An assessment of the native and invasive horticultural plants sold in the mid-Atlantic region

MATERIALS AND METHODS

A group of 14 wholesale nurseries were strategically chosen to best represent the horticultural plant taxa sold throughout the entire mid-Atlantic region. For the purpose of this study, the mid-Atlantic region was defined as the following states: New York, Pennsylvania, New Jersey, Delaware, Maryland, and Virginia. This survey focused on growers of hardy plants (those that persist year after year), as relatively few horticultural annuals are native or invasive. Retail outlets were not surveyed because of a lack of product lists and because the taxa they sell would be included in the lists from wholesale nurseries.

The nurseries included in this survey represent medium- to large-sized businesses that are based in the mid-Atlantic and that ship plants throughout the region. These nurseries were chosen because of their extensive plant offerings and their likeliness to continue in business for the coming decades, thus facilitating repeat assessments. Nurseries were also specifically chosen to include a diverse mix of plant forms (trees, shrubs, perennials, and so forth) and growing methods (container and field production). Table 1 provides a profile of each nursery including location, growing method, and products sold. Together these 14 nurseries well represent the hardy plants grown for horticultural purposes throughout the mid-Atlantic.

Product lists were gathered from each nursery and entered into Microsoft Excel (2016, Microsoft Corporation, Redmond, Washington). Many nurseries were inconsistent in their taxonomy or spelling for the same taxon. Therefore, taxa names required significant editing to normalize discrepancies prior to classification and analysis. Once normalized, reference books by Armitage (2008) and Dirr (2009), as well as the Biota of North America Program (BONAP), the Missouri Botanical Garden (MOBOT) Plant Finder, the Center for Invasive Species and Ecosystem Health, and the USDA PLANTS online databases were used to categorize each taxon based on several subjects. These included nativity, invasiveness, and plant form. These classifications are further defined below.

In this study, a species was categorized as native if it is known to naturally occur within the Eastern Temperate Forest Ecoregion (Omernik 1987). The USDA PLANTS and BONAP databases were used to determine whether a species occurs in the Eastern Temperate Forest Ecoregion. This boundary is admittedly much broader than the mid-Atlantic region surveyed; however, it is the author’s observation that most growers, retailers, and horticulturists in the mid-Atlantic region ascribe the term “native” to the relative geographic area of the eastern US. Since that area does not have explicit boundaries, the Eastern Temperate Forest Ecoregion (ETF) is a more precisely delineated approximation of that geographic range.

Cultivars of species native to the Eastern Temperate Forest were designated as a category separate from native species. In this study, cultivars are defined narrowly to exclude hybrids, which were given a separate category. This approach is a departure from traditional horticulture terminology that technically includes hybrids as a type of cultivar. Our definition includes only taxa that are directly selected from a species native to the ETF. These are often colloquially referred to as “selections,” but that term has no official meaning in the International Code of Nomenclature for Cultivated Plants (2016).

Hybrids of species and cultivars native to the ETF were given their own designation as well. It is important to recognize that nearly all of these hybrids are not naturally occurring; rather, they are derived from direct human intervention and thus should not technically be considered native. Despite this inaccuracy, it is common to see such hybrids lumped in the native category as a horticultural convention, so this category was created to capture those taxa. Another important caveat with assigning nativity to a hybrid taxon is that it is often impossible to know the entire breeding lineage. Therefore, while the taxa in this category are believed to have at least one parent native to the ETF, it is usually not possible to know the full lineage of these plants.

Invasiveness was categorized based on the online database of Invasive and Exotic Species Profiles & State, Regional, and National Lists compiled by Invasive.org, which is a collaborative project between the National Park Service, the University of Georgia Center for Invasive Species and Ecosystem Health, the Invasive Plant Atlas of New England, and the Lady Bird Johnson Wildflower Center. Unfortunately, no universally accepted invasive plant list is available for the entire mid-Atlantic region. Invasive.org does aggregate various state and regional lists into one database. We categorized plants listed as invasive in the states of New York, Pennsylvania, New Jersey, Delaware, Maryland, and Virginia as such in this study.

We also classified all taxa by plant form to provide more detailed analysis. Plant forms were designated based on common horticultural categories and include annual, perennial, graminoid, fern, vine, bamboo, shrub, tree, and edible. The distinction between tree and shrub is somewhat subjective, so we used a mature height breakpoint of 4.5 to 6 m (15–20 ft). Plants taller than this range would be considered trees and shorter plants categorized as shrubs. One exception to this rule was that all Japanese maples (Acer palmatum Thunb. [Aceraceae]) were categorized as trees even if their mature height was shorter than 4.5 m. The edible category also includes plants that could be placed under other categories such as annual, tree, or vine.

However, we decided to group all plants primarily grown for food consumption rather than ornamental purposes into one category.