For years we've hoped that a brisk autumn wind would blow some seeds of Purple Milkweed, Asclepias syriaca, onto Hilton Pond Center, not only because we think it's an attractive blossom but also because it serves as host plant for caterpillars of the Monarch butterfly. We also covet the brilliant orange stands of Butterfly Weed, A. tuberosa, a related plant that flourishes along roadsides near--but not on--the property. We probably could accelerate the advance of these desirable plants by sowing their seeds, but we always seem to forget to collect any pods as they ripen each fall.

All photos & text © Hilton Pond Center

Nonetheless, the Milkweed Family (Asclepiadaceae) IS represented at Hilton Pond Center by one species, Swamp Milkweed, A. incarnata. Over the past decade, breeze-borne seeds of this plant have found their way to the edge of the pond, where moisture-loving Swamp Milkweed has established several small colonies. In some locations Swamp Milkweed may be almost red, but those around Hilton Pond are lavender pink in hue.

Like all its relatives, Swamp Milkweed has a distinctive flower that is shaped rather like an hourglass (right)--wide at the top and base, but constricted in the middle. The "base" is formed by five pigmented sepals that fold away from the rest of the flower, which includes five united petals. Each petal forms a "hood" over a stamen, or "horn," and the relative configurations of these two structures are useful in identifying various milkweed species. In the case of Swamp Milkweed, the horn is longer than its hood and curves away from it toward the flower center.

Within each milkweed flower there are two ovaries, which is why the plant's awl-shaped seed pods occur in pairs. These pods--green and often prickly to the touch (below left, photographed in early September)-- turn brown and brittle with age and burst open to reveal contain many silky-haired seeds that glisten in late summer sunlight (below left). The seeds are dispersed primarily by wind but also float on ponds and streams.

Swamp Milkweed grows to four feet in height (below right), its flowers forming an umbel-like cluster at the top of the stalk; side branching may occur in large, robust plants. Like the rest of its relatives, Swamp Milkweed's showy multiple flowers are easily found by bees and butterflies, which dine on milkweed nectar. But, as noted above, milkweeds play an even more important role for the larvae of some butterflies, most notably the Monarch.

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The milkweed gets its name from its white sap--although Swamp Milkweed is far less sappy than many of its relatives. Milkweed sap is a viscous fluid that flows through the plant's vascular system and oozes out when a stem or flower is damaged (bottom photo). Some animals decline to browse on milkweed foliage because of the bitter sap, which is laden with glycosides that are at least mildly toxic. However, Monarch caterpillars in particular thrive on milkweed and concentrate those glycosides in their own bodies--which in turn makes them unpalatable to potential predators.

We don't know for sure, but maybe the spread of Swamp Milkweed over the last ten years at Hilton Pond Center helps explain why there's been a decline in Blue Jays during that period. After all, as lepidopterist Lincoln Brower once demonstrated on film, the best way to make a Blue Jay throw up--and maybe depart for good--is to feed it a Monarch caterpillar that has been munching on the toxic leaves of milkweed.

All text & photos © Hilton Pond Center

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NOTE: Be sure to scroll down for an account of all birds banded or recaptured during the week, as well as some other interesting nature notes.

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"This Week at Hilton Pond" is written & photographed
by Bill Hilton Jr., executive director of
Hilton Pond Center for Piedmont Natural History.

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Ruby-throated Hummingbird--6*
Northen Cardinal--1
Eastern Towhee--1*
House Finch--21*
Carolina Wren--1*
American Robin--1

* = Includes at least one Recent Fledgling

WEEKLY BANDING TOTAL
(15-21 July 2001)
7 species
31 individuals

American Robin (female)
(Adult male usually has dark
head, female's is paler)

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