by Christos Economou
Oval-leaved milkweed (Asclepias ovalifolia) is Somme’s rarest plant. Northeast Illinois is currently the southeasternmost point of its range, where It has been rare for as long as botanists have thought to look for it. Local expert H.S. Pepoon (writing in the 1920s) never saw it, but mentions collections by botanist H.H. Babcock (from the 1870s), who considered it “a rare occurrence at Stony Island, Glencoe, West Ravenswood.” So far as we know, an original population hasn’t been seen in Illinois in recent decades – except, miraculously, at Somme.
A single plant was found by a volunteer back in the 1980s where a large buckthorn had been cut on the edge of a high quality part of Somme Prairie. My admittedly romantic hunch is that this Somme plant was a relative of Babcock’s Glencoe population; Glencoe is just to the east of Somme, and in the 1870s was the only incorporated town nearby. Whatever the case, the lone Somme plant had been waiting for years to cross with a mate that never came along.
This was a major problem. Many species don’t self-pollinate, and oval-leaved milkweed seems to be one of them. They need to cross-pollinate to set viable seeds, and having genes from two parents helps their offspring develop adaptations that can make them more vigorous. As a result, plant populations with very few individuals (one number I’ve heard is less than ~50) are at higher risk of dying out from inbreeding depression. In other words, with only one individual in the population, if no one did anything to introduce “new blood,” Illinois’ oval-leaved milkweed would eventually die out forever. Luckily, the Chicago region is blessed with many nature people that care – and act.
Ecologist Marlin Bowles (then at Morton Arboretum) took a major initiative. He secured oval-leaved milkweed pollen from the closest known population at a similar latitude all the way in Iowa and used it to hand-pollinate the loner at Somme. The Somme plant thrillingly went on to make one pod full of good seed that was used to propagate new plants, and shortly thereafter died.
Its half-Somme miracle babies were planted out around the high-quality areas of Somme Prairie (near the original) and in a few open places at Somme Prairie Grove with similar conservative associates. Somme’s oval-leaved milkweed was discovered in an area that was historically wide-open prairie, but that in the 1980s was mostly just a few small openings in brushy artificial woodland. Unlike most other species, as the brush was cleared and restoration progressed, the planted Somme Prairie milkweeds increasingly seemed to struggle and ultimately died out.
The ones planted at Prairie Grove fared better – but not much. Regular censuses showed five of the planted milkweeds surviving in 2014. These plants gradually dwindled, until there was only one plant left in 2019. For the last six years, this has been the only plant observed at Somme. It seems to not be doing very well, as best we can tell from the number of flowers it puts up. We wonder anxiously what will happen this year.
Current status
So we are basically right back where we started. We again have a single individual on which all hopes of this species’ continued existence as a wild plant in Illinois are pinned (but now only half-expresses local genetics). Which begs the question: “What, if anything, should we do now?”
There’s so much else to do. Is it right to take time away from helping myriad other species in need to focus on a single species that seems like it might be doomed by climate change anyway, no matter what we do?
The answer is…
We don’t know. There’s a compelling argument that climate change might make any effort to save this species futile. We truly seem to be on the wrong edge of this species’ range given how the climate is expected to change. Like many more northerly species, it’s possible oval-leaved milkweed benefits from steadier and cooler summer temperatures than we are projected to get in the future. And it seems to be struggling even in places such as Wisconsin where it is relatively more common.
On the other hand, it could be wrong to give up on oval-leaved milkweed so easily.
I often wonder if, at least sometimes, we don’t invoke climate change a little reflexively as a major threat to X or Y species, when in the near term a much more likely culprit is simple undermanagement. Climate change is certainly a big problem and will only become more of one in the future. But it seems plausible that milkweed populations struggling in Wisconsin could have more to do with canopy closure or insufficient fire than climate stresses, at least for now. For what it’s worth, I know of one planted population doing just fine a few hours to the south of Somme.
But the major reason to be optimistic is that we may have fundamentally misunderstood this plant’s habitat requirements. Much evidence points to oval-leaved milkweed being more of a woodland and savanna plant than one of full-sun prairies, at least here in Illinois.
For one thing, the historical Illinois reports all seem to be from areas that were probably open oak woodland back in the 1800s, although Babcock makes no explicit comment on habitat. Pepoon also mentions a Mr. Jesse Smith of Highland Park (close to Glencoe), who said he found the milkweed in nearby “woods” in 1925. Woodlands are also where oval-leaved milkweed is usually found in the parts of its range where it is more common. The Wisconsin populations we’ve learned of are all from oak woodlands of one type or another.
