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Harperella: An Endangered Wetland Plant

Arkansas Natural Heritage Commission - Wednesday, November 14, 2018
by Brent Baker

Wetlands can support a variety of plants, and accommodate a number of rare plants as well. Almost 20 percent of the plants considered to be of state conservation concern in Arkansas are wetland species, and another significant percentage occurs in both wetland and upland habitats. These wetland plants are rare for a variety of reasons. Some depend on specific types of wetlands, which are themselves rare (e.g., seeps, wet prairies). Others have declined due to wetland degradation (e.g., pollution, sedimentation) or habitat loss (e.g., conversion of wetlands to agricultural land, urban development, etc.).

Of the vascular plants considered to be of state conservation concern, five are listed as threatened or endangered under the federal Endangered Species Act (ESA). Of those five, two are considered wetland plants: pondberry (Lindera melissifolia) and harperella (Ptilimnium nodosum). Pondberry, an endangered shrub in the laurel (Lauraceae) family, has been discussed on several occasions here in Natural News. This article takes a closer look at the state’s other endangered wetland plant, harperella.

Photo at left — Harperella and the common associate species water-willow (Justicia americana), growing in a rocky stream channel in the Ouachita Mountains. Photo by Brent Baker.

Harperella, also referred to as piedmont mock bishopweed, is an annual or perhaps short-lived perennial, herbaceous (non-woody) plant in the carrot family (Apiaceae). It is known from nine states across the Southeast, primarily from Alabama to West Virginia and Maryland, but also from west-central Arkansas and extreme southeastern Oklahoma. It is considered rare throughout its range. Image below shows the known county distribution of harperella (light green). Image courtesy of the Biota of North America Project (

Harperella is a relatively dainty plant ranging in height from a few inches to a couple of feet tall, with umbels of tiny white flowers in late summer. An umbel is a cluster of flowers in which the flower stems (pedicels) originate from the same central location. Harperella plants, usually found growing together in small to large groups, have a faint dill-like scent. One of the most distinguishing characters of harperella is its hollow, yet septate (cross-partitioned), quill-like leaves which are actually reduced leaf stalks of ancestral compound leaves (“rachis-leaves”). Because of this and a few other morphological and chromosomal characteristics, harperella is sometimes treated in the separate genus Harperella (from which the common name is derived), distinct from the rest of the mock bishopweeds in the genus Ptilimnium, which have compound leaves.

Photo above left — Shown are Harperella's (Ptilimnium nodosum) white flowers and quill-like "rachis-leaves." Photo by Brent Baker.

Harperella is adapted to a narrow range of water depths. It does not tolerate prolonged deep water or excessive dryness; thus, it is vulnerable to drastic changes in natural hydrologic regimes (characteristics and seasonal changes of water bodies). However, it is tolerant of moderate, periodic flooding which allows it to occupy a niche that many other plants cannot tolerate, reducing competition.

Photo at left — Rocky shoal harperella habitat along upland stream in the Ouachita Mountains. Harperella is usually found growing with water-willow (Justicia americana) and sticky hedge-hyssop (Gratiola brevifolia), both also present in this photo. Photo by Brent Baker.

Other threats to harperella’s survival and factors in its listing as an endangered plant include siltation and erosion, declining water quality, disturbance and trampling, and competition from invasive species. Deposition of excess silt from upstream erosion can smother plants, while local erosion can excavate the sediment in which the plants were rooted. Changes in physical (e.g., increased temperature) or chemical (e.g., change in pH) characteristics of water due to environmental and/or watershed changes may have detrimental effects on plants. Trampling of harperella by livestock or disturbance from vehicle and ATV crossings through its habitat can also reduce population numbers. Competition from invasive plant species, such as marsh dewflower (Murdannia keisak), is also a threat to harperella.

Photo at right — Harperella grows in sandy and silty sediment between cobble and gravel in stream channel. Photo by Brent Baker.

ANHC botanists periodically monitor known harperella populations in Arkansas to track population trends and work with other state and federal agencies to protect them. Botanists also search for potentially undocumented populations that might be targets for further protection. Although all known harperella populations currently occupy only two river systems within the Fourche Mountains, seemingly suitable habitat also exists within streams and rivers elsewhere in the Ouachita Mountains as well as in the Arkansas Valley and Boston Mountains. Some of this work is supported through grants from the Cooperative Endangered Species Conservation Fund established under the ESA and administered by the U.S. Fish and Wildlife Service.

Photo at left — Harperella can grow out of cracks in bedrock outcrops in stream channels. The wider leaf plant is water-willow (Justicia americana), commonly found growing with harperella. Photo by Brent Baker.

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