Attack of the Orobanche

Hundreds of little Orobanche uniflora on the mossy cliff ledges

Dozens of little Orobanche uniflora on the mossy cliff ledges

Orobanche uniflora

Since Orobanche uniflora is parasitic, it has no leaves, just these pretty purple tubular flowers.

The genus Orobanche, known as broomrape, is made up of numerous species that parasitize other plants. With vampires and zombies all the rage these days, these plants ought to be more popular. I find them (broomrapes that is, not zombies) really fascinating and am always pleased to find them. I’m not sure if the other plants feel the same, but they don’t look as though they are being harmed by supporting their parasites. Different species of Orobanche use different hosts; some are very picky, while others have broader tastes. Orobanche uniflora is one of those with a number of potential hosts. With the common name of naked broomrape, you’d hardly think it would be such a pretty thing. It is also called one-flowered broomrape since it has only one flower per pedicel.

Probably the best place I’ve ever seen Orobanche uniflora is at Cougar Reservoir, just south of the McKenzie Highway. That was the my destination Tuesday, April 21, along with my friends Sabine Dutoit and Nancy Bray. All of us have been nursing injuries, so easy roadside botanizing was just what we were looking for. The flowers are fabulous in April along the roadside cliffs along the west side of the reservoir. There is more rustyhair saxifrage (Micranthes rufidula) and California mistmaiden (Romanzoffia californica) there than I’ve seen anywhere else. The saxifrage was on the wane, but the mistmaiden was gorgeous. Both these species are hosts for naked broomrape, so, not so coincidentally, Cougar Reservoir is also a haven for this species.

The massive amount of damp, exposed rock along Cougar Reservoir supports many seep-loving species, including California mistmaiden.

The massive amount of damp, exposed rock along Cougar Reservoir supports many seep-loving species, including California mistmaiden.

Orobanche uniflora growing in Sedum spathulifolium, a common host

Orobanche uniflora growing in Sedum spathulifolium, a common host

When we got out of the car at our first stop, right near milepost 55, north of the dam, I started looking for their little purple flowers. It took a few moments to find them, but once we spotted the first few, we were amazed to see how many of them were there. It must be something about the color purple, but it often seems to blend in with gray rocks. While we still couldn’t see them from very far away, up close we could see thousands of them—what a show! The cliffs are at least 100′ high in some places, so who knows how many there might be all together.

In this spot, they appear to be mostly parasitizing the mistmaiden. I’ve only seen this in a couple of other places, but this is clearly their main host here. There were some also popping out of the saxifrages and some in broadleaf stonecrop (Sedum spathulifolium), and farther south along the road these seemed to be their main hosts. These are the species I usually associate with them, but I’ve also found them among small-flowered prairie-star (Lithophragma parviflorum) at Tire Mountain and once in a patch of Olympic onion (Allium crenulatum) at Eagles Rest.

Orobanche uniflora attached to its host, Romanzoffia californica

Orobanche uniflora attached to its host, Romanzoffia californica

Identifying the host plant of an Orobanche can sometimes be difficult. They attach to their host by an underground, root-like appendage called a haustorium, which may travel quite a ways to find a suitable host, so another plant next to a clump of Orobanche flowers is not necessarily the host. The seeds can’t choose where they land, after all. With thousands of Orobanche in front of us, I decided to collect several for the OSU Herbarium (having recently received my yearly permit from the Forest Service), hopefully with the Romanzoffia still attached. Since many of the clumps were growing on the loose rock piles at the base of the cliff, I decided peeling one off a rock would be less disturbing to the area and would mean I wouldn’t risk breaking the haustorium. This was a piece of cake. The Orobanche and Romanzoffia were definitely still attached. Unfortunately, with them growing so thickly, the Orobanche didn’t need to send its attachment very far to find its host, so recognizing the haustorium among the tangle of roots was not as easy as I’d hoped.

As busy as I have been, I didn’t get around to cleaning off the specimens for several days. It was not very easy, and, surprisingly, the Orobanche survived sitting in a tub for several days much better than the Romanzoffia, which collapsed pretty quickly. I had scanned some Romanzoffia californica several weeks before, so I knew it had delicate, translucent roots coming from its white-hairy tuber. Unfortunately, most of the roots and hairs had turned black and shriveled up. The root system of the Orobanche was more extensive than that of the Romanzoffia. Since it gets its nourishment from the host, perhaps the roots function more to stabilize the plant and prevent it from washing off its damp, rocky perch. The Orobanche roots are pale gold like the rest of its underground parts and somewhat fleshy and lumpy. So where are the haustoria? I’m not 100% certain, but I could see some light-colored but not translucent, narrow roots in all the specimens. They are clearly coming off the base of the Orobanche in the photo above. Since they don’t look like Romanzoffia roots or the larger roots of the Orobanche, my guess is these are haustoria.  I would welcome help from more knowledgeable folks, if anyone else is familiar with these fascinating parasitic plants and can confirm or contradict my assumption.

4 Responses to “Attack of the Orobanche”

  • Jack Turner:

    Hi Tanya — You are a gift. Keep looking and writing.

    I think I’ve come across Orobanche at Horse Rock Ridge.

    Jack Turner

  • Wilbur Bluhm:

    Great pix and information, Tanya. You sure found a jewel of a population of the Orobanche, greater than what I’ve ever seen

    Thanks, Tanya.

    Wilbur Bluhm

  • Jennifer Barker:

    Hi Tanya,
    Great story and photos. We have O. uniflora out here in Grant County, mostly in vernal damp-to-wet spots. It seems to like Senecio integerrimus as a host.

  • Crow V.:

    A friend found these along Stevens Canyon Road in Mount Rainier National Park. He also documented Saxifraga rufidula at the same location. I discovered your page following a lead from Mark Turner (“Wildflowers of the Pacific Northwest”) which describes O. uniflora as mycoheterotrophic.

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