FSUS is the only vascular plant flora that incorporates research from the past fifteen years, that covers all of Pennsylvania, is very nearly complete, and includes keys.

A native plant is typically defined as having long-standing natural presence in a region. Some definitions use the criteria of being present before European settlement, others use “thousands of years”. Yes, that’s a long time; but part of the significance of native plants is that they have had a very long time to evolutionarily adapt to local conditions, and to co-evolve with other native species present in the same area. This source explains more.

An introduced species is defined as having arrived through human introduction in the modern era. It may be native elsewhere in North America or have arrived from elsewhere in the world. Species are most likely to naturalize when they originate from areas with similar climates, such as temperate China, Japan, portions of Europe and central Asia. Species originating from distant geographic regions generally have no evolutionary history with native species, and have greatly reduced ecological value as food, habitat, and host plants for our native animals and fungi. Some of them become invasive outside their native ecological context.

Complex and Uncertain Nativity: A small subset of our flora is difficult to categorize as entirely native or entirely introduced to the state. We’ve flagged these taxa in the list (i.e., Native*, or Introduced*) and provide brief explanatory notes about them here.

• "Native*" is assigned when the taxon has been conclusively documented as natively occurring, either now or in the past, in at least one location in the state; and there are also several populations that have naturalized into wild settings from introduced source material and persisted over time. If considering plants for use in restoration, please read the full explanation for these species carefully.
• "Introduced*" is assigned when a taxon is known to be introduced, but there is also unconfirmed potential it could have native occurrences. In a couple of cases, taxa assigned here are thought to be historically native in some part of the state but are largely and widely introduced on the landscape.
• "Uncertain" is assigned in the few cases where nativity cannot be determined because there is either ambiguous material in the historic record, or too little information about a species’ historic presence.

Geographic Concerns: We list whether a species is native anywhere within the bounds of Pennsylvania, but this is only a starting point, and not a very ecologically meaningful one. Pennsylvania crosses several ecoregions; Coastal Plain, Piedmont, Appalachian Mountains, the Western Allegheny Plateau, and the Erie Lakeplain. It is common for species’ historic natural ranges to fall across only part of Pennsylvania; a species with a midwestern range may be native in glaciated northwestern Pennsylvania, but planting it in the Coastal Plain near Philadelphia would move it significantly out of the geographic and ecological context it evolved in. We encourage people interested in learning more about plants in wild settings, and those interested in using native plants for horticultural or restoration purposes, to get to know Pennsylvania’s regional ecosystems and their specific floras. We hope to provide more resources to aid in this in the future.

Synthesizing Many Sources for the Best Dataset: Determining nativity requires research into the historical presence of plants in the state. For the nativity values in this list, we have built on the work of Plants of Pennsylvania (Rhoads and Block, 2007), which in turn built off the atlases of Edgar Wherry (1979) and Rhoads and Klein (1993). We have synthesized this Pennsylvania-specific work with the work of Flora of the Southeastern United States, and PNHP staff have also contributed research, with information available from newly digitized herbarium collections.

A Changing Flora: This list is a snapshot of our state’s flora at the present time. Changes are happening rapidly; more species are being introduced through human activities that move plant material, and native species are also migrating naturally. Migrating species can be added to the list of known native species in Pennsylvania if they meet the following criteria: 1) it is native to a nearby area, 2) it appears to have arrived through natural migration rather than through human introduction, and 3) the population appears persistent. 

This list includes many name changes from those used in Plants of Pennsylvania, the most recent state flora. This document provides more explanation, and species lists for different kinds of changes. In summary, be alert that:

• Many genus names have been changed (e.g., Cornus florida is now Benthamidia florida).
• Some species epithets have changed, including spelling modifications (e.g., Amianthium muscaetoxicum is now Amianthium muscitoxicum), and assignment of new names, usually in conjunction with elevating a variety to species (e.g., Persicaria amphibia var. emersa is now Persicaria coccinea).
• Some species have been divided (e.g., Allium tricoccum is now Allium tricoccum and Allium burdickii).
• In some cases multiple kinds of name change happened at once (e.g., Onosmodium molle var. hispidissimum -> Lithospermum parviflorum).

More on Varieties and Subspecies

Elevation to Species: This list includes many cases where taxa listed as varieties or subspecies in previous floras are elevated to full species. The species names can relate to the variety names in various ways. Examples include:

• Alnus viridis ssp. crispa -> Alnus crispa
• Athyrium filix-femina var. angustum and var. asplenioides -> Athyrium angustum and Athyrium asplenioides
• Arenaria serpyllifolia ssp. tenuior and ssp. leptoclados -> Arenaria serpyllifolia and Arenaria leptoclados

When Subtaxa are No Longer Recognized: In these cases, the name will be on this list with no subtaxon listed, and if “Toggle Name in Plants of PA Flora” button is on, the subtaxa that are no longer recognized will be listed there.

Using ctrl-F to search for the name you are used to in the pdf version of the FSUS will help you sort out these situations.

When there’s Only One in PA: If only one variety or subspecies is on this list, others exist but are not known from Pennsylvania. 

Why are All these Changes Happening?

