Love ’em or Hate ’em: the Milkweeds

True to their name, milkweeds are both milky and weedy. “Milk” refers to the white latex found within, a chemical defense against herbivory. And the plants are weedy: some species are on US state and Canadian province weed lists. Farmers hate them because many species are tall plants with massive root systems that can out-compete crops.  But most of the rest of us love them for attracting bees and butterflies.

I love them because the flowers themselves are fantastically complex.

The inflorescence is an umbel. Each individual flower consists of five sepals and five petals, and in most species five hoods and five horns. The hoods enclose the gynostegium, a complex structure consisting of fused stamens and styles that is unique to the genus Asclepias. It gets even more complicated than that; if you’re interested in the topic, there’s a detailed but not too technical explanation at the Orbis Environmental Consulting website.

The genus Asclepias was once placed in its own family, Asclepiadaceae, which is what you’ll find in older texts. Currently it’s place in the Apocynaceae (dogbane family), subfamily Asclepiadoideae. A dozen species of Asclepias can be found in Maryland, all native and all but one occurring in the Piedmont.

Asclepias incarnata (swamp milkweed) is found in most of the US except Alabama, Arizona, and the Pacific coast states, and in the eastern half of Canada. It prefers wet soils, and can grow two to six feet tall. The leaves are narrower than those of most other milkweed species. In the Potomac Gorge I’ve seen it blooming from late June to late August.

Asclepias quadrifolia (four-leaved milkweed) has a more limited range: it’s found in Ontario and the eastern half of the US, excepting some of the northernmost and southernmost states, and seems to be concentrated in the Appalachians and the Ozarks. It’s endangered in New Hampshire, threatened in Rhode Island, and uncommon in Vermont (per the New England Wild Flower Society). The species prefers drier soils in woodlands. The whorl of four leaves makes it easy to identify.

Asclepias syriaca (common milkweed) is found in the eastern US and Canada and a few western states, generally in full sun on dry or poor soils. In the Potomac Gorge I find it in soil pockets on the bedrock near the river, blooming in mid June to mid July. It’s on several authorities’ weedy plants lists. The flowers are a dusky pink as opposed to the bright pink of swamp milkweed, and the leaves are much broader.

Asclepias verticillata (whorled milkweed) is found in most of the eastern and central US and Canada, in open areas with full sun and poor soils. It’s on several weedy plants lists, but is also threatened in Massachusetts and special concern in Rhode Island. In Maryland it’s S3/watchlist. I’ve never seen it in the wild; the plant pictured here survived the rabbit onslaught in my garden. Note how narrow the whorled leaves are.

Asclepias viridiflora (green comet milkweed) is widespread across the US and Canada, though missing from the West and most of New England. It’s endangered in Florida, threatened in New York, special concern in Connecticut. It’s uncommon in Maryland; look for it blooming from mid June to mid August in dry open areas, especially serpentine barrens.

This last one is not technically a milkweed, but it’s close. Cynanchum laeve (honeyvine) is a sprawling vine that’ll grow up and around everything, so it is weedy. And it does exude latex. And monarch caterpillars (and other milkweed butterfly caterpillars) do feed on it, so you may as well think of it as a milkweed. In Maryland it’s found in the coastal plain, and also in the Piedmont part of Montgomery County. I’ve been seeing it along the banks of the Potomac, twining ’round late-flowering thoroughwort, rosemallows, and anything else it can get to.

Early Spring Aliens

At the end of February, technically still wintertime, I saw this carpet of white on the Cabin John Trail. It wasn’t spring beauties, and it wasn’t leftover snow, either.

This is Galanthus nivalis (snowdrops; Amaryllidaceae), a perennial native to Europe. It naturalizes well, meaning you can plant a few dozen bulbs and enjoy the show year after year as they spread through your garden. Unfortunately, it naturalizes a little too well, and so can be found in woodlands throughout the mid-Atlantic, as well as some other parts of the country.  The National Park Service doesn’t list the species in its manual Plant Invaders of Mid-Atlantic Natural Areas, but the University of Maryland Cooperative Extension considers it invasive.

What exactly does it mean to be “invasive”? I like the USDA Forest Service definition:

An “invasive species” is defined as a species that is

  1. Non-native (or alien) to the ecosystem under consideration; and,
    2. Whose introduction causes or is likely to cause economic or environmental harm or harm to human health.

Whether or not snowdrops cause environmental harm may be just a matter of degree, or time.

