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A bizarre - and rare! - parasite

To a lot of people, this is one of the more despicable plants: Giant Ragweed, Ambrosia trifida. This jumbo native member of the Sunflower Family, Asteraceae, is the most common source of hay fever, its scratchy little pollen particles tickling the sensibilities of many. An abundant species, Giant Ragweed is not normally something I would search out, but it is central to the strange story that follows.

The mighty Great Miami River, not far upstream of its confluence with the Ohio River. Rivers, especially big Mississippi drainage arteries such as these two, serve as major plant migratory corridors; botanical superhighways, if you will. I love exploring habitats such as the above, which are filled with interesting plants.

When Dan Boone - yes, Daniel Boone - called late last week with news of a major find, I had to go. So, on Saturday I was down in Cincinnati to meet up with Dan, Brian Riley, Marjie Becus, and Stan Lockwood. And up the Great Miami we traipsed, to see the exciting and odd find that Jim Decker and Dan had made just a few days prior.

Along the way to our destination, we encountered many interesting plants. This is a type of arrowhead, the Southern Wapato, Sagittaria montevidensis, a rarity of seasonally exposed mudflats. It trails low over the muck, and has very wide lobed leaves. Like all of its ilk, the three-parted flowers are exceedingly showy.

It goes without saying that nearly all of us were excited to encounter this midget - the aptly named Dwarf Bulrush, Lipocarpha micrantha. The tiny spikelets resemble pineapples, and one must look closely to admire their charms. A robust specimen, such as this one, could fit comfortably on a half-dollar coin. Another rarity, Dwarf Bulrush is known from only a smattering of sites in Ohio.

OK, here we are - our destination. Daniel Boone poses in front of an enormous thicket of Giant Ragweed, towering to 15 feet in height. I cannot think of a more shunned habitat than this. Ragweed is quite common on riverine bottomlands, and forms impenetrable jungles largely devoid of other plants. Couple the non-diversity factor with the allergy issues with the accessibility problems, and you've got a situation avoided by all.

Except for some botanists in Indiana, who not long ago discovered a mega-rarity that associates with ragweed, and showed it to Dan. So, he and Jim Decker went looking in Ohio, and Voila!

Now this is a very cool plant: River Broomrape, Orobanche riparia, shooting up between two massive ragweed stalks. There were only two prior Ohio records, both quite old, and back when the plant was considered Orobanche ludoviciana. Recent work has shown that the broomrapes in the O. ludoviciana complex that occur along floodplains differ morphologically and are indeed a different species.

You can see how this botanical oddity could easily be missed. Not only is no one looking for stuff in this habitat, but the ragweed absolutely overpowers the comparatively delicate broomrape, and renders it somewhat invisible.

Upon close inspection, the plant reveals itself to be quite showy. Densely beset with purple and white flowers, and utterly lacking in leaves, this is one distinctive plant.

Here, we've brushed away the soil to reveal the odd relationship between the broomrape and its host, the ragweed. By the way, the origin of the rather unpalatable common name is derived from the Latin rapum, which essentially means "knob", and refers to lumps caused by Orobanche infestation on a species of pea.

River Broomrape, like the rest of the 200+ species of Orobanche, has little to no chlorophyll, and must attach to a suitable host via specialized root structures known as haustoria. It then extracts all of its essential nutrients from the host plant. The large root extending up from the bottom left of the photo belongs to the ragweed. The thick, fleshy yellow stems on the right are the subterranean portions of the broomrape.

In tight on the basal, or lowermost section of the broomrape root. This is the business end; the part that attaches itself to the ragweed and sucks out the goodies.

Seen well from a belly-flopped perspective, the River Broomrape is quite a looker. The flowers are a bit orchidlike, and the entire assemblage is densely beset with glandular pubescence, lending a sticky look to the plant.

It wasn't long before Marjie Becus spotted another broomrape, and in all we found a dozen or so new plants. I suspect that there are plenty more lurking out there. Should you find yourself of a mind, start crashing through thickets of Giant Ragweed along the banks of the Ohio River or the lowermost sections of its major tributaries. You just might unearth some more populations of this very cool plant.

Kudos to Dan and Jim for tracking down River Broomrape, without doubt one of the best botanical finds of 2010.


Russell Reynolds said…
Is there any native plants in Ohio ? haha just learnt of another invasive . Learned thru this I have ragweed here.Golly , all I see around me is invasives as I learn them
Jim McCormac said…
Hi Russ,

We've got about 1,850 native plant species, and perhaps 400-500 well-established non-natives, so the indigenous flora is still sort of winning.

But the ragweeds, weedy as they may be, are definitely native plants!

Wil said…
This is very cool Jim. What is the "known" range for the Broomrape?
Great pics too.
Trendle Ellwood said…
Is ragweed good for anything that you know of, like is it a host plant for any insects? It sure is good for making me miserable, the thought of those giant ragweed plants makes me shiver!
Anonymous said…
Great Story! What a beautiful plant, the Broomrape!
Gary Wayne
Jim McCormac said…
Thanks all. Wil, the broomrape is largely known from midwestern states, with southwest Ohio nearing its northeastern limits. Not known from WV, although it could possibly occur in Giant Ragweed thickets along the banks of the Ohio.

All manner of insects use ragweed, Trendle, at least as cover and possibly a nectar source. And of course the star of this blog, the River Broomrape, requires it!

Holger Uhlich said…
Hi Jim, this is certainly a wonderful find. I deal with broomrapes since 1981. Now I create a wiki about this interesting group. Can I use your excellent pictures of O. riparia for this wiki? Beside of this: According to the original paper O. riparia seems to be parasitic on Ambrosia trifda.
Jim McCormac said…
Hi Holger,

Yes, feel free to use my images on your site, but please credit Jim McCormac as the photographer.

Brian O'Brien said…
Fascinating post - I'll be searching for Minnesota's three native Orobanche species during the upcoming season.

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