Scientific names assigned to plants often change over time as research refines our understanding of individual taxa and how different species (varieties etc.) relate to each other. When writing a flora, authors have to make decisions regarding what names and concepts (including taxonomic level) are best supported by the most recent science, and different floras will almost never match completely. Since the publication of Plants of Pennsylvania, there has been an explosion of molecular and genetic research on plants that has transformed our understanding of taxonomy at many levels, which is why there is a particularly large number of changes. Furthermore, the FSUS team tends to lean towards recognition of proposed new splits, while Plants of Pennsylvania took a more conservative approach.

Matching Plant Names from Different Sources Using Concept Mapping

Flora of the Southeastern United States provides a unique and valuable tool for navigating name changes called “concept mapping”. For each taxon entry, in addition to listing other names used for that taxon, it specifies whether the name as used in FSUS is more inclusive (“ > ”), less inclusive (“ < ”), or equivalent (“ = ”) in concept to prior names used in other floras.

• Allium tricoccum example: FSUS splits this species into two, Allium tricoccum and Allium burdickii. The name “Allium tricoccum” refers to all plants of both species in Plants of Pennsylvania but refers to only a subset of them in FSUS. Same name, different concepts of the species.
• Plants of Pennsylvania uses a broader, more inclusive concept, while FSUS uses a narrower less inclusive concept. This is expressed as:
  Allium tricoccum (FSUS) < Allium tricoccum (Plants of PA).

• This list is limited to vascular plants. Vascular plants include seed plants (flowering plants and gymnosperms) and pteridophytes (ferns and lycophytes). It does not include bryophytes (mosses and liverworts), fungi, or algae.
• Species that are indicated as "not included" in Plants of Pennsylvania were either not yet documented from Pennsylvania at the time of publication (2007), or in some cases are questionably established outside of cultivation.
• There are also many non-native taxa included in FSUS that were not in Plants of Pennsylvania that we do not include in this list. We attempted to add those most likely to raise questions, like species native elsewhere in North America with expanding non-native distributions; these are mostly species not in Plants of Pennsylvania because they had not yet been documented here (sometimes had not even yet arrived). We mostly excluded, as did Plants of Pennsylvania, commonly cultivated plants that may occasionally become established locally, like many garden herbs.
• We excluded some common names that contained derogatory, racialized, or otherwise offensive words. In most cases, alternative common names are available for use (Downing & Frye 2021, Potentially Problematic Common Names in North American Public Gardens).

Get to Know Our Species:

• https://www.paenflowered.org/ Pennsylvania-specific resources, still in development.
• https://gobotany.nativeplanttrust.org/ Great keys and photos! Covers New England, significant overlap with PA.
• https://fsus.ncbg.unc.edu/ Photos, habitat, ID notes, regional mapping, keys, concept mapping.
• https://www.bonap.net County-level mapping of plant ranges in the United States.
• http://floranorthamerica.org/Main_Page Detailed species descriptions, keys that cover all of North America. Still in progress, not all volumes complete yet.
• https://www.minnesotawildflowers.info/ Only partial species overlap, but great source for photos, ID, habitat and natural history descriptions, as well as introductions to the characteristics of groups of plants like grasses, sedges, etc.
• https://www.marylandbiodiversity.com Maryland distribution maps, photos, phenology graphs.

Gardening and Restoration Using Native Plants:

• https://www.panativeplantsociety.org/
• https://extension.umd.edu/resources/yard-garden/native-plants/native-plant-gardening/

Ecological Information:

• https://www.naturalheritage.state.pa.us/communities.aspx Pennsylvania’s Plant Communities
• https://www.dcnr.pa.gov/Geology/GeologyOfPA/Pages/default.aspx Geology Information for Pennsylvania
• https://bplant.org/region/1382 Descriptions of Pennsylvania Ecoregions

Pennsylvania Floras and Atlases:

Wherry, Edgar T., John M. Fogg Jr., and Herbert A. Wahl. 1979. Atlas of the Flora of Pennsylvania. The Morris Arboretum of the University of Pennsylvania, Philadelphia. The original atlas - over a period of 40 years, Wherry and his colleagues gathered plant specimen data from the major PA herbaria and manually placed a quarter of a million dots on over 3,500 maps to show where in the state each species is known to occur.

Rhoads, A.F. and Klein, Jr., W.K.. 1993. The Vascular Flora of Pennsylvania: Annotated Checklist and Atlas. American Philosophical Society, Philadelphia. This updated atlas builds upon the 1979 maps, adding more modern nomenclature and new data. Still the go-to source for the best distribution maps for most species, although it is now missing modern data.

Rhoads, Ann Fowler and Timothy A. Block. 2007. The Plants of Pennsylvania: An illustrated manual. 2nd edition. University of Pennsylvania Press.

Mid-Atlantic Herbaria Portal: Many herbariums have now scanned and digitized their specimens. The Mid-Atlantic Herbaria Portal is a website that allows users to view data from many regional herbaria. They also have a map interface which shows much of the atlas data. However, not all herbaria have digitized their specimens and assigned coordinates to them, so the paper books still include data that are not yet online. Additionally, if a species is listed as rare in a state, data is not displayed for that state.