by Elaine Mills, Extension Master Gardener
Photos by Elaine Mills, unless otherwise credited
As explained in Part 1 of this article, keystone plants belong to native plant genera that play an especially large role in assuring the survival of other species in the ecosystem. Their most critical function is to serve as larval host plants for moths and butterflies (Lepidoptera) whose caterpillars, in turn, are critical components in the food web. Keystone plants may also provide food, cover, and nesting sites to a variety of wildlife. While trees, shrubs, and vines in the woody keystone genera generally provide the greatest benefit, the 21 herbaceous genera that entomologist Douglas Tallamy listed in his book Bringing Nature Home are also valuable plants to include in a home landscape.
Goldenrods
At the top of this list are members of the Solidago genus, which supports 115 species of caterpillars. These summer- to fall-blooming plants are an important late-season food source, attracting many pollinators, and providing critical support to 42 species of pollen specialist (oligolectic) bees, which rely on goldenrod pollen for provisioning nests for their young. The seeds of goldenrods are a high-energy food source for birds in fall and winter, particularly for the American goldfinch, sparrows, juncos, and pine siskins.
Three of the many goldenrods native to Virginia are blue-stemmed, gray, and rough-stemmed goldenrod. The first of these species, Solidago caesia, is a clump-forming plant, 18 to 36 inches tall, whose arching stems are lined with clusters of flowers. It is particularly noted for growing in dry shade at forest edges.
Gray goldenrod (S. nemoralis), a vase-shaped plant of 6 to 24 inches, has one-sided flower plumes on gray-green stems. It does well in sun to part-shade as the flowering component of meadows where it is intermixed with native grasses. This species tolerates rocky and shallow soils as well as drought.
‘Fireworks,’ a naturally occurring cultivar of rough-stemmed goldenrod (S. rugosa), was named as the top performer in trials conducted by Mt. Cuba, the results of which were recently published in their report Solidago for the Mid-Atlantic Region. This three- to four-foot-tall plant features a showy display of golden flowers on long, arching flower spikes and can either be combined with other fall-blooming plants in beds or massed for dramatic effect.
Asters
As a group, native asters serve as larval host plants for 112 species of Lepidoptera. Additionally, they support 22 species of pollen specialist bees. Asters are divided into the Symphyotrichum and Eurybia genera, and they attract high numbers of native bees, flies, butterflies, and skippers for late-season nectar and pollen. When left standing, their seeds provide critical winter sustenance to birds, such as goldfinches, nuthatches, and cardinals.
Asters can fill a variety of roles in the home landscape. New England aster (Symphyotrichum novae-angliae), for example, is a tall, upright, sun-loving species with composite flowers in shades of purple that bloom from August to October. It can be used at the back of mixed perennial borders, in meadows, or at the center of rain gardens.
Aromatic aster (S. oblongifolium), a shorter species around three feet tall, has a dense, mounding, and rather shrub-like habit. It grows in sun to part-shade and tolerates dry, rocky soil and drought. The plant is noteworthy for the length of its bloom period, which can extend into mid-November. This species could be a good choice for rock gardens, border fronts, or a low hedge.
White wood aster (Eurybia divaricata), which measures 18 to 30 inches tall, is an excellent species for woodland gardens where it can spread vigorously to create a ground cover, even in dry, dense shade. Its prolific white, daisy-like flowers bloom in flat-topped clusters from July to October.
Sunflowers
Sunflowers are a third important group of herbaceous plants, hosting 73 caterpillar species. The Helianthus genus is also top-ranked for supporting 50 pollen specialist bees, including mining, fairy, and long-horned species. Sunflowers are another important late-season food resource for pollinators, and they also offer both food and cover to birds and small mammals. Woodland sunflower (H. divaricatus) is an easy-to-grow species, growing two to six feet tall, that tolerates a wide range of soil conditions as well as drought and the growth-suppressing chemical exuded by black walnut. This high-value plant provides floral resources with its showy fall flowers, supplies seeds for songbirds, and offers cover to wildlife with its spreading colonies. It can be used at the back of partially shaded borders, in pollinator gardens, or in naturalized plantings, such as woodland edges.
Other Herbaceous Keystone Plants
Bonesets in the Eupatorium and Eutrochium genera are next on Tallamy’s list, hosting 42 caterpillar species. Common boneset (Eupatorium perfoliatum) and hyssop-leaf thoroughwort (E. hyssopifolium) are not showy, but their dense clusters of small, white flowers draw large numbers of pollinators. They are equally suited to borders, cottage gardens, and meadows. The various tall Joe-pye species, such as Coastal Plain Joe-pye-weed (Eutrochium dubium), are great for butterfly gardens as their large domed clusters of pinkish-purple flowers make ideal landing platforms. The plants can be used in the back of large beds or in naturalized areas, and their preference for moist to wet soil makes them suitable for use in wetlands and rain gardens.
Carex, a large genus with dozens of species native to Virginia, hosts 36 Lepidoptera species. Sedges are wind-pollinated with inconspicuous flowers, but they provide food, shelter, and nesting materials for birds. While many species grow best in shade, Mt. Cuba has conducted trials on tolerance of these plants to sun exposure and mowing. They are ideal “matrix” plants in low-maintenance landscapes. By forming a dense base or ground cover layer, they act as a green mulch, suppress weeds, and control erosion. Three sedge species that illustrate the variation in appearance within the genus are Pennsylvania sedge (Carex pensylvanica) with its thin, arching, grass-like leaf blades, plantain-leaved sedge (C. plantaginea) with its crinkly, strap-like foliage, and Gray’s sedge (C. grayi) with its dramatic, mace-like seed heads.
Spring-blooming plants that rank fairly high in Tallamy’s list include violets (Viola spp.) and wild geranium (Geranium maculatum), which host 29 and 23 Lepidoptera species, respectively. Native violets are especially supportive of fritillary butterflies. They also serve as an early nectar source for bees and butterflies, and birds and small mammals rely on their seeds. The geranium draws bees, syrphid flies, and butterflies to its pink, five-petalled flowers and provides seeds for mourning doves and quail. Both types of plants can be used as edging in borders or planted en masse as a ground cover in woodlands and naturalized areas.
Coneflowers (Rudbeckia genus) host 17 Lepidoptera species and support 29 pollen specialist bees. Their seeds are a wonderful source of late-season nutrition for goldfinches, chickadees, sparrows, and cardinals. Species such as orange coneflower (R. fulgida), brown-eyed Susan (R. triloba), and cut-leaf coneflower (R. laciniata) can be massed in borders, cutting gardens, and meadows.
This composite photo shows the remaining herbaceous genera in Doug Tallamy’s list with the numbers of caterpillars they support. While little bluestem (Schizachyrium scoparium) is the only native grass included, other species such as purpletop (Tridens flavus), switchgrass (Panicum virgatum) and purple lovegrass (Eragrostis spectabilis) also support skippers, satyrs, and wood nymph moths. Grasses are an important component of meadow plantings and the edges of rain gardens, and taller species can be used individually as accent plants or grouped to provide architectural structure or screening in a garden.
See the recorded presentation “Keystone Plants for the Home Landscape” for more details, and many additional photos illustrating each plant and its use in the landscape.