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Fall 2012

Urban Jungle

The changing natural world at our doorsteps | Illustration and text by Patterson Clark      

November 6, 2012

Endangered Harperella: A boost from a hurricane?


Although the Washington area was spared from the worst of Hurricane Sandy's damage, heavy rains pushed many local rivers and streams into flood stage. Severe weather events can put extra stress on endangered species, but one found in small Potomac River tributaries might benefit from a brief flood this time of year.

Harperella nodosum is a small, semiaquatic, annual herb from the carrot/parsley family, which is extremely particular about where it grows. It can survive only on the sun-drenched, damp shoals of small, clean, rocky streams that are sometimes hit with minor floods.

The plant will not tolerate shade, dry spells, too much heat, soft soil, prolonged exposure to deep water, deer grazing, trampling, excessive ice or heavy flooding.

But small floods can strengthen Harperella's populations by scouring algae and other plant competitors from shoals, and by carrying its seeds or sprouts to other ideal locations.

Harperella flowers until the first hard freeze of autumn, but by November some of its seeds have dropped, germinated and anchored themselves in spots near the fading mother plants.

Harperella can also reproduce with small clones, or ramets, which grow on its stem. High water can either carry away ramets or knock over the mother plant, allowing ramets to root in shoals or cracks in the bedrock.

Rooted seedlings and ramets can survive winter to develop into mature plants the next spring.

Elizabeth Fortson Wells, a retired George Washington University botanist, monitors Harperella in the Potomac watershed, analyzing the plant's habitat and reproduction, with the goal of restoring populations within their historic range alongside Potomac River tributaries in Western Maryland and northeastern

Harperella nodosum

Harperella nodosum

West Virginia. Wells has germinated Harperella seeds, planted the sprouts in a variety of sites and studied their progress through the years.

She has concluded that the tender plant can sustain itself in small tributaries, but it can't survive for long on the banks of the Potomac itself, where runoff and flood severity have increased, thanks to deforestation and development. "The major floods in the main stem

of the Potomac River," she says, "are too massive — too fast and too deep."

Harperella has been spotted growing on the Potomac's banks a few times in the past century, but those plants did not last. The rains that Sandy brought, Wells says, are "certain to wash away any seedlings along the Potomac that might have become established this year."