the Milkweeds of Ohio It's been a while since I dedicated a post to the identification and study of a specific group of plants. I meant to get this particular one out this time last year but never got around to it so what better time than now! The milkweeds. One of my favorite genera (Asclepias) in the vascular plant world for their fascinatingly unique flower structure as well as the array of colors and sizes they come in. All are perennial herbs with entire, (mostly) opposite leaves, and have seeds produced in a follicle (pod). Most but not all of Ohio's species have the classic milkweed characteristic of a latex-based milky sap. The genus name of Asclepias was given to the milkweeds by famed botanist and father of the modern binomial nomenclature system, Carl Linnaeus, after the Greek god of healing Asclepius (due to the milkweed's many medicinal uses back in the day). Here in Ohio we have 13 native species in a wide variety of habitats. Some are incredibly common and easily seen in just about every county in Ohio; others more rare with restricted geographical ranges and/or habitat requirements. The specimens and information presented here is of typical Ohio plants that I have come across during time out in the field. All 13 indigenous taxa are present in this post and displayed in alphabetical order by their scientific epithet. Fortunately, our milkweeds are all discernible from one another through photos showing leaves and/or flowers. For that reason, I did not include/use a key and think photos and descriptions will give what's needed to tell them apart. Being a treatment and ID post it's quite long so take from it and use it as you will! It's my hope to clear up and exhibit all Ohio's milkweed species in one easy-to-view and use place. I think each species stands out on its own and none are too hard to tell apart but that's why this post exists. I hope you've enjoyed it and maybe have been introduced to species you didn't know our state had, or perhaps even existed! If you have any comments, suggestions, or complains with the post feel free to let me know. I'd like it to be as helpful and accurate as possible! Posted by A.L. Gibson
Up first is one of our more uncommon and earliest blooming species: the clasping or blunt-leaved milkweed (Asclepias amplexicaulis). It
also happens to be one of the most obvious and easily distinguished of the milkweeds. It typically grows upwards of two-four feet tall
with a single terminal inflorescence at the end of a naked stalk well above the highest pair of leaves.
Speaking of leaves, this is the only species that exhibits entirely sessile, clasping leaves that hug the stem. No other Ohio milkweed has
clasping leaves this obvious and noticeable. Another intriguing aspect to A. amplexicaulis is the color variation of its inflorescences. They
can range from a wine-red color (shown above) to a yellowish tan (show on the right).
Clasping milkweed is a species of concern in Ohio and has two distinct population centers: the northwest corner and the south-
central/southeastern areas. It grows in a variety of dry, sandy/gravelly, and open habitats like roadside/embankments, fields, meadows,
prairies, and dunes.
Poke Milkweed (Asclepias exaltata)
Poke milkweed (A. exaltata) is one of our green-flowered milkweeds but still quite easily told apart by its habitat, leaves, and
flowers. This species can grow upwards of four feet tall and can have multiple flower umbels that tend to droop and hang over. Each
individual inflorescence is a very unique combination of green and white and hangs on a long peduncle in the open, lax umbel. Poke
milkweed's leaves are rather large and more or less evenly sized up the stem. They are similar in size and shape to pawpaw (Asimina
triloba) leaves to me.
Poke milkweed seems to be the most shade-tolerant of all our species. It grows in woodland settings from dry, upland forests to more
mesic deciduous forests in a variety of soil conditions. I have seen this species growing and flowering (the specimen above right) in the
dense shade of a sandstone gorge as well as a rocky, upland oaks woods under similar lighting conditions. Few other milkweed species
grow in full shade conditions. Poke milkweed is widely-scattered throughout the state but is predominately found in the eastern and
south/eastern counties. It blooms from late May through June and into July.
Tall Green Milkweed, Prairie Milkweed (Asclepias hirtella)
An uncommon and distinct species of mid-late summer is tall green milkweed (A. hirtella), otherwise known as prairie milkweed. Its small, spherical
umbels of tiny greenish-yellow flowers look like mini pompom balls or exploding fireworks and occur along the top third of the densely pubescent
stem. Tall green milkweed's leaves are very narrow and linear in shape with an alternate attachment. It's a very attractive species when seen in full
bloom and clumped together.
