Keystone Shrubs of Pennsylvania

Keystone species are native plants that provide the maximum habitat benefit to local wildlife. Research shows that just 5% of a region's native plants host 70–75% of butterfly and moth species, while also supplying essential pollen to specialist bees. Because these insects form the foundation of the local food web—feeding birds, bats, and other wildlife—keystone plants have an outsized ecological impact compared to other native species. Keystone designations are made at the ecoregion level because these relationships shift across the continent, meaning the right species varies by area.

The following chart provides a comprehensive list of the keystone shrub species for each of Pennsylvania's five Level 2 ecoregions.

Let's look at these species, their growing conditions, and their importance to our ecosystems in Pennsylvania.

Alnus serrulata is commonly known as smooth alder. It is in the birch family (Betulaceae). This shrub grows 10 to 15 feet tall and 8 to 15 feet wide. Plant in full sun to partial shade in any moist to wet soils. It can be used in naturalized areas and along streams and ponds. The specific epithet "serrulata" refers to the serration on its leaf margins (edges). Smooth alder is the host plant for the only obligate carnivorous butterfly in North America, the harvester butterfly (Feniseca tarquinius). The adult butterfly does not feed on the flower's nectar; instead, it feeds on aphid honeydew, and their carnivorous larvae feed on the aphids themselves.

Baccharis halimifolia is commonly called saltbush, groundsel, or silverling. It is in the daisy family (Asteraceae) and is found in coastal areas in salt marshes, sand, or disturbed sites. It is now found in the ditches along major roadways as the runoff from the application of salt has affected those soils. Saltbush is multi-trunked with an airy, open habit growing 5 to 12 feet tall and 4 to 7 feet wide. It prefers full sun to partial shade and is a tough plant because it is tolerant of heat, drought, and saltwater. It can be used in rain or bog gardens, along streams or ponds, or in naturalized areas. It has white/cream/tan flowers and is the host plant for numerous moth species. It can spread aggressively; weed it by hand to control its spread. 

Dasiphora fruticosa is commonly called shrubby cinquefoil or prairie weed, and is a member of the rose family (Rosaceae). It grows up to 2 to 4 feet tall and 3 to 5 feet wide. It will flower better in full sun, but tolerates partial shade and is found in sunny, moist thickets and wet prairies. This shrub can be grown in the cooler regions of Pennsylvania from USDA Plant Hardiness Zones 2 to 7. The species has attractive, five-petaled, saucer-shaped flowers that are bright yellow with a cluster of yellow stamens and pistils in the center. Shrubby cinquefoil flowers from June to September. This shrub tends to become ragged-looking, but can be pruned to keep a better shape. There are many cultivars with different colored flowers. It can be used in a mixed border, as a foundation plant, or even as a hedge. Shrubby cinquefoil is the primary larval host plant for several specialized insects. Most notably, its foliage is the exclusive food source for caterpillars of the endangered Clayton's copper butterfly (Lycaena dorcas claytoni), as well as the more widespread dorcas copper butterfly (Lycaena dorcas).

Prunus americana is also known as the American plum or American wild plum and is in the rose family (Rosaceae). It is grown as a small tree or shrub. As a tree, it tops out at 10 to 20 feet tall with a broad, spreading crown. As a shrub, it suckers freely and can form large colonies or thickets. Remove suckers to control its spread. White flowers appear in early spring, followed by showy and edible fruit. It can be grown in average, well-drained soils in full sun to partial shade. Use this as a specimen plant, in a mixed border, or in a naturalized area. This plant provides nectar for pollinators and is a larval host plant for several species of butterflies. Its fruit is eaten by songbirds, other birds, small mammals, and deer. Its foliage and small twigs are also browsed by deer. The pit of the plum fruit is poisonous to humans and some pets.

