Pollinators are an essential part of our gardens, the ecosystem, and the United States economy. One in three bites of food you eat depends on pollinators. Honeybee pollination adds more than $15 billion to the value of agricultural crops in the United States each year, with another $9 billion coming from pollination by other species. Pollinator populations have been declining after decades of stress related to loss, degradation, and fragmentation of habitat; reduction in the number and quality of food sources; a lack of sites for breeding, nesting, and roosting; and improper use of pesticides and herbicides. Gardeners can be part of the solution to pollinator loss by creating landscapes that support pollinator health.
From butterflies and bees to beetles and birds, many different kinds of pollinators have evolved within their ecosystems, building unique relationships with plants. When gardeners think of designing landscapes for pollinators, they may imagine plantings of floriferous herbaceous beds; however, trees and shrubs are essential components of the habitat required to support a wide variety of pollinators.
For successful pollination, a pollinator must find a flower with a structure that matches its body. Consider a butterfly feeding on a daisy-like composite flower. The butterfly will gracefully land on the inflorescence and elegantly unfurl its proboscis, which it precisely inserts through the long narrow tube of a central disc flower to drink nectar hidden inside. During this process, a cleverly positioned anther (male flower part) rubs against the butterfly depositing pollen. After drinking nectar, the butterfly flutters away to the next bloom where the pollen will be brushed against the stigma (female flower part).
Now consider beetles, which are sometimes referred to as “mess and soil” pollinators because of how they blunder their way through blossoms searching for food. Beetles are important pollinators for flowers like those of magnolia species, often arriving early in the season when temperatures are still too cool for most other pollinators. Since beetles did not originally evolve as pollinators, plants had to adapt to find a way to lure these insects. Most early-season beetles are attracted to rotting materials, as many beetle grubs are decomposers of decaying wood and plant tissue. The strongly fruited or slightly fetid smells associated with magnolias play on these preferences.
Beetles don’t possess special pollen-collecting features. Instead, with magnolias, they get covered in pollen while chewing on anthers and tepals (the term for undifferentiated petals and sepals). They then carry the pollen to the next flower. The process may not be as refined as that of a butterfly, but it is just as necessary for certain plants. Beetles are ancient and rank among the earliest evolving pollinators. Therefore, their correlation as the pollinator of ancient plants like magnolias makes evolutionary sense. Beetles and magnolias existed before bees and butterflies, and though bee pollination has been observed on several magnolia species at the Arnold Arboretum (see Juan Losada’s 2014 article in Arnoldia), the unique connection between these beetles and plants was developed well before other players arrived to the pollination game.
At the Smithsonian Pollinator Garden in Washington, DC, we showcase trees, shrubs, and herbaceous plants that support a wide array of pollinators and other wildlife. The garden occupies a relatively small footprint in the urban landscape. In a 400-by-40-foot space, we cultivate more than two hundred types of plants, including many woody species. Diversity and seasonality are among the most important factors when choosing plants for pollinators. Plants with high wildlife value and great aesthetics help our public landscape achieve our goals of creating a healthy ecosystem, while both attracting and educating our visitors.
Selecting Shrubs for Pollinators
The evolution and lifecycle of shrubs make them a particularly important part of a wildlife garden. All of the shrubs on this list are tough and adaptable in Mid-Atlantic gardens and often beyond. In addition to producing beautiful pollinator-attracting blooms, many have multi-season horticultural impact. When choosing the best combination of plants for the landscape, be sure that you plan for a garden that serves pollinators throughout the seasons. Include shrubs that flower early in spring, as well as others that will attract heavy pollinator activity in summer or during the bustling fall when pollinators are building their winter reserves.
Witch-hazel species bookend the pollination season in our gardens. Common witch-hazel (Hamamelis virginiana, USDA Hardiness Zone 3–8) is one of the last plants that will bloom each year. When the days grow short and little else is flowering, the strap-like petals and strong fragrance of witch-hazel flowers draw pollinators like owlet moths, and potentially gnats and late-season bees scavenging for food. At the start of the year, Ozark witch-hazel (Hamamelis vernalis, Zone 4–8) is one of the first blooms to greet pollinators.
Virginia sweetspire (Itea virginica, Zone 5–9) is another versatile spring-blooming shrub in the Smithsonian’s landscape. It is prominently featured at the entrance to the Pollinator Garden in the shade of a black gum tree (Nyssa sylvatica, Zone 3–9). We have also used it in a full-sun planting, where it retains a slope along a parking lot. It thrives in both locations, but the blooms are best in full sun. Its spires of fragrant white blossoms appear in early summer and draw nectar-loving insects like butterflies and bees, including native bumblebees and sweat bees. Best planted in a mass, the plant’s rich red to purple fall color will persist into the winter in southern areas as a semi-evergreen plant. If some of the foliage has been damaged by insects, it might be a sign of more pollinator activity, as this plant is a host to the American holly azure butterfly (Celastrina idella).
