A native plant is a plant that originated and then evolved in a certain geographic area or range. For instance, some plants are native to New England, but not to the rest of North America. Likewise, some plants are native to most of the Eastern U.S. but have not been documented in New England— so whether or not a plant is “native” depends on the geographic range and scale that one is talking about. 

Native plants are important because they contribute to the diversity of the ecosystems to which they belong. Native plants and native insects evolved alongside each other; those insects have adapted the ability to digest the chemicals produced by specific native plants, and have incorporated these plants into their life cycles. Insects use the leaves and stems of native plants, shrubs and trees as host plants upon which their larvae feed. When an insect lays its eggs on a leaf, the eggs hatch and begin to eat right there! Native plants also provide pollen for bees — many of which need a specific pollen for their nests — and other food (in the form of nectar, seeds, fruits and nuts) habitat for other native wildlife, especially birds.

More than 80% of pollinators are “specialists” and need a specific native plant genera to survive. For example, Monarch butterflies cannot successfully reproduce without Milkweed (Asclepias genus), because Monarchs are able to sequester the toxic compounds in Milkweed’s eponymous milk (latex). Likewise, many species of bumblebees evolved tongues of just the right length to reach nectar deep within specific flowers—and seek out a special pollen to put in their nests for emerging larvae to eat.  Without these flowers, the adult bumblebee specialists are not able to obtain enough energy and food for life. Native insects that are “generalists” (for example Eastern common bumblebee, Eastern tiger swallowtail, some moths and wasps) can rely on a number of native plants for food and energy. And these plants can use an array of native insects for pollination. Native plants and their pollinators make up the heart of local food chains: without native plants, native insects cannot thrive, resulting in other insect carnivores, amphibians, birds, mammals and apex predators unable to have enough to eat. 

“Ecological value” is a term used to refer to how much a species of plant contributes to the food web and ecosystem.  How many insects does it feed, not just by producing nectar but also by serving as a host plant for insects at different life stages? How many other species in the food web depends upon it? Does it provide habitat for birds and mammals? Does it prevent erosion, suck up or slow down water to reduce runoff? Does the plant help fix nitrogen into the soil? In other words, how much can the plant contribute to the overall diversity of its ecosystem? Ecological value can be calculated in many different ways, but an easy way to estimate its importance is to count how many species of butterflies, moths (Lepidopterans), bees, birds and mammals it supports.  When it supports many across these categories, it is a keystone species, for example native oak trees are keystone species. They support over 500 species of lepidoptera and the many birds and mammals that depend upon larvae and habitat the tree provides.

A cultivar is a plant that has been intentionally bred by humans to have certain desirable traits. All the produce that we eat has been cultivated for many years to be bigger, juicier, faster growing, more disease-resistant, or more able to withstand import and export so that they’ll look appealing by the time they hit supermarket shelves. Many garden plants sold commercially are cultivated; nurseries select for traits like bigger flowers, splashy colors, disease resistance, and tidy shapes. You can usually tell that a plant is a cultivar by looking at the label: if the plant’s name includes a portion in single quotes, it’s a cultivar. For example, a label that says Coreopsis verticillata ‘Zagreb’ indicates that you’re looking at a cultivated form of Coreopsis verticillata. Some of the plants sold by nurseries that claim to offer native plants are cultivated forms of plants native to New England. These plants are known as “nativars,” a portmanteau of “native”  and “cultivar.” 

Strictly speaking, no, but it’s a loaded question. For instance, it depends on whom you ask; conservationists argue that because the cultivated plant has characteristics that are not the result of natural selection, but rather of human selection, it cannot be considered native, even if it is derived from a native plant. Ecologists might say “no, but the implications aren’t clear-cut” because there is a (rightly) assumed equivalence between “native” status and ecological value. The answer depends on whether the cultivated form of a native plant can support the same native insects in the same or additional ways. A few studies (linked below) have begun to compare the value of cultivars to their “straight species” counterparts, but more research is needed to answer that question for each cultivar of our many native plants. There is also concern that nativars may disrupt the gene pool of native populations by interbreeding or hybridizing with wild/uncultivated plant populations, introducing potentially detrimental cultivated traits into populations of natives. In general, it’s better to cautiously assume that a cultivar is going to be less ecologically valuable than a regular native species, but that doesn’t mean that the cultivar is outright harmful.

Examples of research on the value and ecological effects of nativars: 

https://doi.org/10.1093/ee/nvy057

https://doi.org/10.17660/ActaHortic.2015.1085.99http://udspace.udel.edu/handle/19716/21442

https://www.zotero.org/yardmap/items/itemKey/SVSBQ6V8

Well, what is the result of avoiding it? If you choose a different native plant to put in the same place, then there’s no contest: go with the native and not the cultivar. However, if the option is to leave the space bare or to plant something non-native or purely ornamental, it makes sense to plant the cultivar of the native plant. Purists make a compelling argument that we *must* plant natives in order to do our best to improve the likelihood of long-term ecological survival. Birds, mammals, insects, and ultimately, humans rely upon the survival of a broad range of genetically diverse native plants. Likewise, it is advisable to prevent cultivated genetic traits from working into the native plant gene pool, as these traits may compromise plant populations’ overall fitness and resilience to disturbance.

That said, cultivated forms of certain plants may be more practical in a garden setting: for instance, cultivars of suckering (spreading) shrubs that spread less aggressively or come in shorter forms may be preferable to gardeners or designers.
Generally, it’s better to stick to straight-species native plants; nativars should be seen as reasonable alternatives when the native species is unavailable or impractical within the garden context.

More resources and in-depth discussions of the nativar issue:

http://content.yardmap.org/learn/nativars-native-cultivars/ (this one is particularly useful, explains which traits to avoid in cultivars and gives a recap of the research done so far)

http://www.fort-worth-metropolitan-area.com/nativeplantwildlifegardencom/the-nativar-dilemma/

https://www.nwf.org/Magazines/National-Wildlife/2016/JuneJuly/Gardening/Cultivars

https://www.ecolandscaping.org/01/native-plants/the-role-of-native-cultivars-in-the-ecological-landscape-evaluating-insect-preferences-and-nectar-quality-in-phlox-and-its-cultivars