Maximize Biodiversity: Use Keystone Plants in Your Garden

by Joan McIntyre, Elaine Mills, & Leslie Cameron, Extension Master Gardeners

Nature is in trouble, but we can all play a role in helping to restore essential ecological services in our urban and suburban landscapes, including those of us with modest garden space.

A familiar insect-plant relationship – Monarch butterflies and milkweed plants

A monarch visiting Asclepias incarnata — Native *Asclepias incarnata* (Swamp Milkweed) with monarch butterfly in July.
Photo © 2015 Henry Staples and Claudia Gerwin

Monarch caterpillar eating host plant Asclepias tuberosa — Monarch caterpillar (*Danaus plexippus*) eating host plant
*Asclepias tuberosa* (butterfly-weed) leaves in September.
Photo © Mary Free

Native plants play a critical role in supporting the survival of other species in the ecosystem, but not all native plants are equal. Keystone plants will support the most wildlife species and should be an important part of our layered landscape. Action 3 in our Restoring Nature series emphasizes identifying and adding keystone plants to our landscapes and yards.

Entomologist Douglas Tallamy’s research has shown that native plant species support our Lepidoptera (butterflies and moths), whose caterpillars play a critical role in the food web. In particular, caterpillars frequently require foliage from specific species of native plants during this stage of their development. Caterpillars, in turn, are eaten by many other insects, birds, and animals, transferring energy from plants to other species. For example, 96 percent of our songbirds feed them to their young.

A familiar example is the critical insect-plant relationship between monarchs and milkweeds. Monarch caterpillars feed specifically and exclusively on plants in the milkweed (Asclepias) genus. Willows, plants in the Salix genus, are an even more dramatic case in point as they serve as larval host plants to 455 Lepidoptera species, including the eastern tiger swallowtail butterfly.

A bright green caterpillar resting on a leaf, with a quarter placed next to it for size comparison. — Oaks are larval host plants for the caterpillar of the polyphemus moth,
David J. Moorhead, University of Georgia, Bugwood.org

A close-up view of a large moth resting on the ground, showcasing its striking wings with unique patterns of eyespots and colors, surrounded by dried leaves. — Adult polyphemus moth
Steven Katovich, Bugwood.org

Because keystone plants support a large number of Lepidoptera species, make them the centerpiece of your garden plan and start by identifying which ones you already have in your yard. Woody plants support more species of moths and butterflies, so it is especially important to include them in your home landscape. The relationships between plants and wildlife are different in different areas. Ecoregions are large geographical areas that incorporate similarly functioning ecosystems, and choosing keystone plants suitable for the ecoregion is important in maximizing ecosystem support.

This chart is from the recently published book, Nature’s Action Guide, lists the most valuable woody and perennial native plant genera for supporting biodiversity in the mid-Atlantic.

A chart listing the 20 most valuable woody and perennial native plant genera for supporting biodiversity in the mid-Atlantic region, including common names and associated Lepidoptera species counts. — List of keystone plants determined by Doug Tallamy’s research, provided courtesy of Sarah F. Jayne, author of Nature’s Action Guide

The top 20 genera of woody plants start with Quercus (oak), which support 534 Lepidoptera species, then Prunus (black cherry), which support 456, and Salix (willow), which support 455, down to Castanea (chestnut) at the bottom, which support 125 species. The top 20 perennials start with Solidago (goldenrod), which support 115 species, then Aster (asters), which support 112, and Helianthus (sunflower), which support 73, down to Lobelia (cardinal flower), which support 4 species. (Note that the genus name Aster, which is used in the Nature’s Action Guide list, has recently been recategorized as Symphyotrichum and Eurybia.)

Close-up of a flowering plant with bright yellow flowers and green leaves, with a bee visible in the background. — *Solidago* is top-ranked herbaceous plant genus.
Photo © Elaine Mills

A close-up of a bumblebee collecting nectar from purple flowers with yellow centers. — Asters are plants in the *Symphyotrichum* and *Eurybia* genera. Photo © Elaine Mills

A close-up of a bee on a vibrant yellow flower, surrounded by green leaves and additional yellow flowers in the background. — *Helianthus* species also provide larval host support.
Photo © Elaine Mills

As you explore native plant options, species with only a handful of insect partners are still valuable as many Lepidoptera species are dependent on one or two plants. The monarch for example, can only reproduce on milkweed species.

To learn more about native plants and their wildlife benefits, here are three handy tools to help you determine exactly which species in each plant genus are native to your area, including which are considered keystone plants:

The National Wildlife Federation (NWF) Keystone Plants by Ecoregion and Host Plants by Ecoregion.
The NWF Native Plant Finder, a database of genera of native flowers, grasses, trees, and shrubs searchable by zip code and ranked by the number of butterfly and moth species that use them as host plants for their caterpillars.
The Biota of North America Program (BONAP), a plant atlas that indicates whether plant species are native in a particular area, down to the level of county. The BONAP atlas can confirm that potential keystone plant choices are indeed native to your specific ecoregion.

A list published in the Wild Ones Journal expands on Doug Tallamy’s original keystone plant list, including more than 60 additional native “woodies” (trees, shrubs, and vines) that serve as larval host plants for Lepidoptera.

An educational page featuring images of native caterpillars, moths, and butterflies alongside their host native plant species. The layout includes various species' names and related information, including the Double-toothed Prominent caterpillar and Honey Locust Moth caterpillar. — Provided courtesy of Wild Ones.

Additionally, you can consult these sources to learn more about characteristics of keystone plants, including what soil, light, and moisture conditions they prefer:

MGNV, Tried and True Native Plant Selections for the Mid-Atlantic
Plant NOVA Natives, Native Plants for Northern Virginia or download the mobile search app

SMALL SPACE /BALCONY GARDENING TIPS:

Even if you don’t have room for an oak or willow tree, you still have numerous native plant choices that will provide nectar and support caterpillars in small spaces and containers.

For shrubs you might consider blueberries (Vaccinium), berries (Rubus), such as raspberries (Rubus odoratus), or roses (Rosa), which rank high in terms of larval host support. Other shrubs include inkberry holly (Ilex glabra), shrubby St. John’s-wort (Hypericum prolificum), or fragrant sumac (Rhus aromatica). Cultivars of these and other native plants that are compact may be more suited to containers.

You can grow Coral Honeysuckle (Lonicera sempervirens) on a trellis.

Many high-value perennials work well in a container—for example, goldenrod (Solidago), wild strawberries (Fragaria virginiana), various asters, butterfly-weed (Asclepias tuberosa), and wild geranium (Geranium maculatum). Native grasses and sedges can serve as either container “thriller” or “spiller” elements, depending upon their height.