How Much Native Vegetation Should Homeowners Plant?

by Elaine Mills, Extension Master Gardener

Native plant aficionados often converse amongst themselves regarding the ideal proportion of native species for their gardens. They also want authoritative information to share when answering questions from gardeners who are just being introduced to native plants.

Nationally, much of the discussion centers on the often-cited article “Nonnative plants reduce population growth of an insectivorous bird” by entomologists Desirée Narango and Doug Tallamy of the University of Delaware and Peter Marra of the Smithsonian Migratory Bird Center. In that paper, they describe the results of their work in conjunction with Neighborhood Nestwatch, a citizen science program that monitors breeding birds on residential properties in the metropolitan area of Washington, D.C.

The team placed nest boxes for chickadees, a species commonly found in urban and suburban environments, in the yards of study participants. Then, from 2013 to 2016, they identified and quantified the plant species for each site, monitored the abundance of insects (caterpillars and spiders), and collected data on the birds’ occupancy and reproduction. At the end of the study period, the team determined that yards with a predominance of nonnative trees and shrubs were less attractive to the birds and resulted in fewer young that fledged. They concluded that a biomass of at least 70 percent native plants is required to provide sufficient caterpillars (6,000 to 9,000 per clutch) to maintain the chickadees’ population.

In subsequent webinars, such as “Bringing Nature Home: The Importance of Native Plants,” Tallamy went on to suggest that while the overall goal of home landscaping should be to support declining populations of insects and birds by using native plants, there is room for compromise. He states that up to 30 percent of woody plant biomass could be nonnative without destroying the integrity of the food web, specifying that the nonnative plants should not be invasive species.

Local discussion has arisen around a recent blog post, “How Many Natives Do I Need?” by Matt Bright, executive director of Earth Sangha, in which he asks if there are “benefits to going beyond 70% native cover.” In taking a closer look at the results section of Narango’s paper, he points out that the birds’ population growth is only sustainable when the amount of nonnative plant biomass is less than 6%. His analysis indicates that “it’s more valuable to transform habitat from 75% native to 100% native than to transform a site from 0% native to 25% native.”

While the scientific results show that a higher percentage of native plants would be most beneficial for wildlife, few gardeners in any given neighborhood may have reached the initial goal of 70 percent. It is important to recognize that there can be value in encouraging the use of native plants across a wide spectrum of homeowners.

Some residential properties with small lots cannot accommodate many large trees and shrubs. Other homeowners may have mature nonnative trees that would be inconvenient or very costly to remove. Even gardeners who have a majority of native plants feel reassured knowing that it can be acceptable to include some non-invasive longtime favorite plants that they enjoy simply for their beauty or because of a sentimental attachment. With time, any of these gardeners may take additional steps toward a higher goal, such as replacing less desirable plants when they die.

It may ultimately be more important from an ecological standpoint to encourage the collective use of native plants in more settings than for individual gardeners to reach the goal of 100 percent biomass on their own properties.  We need to support Doug Tallamy’s goal of enlisting as broad a coalition of average home gardeners as possible to take the right steps in support of the environment. As he explains in the mission statement for his Homegrown National Park (HNP) initiative, he sees “small efforts by many people” as the solution to creating new ecological networks. Native plant enthusiasts can assist by acting as ambassadors, modeling sustainable landscaping principles, and educating their neighbors and colleagues.

Here are some guidelines for taking those “right steps” in our own yards and a few suggestions for ways to encourage the wider use of native plants.

(Links to helpful websites, articles, and videos can be found under Resources below.)

• Maintain the health of existing canopy trees, especially native oaks, by removing any invasive vines, watering during droughts, avoiding soil compaction around tree roots, and consulting a certified arborist regarding any damage or necessary pruning.
• Consider adding native “keystone” tree species, such as black cherry, willows, birch, poplars, hickories, or hawthorns, that support the greatest number of butterfly and moth caterpillars, while also offering nectar and pollen to pollinators and fruit and nuts to other wildlife.
• Replace invasive shrubs with native species, including “keystone” blueberry and rose, over time to increase cover and nesting places as well as food resources.
• Add native herbaceous plants, especially those in the goldenrod, aster, and sunflower families, both in the ground and in containers.
• Use explanatory signage and labeling of native plants that are visible to neighbors and can start conversations.
• Offer to help neighbors remove invasives and share native plants when rightsizing the species that have multiplied.
• Get involved in community plantings and join HOA grounds committees to educate these groups and make sure native plants are prioritized.
• Provide guidance to schools and churches on the benefits of including native plants and volunteer in planting efforts.

Resources

• See the Trees Are Good website for information on tree care and locating a certified arborist.
• See presentations on  “Keystone Species of Native Plants”,  “Celebrating Native Trees” , and “Native Trees: How to Select, Plant and Transplant” for assistance in tree selection.
• See the presentations on “Invasive Plants and Native Alternatives” and “Overused Foundation Plants and Native Alternatives” for recommendations on replacements.
• See “Keystone Species of Native Plants”, the National Wildlife Federation’s Native Plant Finder, and Container Gardening with Keystones on the Homegrown National Park  website for the herbaceous species most supportive of wildlife.
• See information on MGNV’s Small Trees Make Big Canopies program as a source of free native tree saplings.
• See the Plant NoVA Natives website for lists of Native-Only Sellers and Local Native Plant Sales.