Prunus caroliniana is also known as Carolina cherry laurel and is grown only in the warmest coastal regions of Pennsylvania, in USDA Plant Hardiness Zones 7a to 10b. The Carolina cherry laurel is a broadleaf evergreen, woody shrub to small tree, growing to 35 feet tall and 15 to 20 feet wide. It prefers full sun but can tolerate partial shade if it grows in moist, well-draining soil. Established plants have good drought tolerance. It is easy to transplant, can withstand heavy pruning, and is moderately salt-tolerant. When crushed, its fresh leaves have a maraschino cherry fragrance. The flowers are inconspicuous yet fragrant. Carolina cherry is a host plant for many butterflies, including the coral hairstreak (Satyrium titus), red-spotted purple (Limenitis arthemis), spring/summer azures (Celastrina spp.), and viceroy (Limenitis archippus). This is also a larval host plant to the eastern tiger swallowtail (Papilio glaucus).  Its fruits are eaten by songbirds, quail, wild turkeys, foxes, raccoons, and many other small mammals. White-tailed deer browse the foliage. Its leaves, seeds, and stems are poisonous to humans, cats, and dogs.

Prunus virginiana is commonly known as chokecherry, also spelled choke cherry. It is a large deciduous shrub or a small tree, growing 20 to 30 feet tall and 18 to 25 feet wide. Growing in the wild, it can form very dense thickets—a safe habitat for mammals and birds. It gets its name from the bitter taste of the berries. The berries can, however, be harvested and cooked into jams, jellies, and pies. Chokecherry prefers full sun and soils with good drainage. It can be grown in Pennsylvania from USDA Plant Hardiness Zones 2 to 7. The cultivar 'Shubert' is commonly found and has very striking red summer foliage. Chokecherry is a larval host plant for the eastern tiger swallowtail (Papilio glaucus).

Quercus prinoides is also known as dwarf chinkapin or dwarf chestnut oak. This small oak tree or shrub grows to 12 to 25 feet tall and equally wide. It is easily adaptable to sunny or partly shaded sites in the landscape and tolerates poor, dry soils. It has a deep taproot and is, therefore, difficult to transplant. The acorns are a primary food for many bird species, including wood ducks, ruffed grouse, quail, wild turkey, grackles, jays, nuthatches, thrashers, titmice, towhees, and woodpeckers, and are also eaten by bears, raccoons, squirrels, chipmunks, and deer. This oak tree is also a host plant for more than 400 species of caterpillars, butterflies, and moths. Because of its ecological value and smaller size, it is a great alternative to a more substantial oak tree. Plant it as a specimen or in a naturalized or woodland setting.

Salix discolor is commonly known as pussy willow. This shrub grows to 10 to 20 feet tall and 8 to 12 feet wide. It has a rapidly spreading, shallow root system that can make it difficult to garden underneath. Pussy willow tolerates deer, erosion, wet soil, and can even grow under black walnut trees. This shrub can be grown in the cooler regions of Pennsylvania from USDA Plant Hardiness Zones 2 to 7. The pussy willow is best known for its flowers—the fuzzy, silvery, soft catkins. Pussy willows are dioecious (meaning male and female flowers grow on separate plants). The male flower is showier than the female. The soft catkins emerge on bare branches in late winter and early spring. The soft, furry hairs of the catkin insulate the delicate reproductive parts from the early frost as they emerge on bare branches. This shrub can be pruned severely after flowering. Because they bloom so early, these flowers provide a crucial first food source for bees and other pollinators. It is also a very important larval host plant, supporting dozens of native butterfly and moth species, including the mourning cloak (Nymphalis antiopa) and viceroy (Limenitis archippus). Pussy willow also supports numerous specialist bees.