Tall green milkweed is restricted to largely the same areas as the aforementioned clasping milkweed in the south/east and northwest. It grows in
mesic-damp fields, prairies, and roadsides and typically blooms late June into August. Worth noting is the existence of the longleaf milkweed (A.
longifolia); an excruciatingly similar looking species of the Atlantic coastal plain and Gulf states. Some authors and taxonomists keep them separate
species; others recognize them as subspecies. I won't get into the differences here as only A. hirtella occurs in Ohio.
Swamp Milkweed (Asclepias incarnata)
Swamp milkweed (A. incarnata) is one of our most common species and can be seen in literally any of Ohio's 88 counties come mid-late summer. The
plants can vary greatly in size depending on the habitat conditions from two to over five feet in height. They are easily identified by the terminal
flower umbels consisting of dozens of small, bright pink flowers. The leaves are oppositely arranged and are narrow to widely lanceolate in shape
with little to no petiole (and somewhat clasp the stem).
Swamp milkweed is a species that loves to keep its feet wet or at least moist. It grows in a very wide variety of hydric-mesic habitats and occurs in
high-quality to degraded environments. Floodplain forests, thickets, swamps, wet prairies/meadows/fields, roadside ditches, fens, marshes, and
along just about any waterway all are places you're likely to come across swamp milkweed.
Swamp milkweed is incredibly common across its wide range in North America. It's typically a mid-late summer bloomer here in Ohio with flowers
coming on in early July through August.
Purple Milkweed (Asclepias purpurascens)
Relatively common and nothing too special in its own right but purple milkweed (A. purpurascens) is a stunning plant that lights up the
roadsides and forest margins come early summer. Its deep, vividly purple-pink colored umbels of flowers occur at the apex of the central
stem. The large ovate-oblong lanceolate leaves are oppositely arranged and very similar to the common milkweed (A. syriaca).
Purple milkweed occurs throughout Ohio but is chiefly found in the south-central counties of the state. It blooms from late May into
July. This species is more adaptable and tolerant of shade and grows in open woods, thickets, forested margins, and roadsides.
Four-leaved Milkweed (Asclepias quadrifolia)
Typically our earliest and one of our smallest milkweeds to bloom is the aptly-named four-leaved milkweed (A. quadrifolia). Looking at the
photograph above left it isn't too difficult to see how it got the name 'four-leaved' milkweed as the lower leaves tend to form a whorl of four (rarely
three or five) leaves. The fewer-flowered terminal umbels occur typically in ones or twos with a light pink-white corollas. This is overall one of the
most glabrous (hairless) milkweed species.
Four-leaved milkweed is a frequently encountered species throughout Ohio, excluding the northwest quarter. It most commonly occurs in the
southern and eastern counties in dry-mesic woodlands and/or forest openings, borders, and margins. It blooms from mid-May to June and is one of
This next species just so happens to be your blogger's personal favorite: Sullivant's milkweed (A. sullivantii). This particular species was named after
the great Ohio botanist and bryologist William Starling Sullivant. Its large, terminal umbels contain fewer flowers (less than 20) than most other
milkweeds and have one the largest individual flowers. The flowers are a unique bubblegum pink color. Leaves are nearly sessile on very short
petioles and held +/- erect.
It can be confused with the common (and purple) milkweeds but can be told apart in a few key ways:
Sullivant's milkweed is an uncommon species in Ohio due to being a prairie obligate plant that only occurred in Ohio's historic tall grass prairie
openings. Much of Ohio's prairie landscape has been lost to the ages by development and agriculture and with it went this milkweed. It still occurs
throughout central/north-central and northwest Ohio, in prairie pockets, fence rows, roadside banks, and dry fields. It typically begins to flower in
late June and will bloom into August.
