The Case for Recognizing Persicaria amphibia and Persicaria coccinea as Distinct Species

The following information is presented to the community so that they may know more about these beautiful plants and appreciate their unique attributes and inherent value as living beings worthy of respect and protection.

I began studying Smartweeds, Persicaria, over a decade ago when I found a plant right outside my office door and all over New York City that had been in North America for fifty years, yet was unrecognized by all botanists (Persicaria extremiorientalis). Since then, I have collected hundreds of Persicaria specimens and examined thousands more in major herbaria. I have read the literature old and new, been a peer-reviewer for journals and corresponded with the handful of Polygonaceae specialists practicing today. I know the North American species pretty well and can recognize most from photographs.

Smartweeds are a genus of about 100 species primarily of the north temperate zone of both hemispheres. Most species are annuals with simple, alternate, entire ovate or elliptic leaves and spicate inflorescences and flowers of usually five perianth parts. Hybridization, introgression and polyploidy are especially common in the core “Eupersicaria” group of the genus. There are few autapomorphies (unique traits) that clearly distinguish one species from another. Often a suite of characters are necessary to define a species and distinguish it from others. In addition, the species can be quite variable and morphologically plastic in response to environmental conditions, especially periodic inundation (as are the two species discussed here). The Pale Smartweed Persicaria lapathifolia and Lady’s Thumb Persicaria maculosa can also form inflated, floating stems when flooded

The “Amphibious” Smartweeds, Persicaria amphibia and Persicaria coccinea attracted my attention early on because they are such beautiful plants, yet no one seemed interested in them, perhaps because of their tortured taxonomic history. The more I learned about them and the sorry state of our professional conclusions, the more I wanted to “understand” them and reveal their unique qualities and relationships to each other and their surroundings. Ultimately, I want to protect these plants as much as possible from further human harm.

Water Smartweed, Persicaria amphibia var. stipulacea (photo by Reuven Martin)

Scarlet Smartweed aka Longroot Smartweed, Persicaria coccinea (photo by Henggang Cui)

Persicaria amphibia var. stipulacea and Persicaria coccinea (Persicaria amphibia var. emersa) are perennial North American natives that inhabit high-quality, oligotrophic wetlands (especially Persicaria amphibia). They are both adapted to fluctuating water levels (hence the “amphibious” epithet). Persicaria amphibia is normally an aquatic with floating leaves, but when stranded on dry banks can grow aerial shoots (with flared ocreae). Persicaria coccinea is normally a palustrine species with aerial shoots, but can tolerate flooding for periods of time and may sometimes develop floating stems and leaves. Persicaria amphibia var stipulacea does not grow south of the Laurentide Ice Sheet except in the mountain west and Mexico. The fidelity is amazing as can be seen in this map. Persicaria coccinea grows nearly throughout North America except the southeast coastal plains and Mexico.

The vast majority of specimens across North America will key out clearly with the key below. But there are populations that don’t, especially in the mountain west. These anomalies are probably genetic mixtures from hybridization and introgression, both phenomena common and well-documented in the genus. Each species’ extreme anatomical plasticity and the existence of intermediate specimens has thrown botanists for 200+ years into taxonomic fits, lumping the entire range into one artificial super-species (e.g., R. Mitchell) or dividing every minor morphotype into a separate species (e.g., E.L. Greene). Persicaria amphibia has over 100 heterotypic synonyms just in North America!; and Persicaria coccinea almost as many. The hypothesis that there are two distinct species and one or more hybrid swarms (cited below) is the most plausible and parsimonious way to make sense of these beautiful and important plants. It is also the only way to ensure that each species (and its genetic diversity) is wisely and effectively conserved.

Key to the species (currently treated as varieties in most works, but see Reveal, J. L. & D. E. Atha. 2012. 8. Persicaria (L.) Mill. Smartweed, pp 236–250. in Cronquist et al. (eds), Intermountain Flora. The New York Botanical Garden Press, Bronx, NY).

1a. Plants palustrine, usually with emergent leafy stems; ocreae never with flared apices; aerial leaves petiolate with acuminate tips; inflorescence spikes terminal, usually 2 (unequal), > 4 cm long...… Persicaria coccinea (Persicaria amphibia var. emersa).

1b. Plants aquatic, usually with floating stems and leaves; ocreae with flared apices (when stranded); aerial leaves (when present) nearly sessile with somewhat cordate bases and blunt apices; inflorescence spikes usually 1, < 4 cm long...…Persicaria amphibia var. stipulacea.


  1. Distinct geographic ranges. This is becoming more and more clear as the observations accumulate. There are now over 2000 observations of the two species and they clearly have different distributions. There are no Persicaria coccinea in the far north. And there are no Persicaria amphibia south of the Laurentide ice sheet in the eastern US.
  2. Distinct morphologies. The vast majority of plants clearly exhibit a number of discontinuous character states consistent with one species or the other.
  3. No single plant has ever been found to possess the characters of Persicaria coccinea at one end and Persicaria amphibia at the other, even though there is ample opportunity for them to do so based on level of inundation. There are many examples of Persicaria amphibia with floating leaves at one end and erect shoots at the other. I have seen most of the herbarium specimens of both species in North America and all the iNaturalist observations and I have never seen a plant with erect shoots and long inflorescences on the stem portion out of water and oblong floating leaves and short inflorescences on the stem portion in the water.
  4. If they are one species with blended genetics, how can it be that no Persicaria coccinea like plant has ever been found with flared ocreae? That character is found exclusively in Persicaria amphibia var. stipulacea.
  5. The presence of intermediates does not "prove" they are a single species. The most parsimonious explanation is that the intermediates are hybrids. To consider them as one species is the least good explanation. R.S. Mitchell did not consider this possibility (as the null hypothesis) when he lumped them for his Ph.D thesis in 1968.
  6. Taken all together these data are consistent with the consensus definition of a species in botany.
  7. Lumping them as a single species has very serious conservation implications. Conservation plans should conserve distinctive genetic lineages and conflating the two species could lead to the extinction of one or the other in the false belief that the "species" is preserved by the presence of at least some Persicaria amphibia s.l. We all know that most people ignore varieties and even heritage botanists and environmental surveyors will use the species name for convenience or uncertainty. U.S. Army Corps of Engineers data forms often omit subspecies or varieties, compromising the integrity and usefulness of EIS surveys that might include “Persicaria amphibia”
פורסם על-ידי danielatha danielatha, אוקטובר 20, 2020 07:08 אחה"צ


I so appreciate you, Daniel. :) Thanks for this detailed journal entry!

And I'm sure you've seen it before, but it reminded me of this rant:

Keep up the great work, Daniel.

פורסם על-ידי sambiology לפני יותר משנה (סמן)

Thanks, Sam. That was a creative use of a horrific episode in human history. Nothing like the trouble we cause.

פורסם על-ידי danielatha לפני יותר משנה (סמן)

A condensed synopsis is on Wikipedia. Thanks also to @crwrcwamt and @er1kksen

פורסם על-ידי peterwchen לפני יותר משנה (סמן)

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