Minnesota Wildflowers lists oval-leaved milkweed’s habitats as “dry sandy soil, prairies, open woods, roadsides,” while the Illinois Wildflowers website says, “hill prairies and dry sand prairies, typical savannas and sandy savannas, and openings in upland oak woodlands.” When we see habitat lists like these, we usually wonder how close the “prairies” mentioned are to groves of trees. It seems an important adaptation for many savanna plants that they can cope with different sun situations as the savanna canopy is naturally variable and dynamic. It seems normal that some parts of a population sample sunnier areas and some shadier. We might imagine then that at least some of the "prairies" cited could be better conceived of as “open savannas.” That is, very sunny, open places full of “prairie” plants – but near trees which have meaningful ecological implications when compared with a truly treeless prairie. An “open savanna” area is where our last surviving milkweed is now.
The association with dry places provides another clue. It seems that, like many other short conservative species, oval-leaved milkweed today is surviving better in drier areas where vegetation cannot grow as rank as in more fertile areas and the dry soil restricts tree growth, enabling more light through the sparser canopy.
Somme doesn’t have any truly dry areas. But over time we’ve come to suspect that many such low-growing conservatives were once also components of less dry (i.e. mesic) areas as well, at least where high ecosystem quality promotes lower vegetation height through competition, soil dynamics, etc. Case studies are plants like Canada hawkweed, June grass, Seneca snakeroot, short green milkweed, and porcupine grass, all of which occur in some very high-quality mesic areas but are normally associated with dry places today. It seems we ought to add oval-leaved milkweed to this list too.
It could be then that our original oval-leaved milkweed set up shop at Somme Prairie because of the prairie’s (thankfully temporary) degradation by brush, which provided both high-quality and part-shade habitat. Quite correctly, the Somme/Iowa plants were planted in areas where “prairie” associates of the original plant – prairie dropseed, June grass, leadplant, cream baptisia, and prairie gentian – were well established. But maybe such places are just too sunny for the milkweed to really thrive.
In that case, it’s possible that more appropriate habitat would be a bur or very open white oak woodland, with more consistent associates being low-growing woodland and savanna conservatives like Penn sedge, wood rush, bastard toadflax, wood betony, savanna blazingstar, and meadow parsnip. At Somme we refer to this as “closed savanna” and the intermediate areas where these species begin to truly intermingle with the more classic “prairie” plants as “medium savanna”. Based on what we know these seem like the sort of places oval-leaved milkweed would like. As these “intermediate” areas have become a major focus for us in recent years, we’re optimistic Somme could continue to be a good home for it.
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| An open woodland with low-growing woodland and savanna conservatives species |
A way forward?
This all suggests a possible course of action. First: find a nearby wild population from which to obtain seeds to propagate a companion for the last of the Illinois oval milkweeds. For this we would have to make an exception to the longstanding Somme rule of sourcing from wild populations within 25 miles of Somme. But under the circumstances, and having already made this exception for this species, we think it’s warranted.
With those in hand, we would next plant out the companions in part-shade areas near our loner, within a reasonable distance that there will be cross-pollination, and/or attempt to cross-pollinate them ourselves just to be sure. Though milkweeds aren’t usually targets for deer, we’d pop cages on them to make them as secure as possible. Then, if we successfully obtain pods from the original plant, we would take the seed and add it to our woodland, closed savanna, and medium savanna “lo-pro” mixes, making sure to plant some of the mixes near our established plants too so that any offspring would be more likely to cross-pollinate the originals. Then we’d wait. Perhaps, with a little TLC, in 5 or 10 years we’d have an increasing, somewhat-original, wild population of oval-leaved milkweed again in Illinois, for the first time in a century. That would be a win for conservation.
Will this be the year oval-leaved milkweed begins that comeback? Possibly. Many things might prevent it. Our plant might die on us before it can make seed. We might fail to find a suitable donor population, or it may fail to make healthy seeds. Red tape might slow the project down. Or we honestly might just get too busy with the million other things that need doing.
As always, we will try our best. But whatever happens, we are happy to have been able to share some time on Earth with this little plant, holding out here miraculously against all the odds.
Special thanks...
...go to the many people and groups who've been involved in the drama of trying to help oval-leaved milkweed over the years, including the Morton Arboretum, Chicago Botanic Garden, FPCC ecologists Anna Braum and Rebecca Collings, Marlin Bowles, Stephen Packard (also for helpful edits), Eriko Kojima, and Matt Evans.
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