I didn’t know that snowdrops were a problem before researching them for this post. Now there’s another item on my garden to-do list: pull out the snowdrops.

When it comes to Vinca minor (periwinkle; Apocynaceae), though, there’s no doubt. At least twenty-two authorities consider it invasive. It’s easy to find at this time of year. If you follow some of the footpaths near Carderock to the base of the climbing wall, you’ll find vast slopes of the stuff, competing with the Dutchman’s breeches, squirrel corn, Virginia bluebells, and trout lilies that belong there. The plant roots easily at the stem nodes, and since the long stems trail along the ground, it spreads quickly. This characteristic, along with evergreen leaves, pretty purple flowers, and tolerance for a variety of growing conditions, makes it a popular groundcover in home gardens. Please don’t plant it.

next time: more aliens


Monarchs (A Post for Cheryl)


Ta-da!  Here it is, the current darling of conservation, the monarch butterfly (Danaus plexippus).  I promised a friend I’d write about them if I ever actually saw one.  So here you go, Cheryl, this post is for you!

This one was in the Smithsonian butterfly garden on the National Mall, feeding on butterfly milkweed (Asclepias tuberosa).

The monarch is easily mistaken for another butterfly, the viceroy.  The two look a lot alike; the major difference is a dark lateral stripe toward the rear of the viceroy’s hindwings.  I don’t have a picture to illustrate that but have a look at the Maryland Butterflies website.

I’m no entomologist so I’ll stick with a few basic facts.

There are two populations of monarchs, one east of the Rocky Mountains and one to the west.

Most of the eastern population overwinters in Mexico, though some will hibernate on the Gulf and southern Atlantic coasts.  Adults already in the tropics do not migrate.

Monarchs lay eggs on milkweeds (Asclepias species, and two other species in the Apocynaceae).  The caterpillars go through five instars before pupating and emerging as an adult butterfly.  The adults will feed on a variety of plants, including species in the aster, carrot, pea, and mustard families.

Monarchs will produce one to six broods during the summer.  Most of the butterflies live only a few weeks; it’s the last brood of the year that migrates south to overwinter, mate, and return north, laying eggs on the way.

The eastern population has declined by more than 90% since 1995, which is about one billion individuals.

The major threat to monarchs is habitat loss, which takes two forms: destruction of the forests in which the monarchs overwinter, and destruction of milkweeds, the sole larval host plant.  Milkweeds are considered nuisance plants by farmers and many homeowners.  Another threat – and another reason to hate invasive species – is the increasing presence of two species of Cynanchum, exotic alien plants that look like the native Cynanchum laeve (which is a larval host plant) but are poisonous to the larvae.

Other species of butterflies migrate, but the monarch is the only one that migrates both ways.

For more detailed information check out any of the following sites:
USDA Forest Service
Butterflies and Moths of North America
Monarch Watch
The Xerces Society


Flower of the Day: Climbing Dogbane


Thyrsanthella difformis
Trachelospermum difforme



There are places within my regular hunting grounds where I’m reluctant to go.  Down rock scrambles that were easier before my knees got bad, before I started schlepping around a daypack stuffed with photography equipment… areas covered in poison ivy, part-time islands…  But the effort can be rewarding (see my recent post about blue false indigo).  So it was last week when I went to check on culver’s root (still there, not yet blooming).  Slick wet rocks, poison ivy everywhere, some invasive aliens, nothing terribly interesting… hey, what’s this?


This is climbing dogbane, a low-growing vine of the southeastern US that is rare in Illinois and endangered in Maryland.  Not only endangered, but listed by the Maryland Department of Natural Resources as S1, which is defined:

“Critically imperiled in Maryland because of extreme rarity (typically 5 or fewer estimated occurrences or very few remaining individuals or acres in the State) or because of some factor(s) making it especially vulnerable to extirpation. Species with this rank are actively tracked by the Wildlife and Heritage Service.”

There’s not a lot to say about it.  It isn’t particularly beautiful.  There’s almost no useful or interesting information on the internet about it, except at  It isn’t in any of my books; someone in an internet plant group identified it for me.

And I don’t have very good pictures, because I slipped on wet rocks climbing down there, my knees were aching, I was precariously balanced on more wet rocks with no place to set the tripod, and oh, guess what the climbing dogbane was climbing on?

Poison ivy.


Nonetheless, I am going back there in a few days to try to locate it again.  This time I’ll geotag it and report the find to the DNR.