Sunday, October 18, 2015

Just a Glimpse of Milkweed Diversity

swamp milkweed, Asclepias incarnata
swamp milkweed, Asclepias incarnata
Milkweeds, genus Asclepias, the dogbane family, Apocynaceae, are wonderful plants. They are such a distinctive group that they are easy for botanical beginners to recognize. Consequently, I have been taking photographs of milkweeds since I was a graduate student who could identify only one or two wildflowers.

Why is recognizing milkweeds easy? First, the milkweed flowers have a distictive shape not shared with other plants (see photo above and look at other photos. Read about it on the Xerces Society site link or Orbis). Second, milkweed fruits are tubular pods that open to release flat, oval brown seeds attached to a white pappus to carry it on the wind, again unlike other plants around it. Finally, most (but not quite all) milkweeds have milky sap, a third distinctive characteristic.
milkweed releasing seeds
milkweed releasing seeds
There are over 100 milkweed species, all native to the Americas. Most regions of the United States have several native species. (Nice maps: link)

showy milkweed, Asclepias speciosa
showy milkweed, Asclepias speciosa
Beyond the shared characters that make them recognizeable, milkweeds have fascinating diversity. Some have pink flowers, but on others the flowers are purple...or white...or orange.

butterfly milkweed, Asclepias tuberosa
butterfly milkweed,  Asclepias tuberosa
Some milkweeds are three or more feet high, others barely a foot. The sand milkweed, Asclepias arenaria, sprawls, so it always looks like something broke its stem and it fell over.

sand milkweed, Asclepias arenaria
sand milkweed, Asclepias arenaria
sand milkweed, Asclepias arenaria
sand milkweed, Asclepias arenaria
The dwarf milkweed, Asclepias pumila, stands at most a foot high. It is not common but when I found it, often there was an extensive clone, a patch with lots of the small shoots. My picture shows it flowering, but often it did not. Notice that its leaves are thin and narrow (linear) and that sand milkweed's leaves are much bigger and oval. And yet they grow in the same prairies.

dwarf milkweed, Asclepias pumila
dwarf milkweed, Asclepias pumila
Swamp milkweed, Asclepias arenaria, is a tall handsome plant that grows in wetlands. In western Nebraska I often saw it in marshes. And that's a good description, I'd see it but not be able to walk out to it unless I wanted to soak my hiking boots.

swamp milkweed, Asclepias incarnata
swamp milkweed, Asclepias incarnata
swamp milkweed
swamp milkweed, Asclepias incarnata
The narrowleafed milkweed, below, was not common in western Nebraska, so I was always pleased to find it. The pale flowers are a modified version of the typical milkweed shape. My picture doesn't show its size, but it was normally 1-2 feet tall, usually just the a single stem, rarely with as many flowers as in the photo I chose.

green comet milkweed, Asclepias viridiflora
narrowleaf milkweed, Asclepias stenophylla
Here's an unidentified milkweed. Maybe a showy milkweed with only a few flowers, but probably another species entirely. I know the most if not all the milkweeds of Nebraska and eastern Colorado, but I'd only have to go into the Dakotas or southern Kansas before there are species I have never seen. In California or Florida or New York...well, milkweeds "look like milkweeds" but there are many different quite wonderful variations on that theme.

milkweed, Asclepias sp.
The photo below is Asclepias curissavica, called the tropical milkweed or blood flower. I took the photo in Costa Rica years ago, pleased to have said "that looks like a milkweed" and been correct. The tropical milkweed has been widely planted and today can be found all over the world. It makes an attractive garden plant. It grows easily from seeds and can be raised as an annual if you start it indoors. (link) Since it is a host for monarch butterflies, growing blood flower is another way to help increase the number of milkweeds available to monarch butterflies (useful discussion: link). (Better photos than mine: link)
Asclepias curisavica, tropical milkweed
Asclepias curissavica tropical milkweed also called blood flower
The story of milkweeds and monarch butterflies (Danaus plexippus) was being worked out when I was a student. Lincoln Brower's research group showed that monarch butterflies tasted bad and predatory birds such as bluejays quickly learned not to eat them. (If they did eat a monarch butterfly or caterpillar, poisons in the monarch would cause them to vomit it back up. Like people, birds remember and avoid foods that make them vomit.) The poisons, it turned out, came from the milkweeds the monarch caterpillars ate.

showy milkweed, Asclepias speciosa
showy milkweed, Asclepias speciosa
The milkweed plant's "point of view"on this is not sympathetic to monarch butterflies. To a milkweed, monarch butterflies are nasty animals that devour their leaves. The poisons in the milkweeds evolved to discourage being eaten by herbivores such as monarch butterflies. However, monarch butterflies, like many specialist herbivores, are adapted to safely eating milkweed leaves, poisons and all.

This story is ongoing: mutants among the milkweeds poison monarchs, mutants among the monarchs eat those poisonous milkweeds without injury. Consequently, some milkweeds are more toxic than others, some to the point that monarch butterfly caterpillars do not grow well on them. The toxicity of the milkweeds varies between milkweed species and between plants within a species. The milkweeds that conservation organizations are urging us to plant to help the butterflies are milkweed species that have a good balance of poisons: not enough to poison the monarch caterpillars but enough that the caterpillars will sequester poisons that discourage birds and other predators from eating them.

Milkweeds have gone from "those weeds" to a "save the monarchs!" cause. I'd recommend taking time to admire the milkweeds themselves--for the variety of colors, shapes and sizes despite remaining recognizable--and as part of a very neat story, where monarch butterflies usurp the poisons intended to poison them as plant-predators as protection from their own predators.

My photos are of some of the Nebraska and Colorado milkweeds. If you aren't there, your native vegetation will reveal recognizeable but different milkweeds. Enjoy the discovery!

Blog post on which milkweed to grow to aid monarch butterfly populations link

Comments and corrections welcome.

Kathy Keeler

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