Salix nigra is commonly known as black willow. It is a medium-to-large, fast-growing tree that can reach heights of up to 100 feet. However, it can be pruned to the ground every 2 to 3 years and kept as a shrub. Black willow grows in full sun to partial shade but needs consistently moist to wet soils. It is not recommended for use as a specimen in residential landscapes for many reasons. But if you have a large property with a wet area that needs erosion control, the shallow root system of the black willow will do the trick. This tree serves as a host plant for the larvae of several butterflies, including the eastern tiger swallowtail (Papilio glaucus), mourning cloak (Nymphalis antiopa), and the gorgeous red spotted purple (Limenitis arthemis astyanax). It supports many specialist bees. Its buds and catkins are an important early spring food source for many birds.

Vaccinium angustifolium is commonly called lowbush blueberry. It is a small, low-growing shrub from 0.5 to 2 feet tall and wide. Use it as a groundcover near the patio, in the border, along pathways, and in a woodland setting. It grows in dappled sunlight to deep shade but must have well-drained, acidic soil. While you could grow it solely for its beautiful fall color, its true value is its flowers and berries. Butterflies are attracted to the blooms. Native bees pollinate its flowers. Its berries are relished by birds and other wildlife. Humans love the berries, too, if not eaten by birds and other mammals. There are many cultivars. It is also a valuable host plant, supporting over 200 species of caterpillars, butterflies, and moths—including the brown elfin butterfly (Callophrys augustinus), Henry's elfin (Callophrys henrici), and the banded tussock moth (Halysidota tessellaris).

Vaccinium corymbosum is commonly called northern highbush or rabbit-eye blueberry. It is an upright, slow-growing, multi-stemmed shrub found naturally in bogs, swamps, and high-elevation forests. It needs full sun to partial shade, and moist, acidic soil, with a pH of 5.0 being ideal. Highbush blueberry tops out at 6 to 12 feet tall and almost as wide. It is valuable to humans for its beautiful fall color and edible fruit, and there are many cultivars. Plant more than one cultivar for more and larger berries. Use it to replace invasive species such as burning bush (Euonymus alatus) and Japanese barberry (Berberis thunbergii). It is valuable as a keystone plant for many reasons. Its flowers attract butterflies and bees. It is a host plant for the brown elfin butterfly (Callophrys augustinus). Adult brown elfin butterflies also feed on the flower nectar. Its fruit attracts small mammals, ruffed grouse, wild turkey, black bears, squirrels, and many songbirds. The twigs provide food for deer and rabbits. Members of the genus Vaccinium also support many specialized bees. 

Vaccinium macrocarpon is also known as the American cranberry or the large cranberry. It is a low-growing vine, ground cover, or trailing, evergreen shrub. It needs full sun and moist but well-drained soil. It must have acidic soil, in the range of pH 4.0 to 5.2. This shrub is hardy in the cooler regions of Pennsylvania from USDA Plant Hardiness Zones 3 to 7. The plants are grown commercially for the berries, which mature in the fall, just in time to be part of American holiday traditions. Grow it ornamentally as a ground cover for boggy or marshy areas, or near a pond. As a keystone plant, its berries are eaten by birds and small mammals. It also attracts pollinators and is an important larval host plant for several butterfly and moth species, notably the highly specialized bog copper butterfly (Lycaena epixanthe), the chain-dotted geometer moth (Cingilia catenaria), and the eastern comma butterfly (Polygonia comma). Members of the genus Vaccinium support many specialized bees.

Vaccinium pallidum is commonly known as hillside blueberry or dryland blueberry. It is usually found in dry woodlands, on slopes, in clearings, and in thickets. It grows 2 to 3 feet tall and as wide, with a rounded form. It has beautiful red-to-orange fall color.  It can form clumps as it spreads through underground runners. It prefers well-drained, acidic soil and does best in full sun, yet tolerates shade. This is a high-value wildlife plant—just like the highbush blueberry. Its berries are eaten by songbirds, small mammals, ruffed grouse, wild turkeys, and black bears. The shrub is a host plant for the brown elfin butterfly (Callophrys augustinus). Members of the genus Vaccinium support many specialized bees. There are a few cultivars, as it is typically sold as a species native plant. Grow this in small groups in a naturalized, woodland, or native plant restoration garden.