Sullivant’s and Commons and Purple Milkweed can be confused, but can be told apart in a few key ways:
Sullivant's Milkweed Purple Milkweed Common Milkweed
- starts blooming in late June - starts blooming in late May - starts blooming in mid June
- prairie obligate species, needs full sun - grows in woods/shade - grows just about anywhere/weed
- corollas large (10-12mm) - corollas (8-10mm) long - corollas (7-9mm) long
There's no mistaking this species of milkweed. Not even cruising down the highway at 70 mph will you fail to miss the butterfly milkweed
(A. tuberosa) when in full, glorious flower. The small, bright orange colored flowers in the numerous erect umbels are like crack-cocaine to passing
butterflies; you almost never see a lonely looking orange milkweed. The linear-oblong lanceolate leaves are scattered in an alternating fashion along
the roughly hairy stem.
Some specimens of butterfly milkweed will have flowers that exhibit a more yellow color rather than the typical orange. Regardless, there are no
other milkweed species in Ohio with bright orange or yellow flowers. An important characteristic and trait of this species is its clear colored sap. This
clear sap also has lower levels of the natural latex in its sap, which allows for butterfly caterpillars (like the Monarch) to have a lower mortality rate in
their early instars. Species with milky sap have higher levels of latex and cause the young caterpillars to get stuck and drown when eating the leaves
or especially when severing the main leaf vein to drain the leaf of sap.
Butterfly milkweed is another incredibly common species that can be found in just about every county of Ohio in many open habitats. Dry fields and
roadsides are heavily utilized by this species. It can be found in both high-quality and disturbed, degraded ecosystems.
White Milkweed, Red-ringed Milkweed (Asclepias variegata)
Moving on with this mega post on Ohio's milkweeds is one of our most stunning taxa. White milkweed (A. variegata) is another shade tolerant
species whose snow-white flowers stick out like a sore thumb in the shadows of its woodland home. Its broadly elliptic leaves are arranged
oppositely on the stem and sit on long petioles. The flower umbels are terminal and erect with each individual flower exhibiting a charming purplish-
red ring at the base of the hood. This is our only large statured white milkweed.
White milkweed is restricted to south-central Ohio and only extant in a handful of counties. It grows in dry, rocky woodlands, roadsides, and
thickets. The best place to see it in our state is to drive through the ridge top roads of Shawnee state forest come late spring and early summer. It's
an early blooming species that flowers from late May through June.
Whorled Milkweed (Asclepias verticillata)
Ohio's daintiest species might also be the most charming in the whorled milkweed (A. verticillata). It gets its name from the needle-thin, linear leaves
that whorl around the stem. The small, few-flowered umbels droop from the apex of the stem and rarely stand more than a couple feet
tall. Whorled milkweed's flowers are not nearly as white as the aforementioned white milkweed; I'd call them more of a cream color.
Whorled milkweed is a more western species that only gets into Ohio in a scattered fashion. It prefers dry, open/sunny conditions where it occurs in
dry fields, roadsides, and prairie openings. It's one of the later flowering species that doesn't really kick in until July and blooms into September.
Green-flowered Milkweed, Short Green Milkweed (Asclepias viridiflora)
The green-flowered milkweed (A. viridiflora) is another one of our uncommon species that has a seemingly random scattering of occurrences
throughout Ohio. It may be small but it's easily distinguished by its green colored flowers, horn-less corollas, and small, sessile umbels in the leaf
axils. This species seems to have a weak stem as I rarely ever see it standing erect but usually bent over and laying atop nearby vegetation. Its all-
green color and small stature help it to easily blend in and be overlooked.
Green-flowered milkweed grows in dry, open areas of grassy fields, slopes of goat prairies, roadside banks, and prairie openings. It tends to grow in
higher quality environments than disturbed ones. It blooms later in the year from late June into August. Best place to see it with any consistency is
the limestone barrens and prairies of Adams county (e.g. Lynx Prairie).
The last and final species of Ohio native milkweed certainly isn't last in looks. Right behind the Sullivant's milkweed on my list of favorites is spider
milkweed (A. viridis); a showy and peculiar species of extreme southern Ohio. There's no other milkweed that looks like this species with its large,
open green flowers. It rarely grows more than a couple feet tall and is our most geographically-restricted species.
Spider milkweed is a southern species that barely makes it across the river and into Ohio at the northeastern fringes of its range. It's locally common
in the Adams county area in the prairie openings and limestone barrens as well as roadsides and dry fields. It is one of our earliest blooming species
that flowers starting in mid-late May into mid-June.