Dwarf fothergilla (Fothergilla gardenii, Zone 4–8) has showy bottlebrush inflorescences that emerge from March to May before their leaves. They attract bees with their white color and pleasant scent, and then offer their pollinators a sweet nectar reward for those that get past their dense tangle of anthers. Ornamentally, it is a great three-season plant with nice foliage that becomes particularly attractive in fall, when it turns brilliant shades of yellow, orange, and red. Consider pairing fothergilla with oakleaf hydrangea (Hydrangea quercifolia, Zone 5–9), which likes similar growing conditions along a woodland edge. Its flowers appear from May to July, after fothergillas have finished, and draw later-emerging wasps and flower flies, along with the aforementioned bees.
Not all plants will contribute as much to the aesthetics of your landscape as they will to pollinators. Spicebush (Lindera benzoin, Zone 5–9) is not often the focal point in a garden, but it offers a full package of horticultural benefits. It is tolerant of deer, drought, heavy shade, and clay soil. Green-yellow flowers appear in early spring before leaves emerge, and while the flowers are small, they have garnered enough attention for this native woodland understory shrub to be called the “forsythia of the wilds.” The plants are dioecious, requiring small bees and various flies to move pollen from the larger, showier flowers on male shrubs to those on the separate females, providing a critical resource for native pollinators when many food sources are not available on the landscape. Once pollinated, the female shrubs produce red drupes that are a good food source for birds and a possible nutmeg substitute for bakers. The plant also features aromatic leaves that turn an attractive yellow in the fall.
Spicebush is one of the few host plants used by the spicebush swallowtail (Papilio troilus), a well-recognized visitor of gardens and natural landscapes in the eastern United States. The adult female spicebush swallowtail has evolved to recognize specific compounds on the surface of its host plant before laying eggs, to ensure a suitable food source for maturing larvae. Other related hosts to spicebush that can support the native spicebush swallowtail include sassafras (Sassafras albidum, Zone 5–9) and redbay (Persea borbonia, Zone 7–11).
Red chokeberry (Aronia arbutifolia, Zone 4–9) supplies a great deal of value to wildlife in our gardens. Butterflies and native bees, such as mason bees, mining bees, and bumblebees, visit its clusters of white flowers from March to May. Chokeberry foliage turns stunning shades in fall and provides a food source to some hairstreak butterflies and moths, including bluish spring moths (Lomographa semiclarata) and praeclara underwings (Catocala praeclara). This herbivore activity can be observed as typical chewing damage along leaf margins. True to their name, red chokeberry fruits (though actually pomes and not berries) have a dry, astringent taste for birds and humans alike. The fruits persist from summer into the winter, and after a long period of exposure to cold weather, the fruit becomes more palatable. This makes chokeberries an important late-season native food for birds after other food sources are exhausted. The fruit’s persistence through late winter also makes it a beautiful ornamental plant in the winter garden. Both red chokeberry and black chokeberry (Aronia melanocarpa, Zone 3–8) are useful for mass plantings or for mixing into a naturalized perennial border. Running serviceberry (Amelanchier stolonifera, Zone 4–8) is another plant that provides showy white blooms for pollinators in May, edible berries in summer, and striking fall foliage.
Hollies (Ilex spp.) are a strong draw for pollinators in late spring and early summer. Their small scented flowers often go unnoticed by garden designers, but they effectively draw the attention of bees and flies. The prolific and showy fruits of winterberry holly (Ilex verticillata, Zone 3–9) are another credit to good cross-pollination required for these dioecious plants. At the Pollinator Garden, we recently installed a grouping of dwarf American holly (Ilex opaca ‘Maryland Dwarf’, Zone 5–9), which provides the classic evergreen holly appearance in a more compact space. Other hollies like inkberry (Ilex glabra, Zone 4–9) may not have showy flowers or fruits, but you’ll still find them covered in bees and later with birds looking for a snack. The same is true for the common wax myrtle (Morella cerifera, Zone 7–10).
Shrubs can also fill a flowering lag in the summer landscape. Buttonbush (Cephalanthus occidentalis, Zone 5–9) is a captivating and attention-grabbing plant for sun to part shade. Planted at the entrance to our Pollinator Garden, buttonbush draws the interest of visitors with its unique round flower heads. The flowers are also a magnet to bees and butterflies in June, just as our hot DC summers begin to peak. Additionally, the flowers of American beautyberry (Callicarpa americana, Zone 6–10) often go unnoticed by gardeners in June and July but draw bees and butterflies for pollination, enabling the glossy purple fruit that gardeners and birds adore. In full sun, bluebeard (Caryopteris × clandonensis, Zone 6–9) and leadplant (Amorpha canescens, Zone 2–9) are other great summer pollinator plants and can mix nicely into a perennial border due to their smaller habits.
Carolina allspice (Calycanthus floridus, Zone 4–9) is also called sweetshrub and strawberry bush because of how the bloom fragrance combines hints of pineapple, strawberry, and banana. Similar to magnolias, Carolina allspice has tepals and evolved long before bees and butterflies entered the landscape. As such, its flowers are predominantly pollinated by sap beetles, though they are attractive to other local pollinators as well. The beetles are drawn by the scent of sweet fermentation, and they work their way into the shade of the overlapping tepals to find food from April to July. The flowers are easy to enter but difficult to depart. Once trapped inside, the beetle picks up pollen. After the flower further matures, the inner parts of the flower fold back to release the beetle. By that point, the stigmas will have already withered, and the beetle will move on to another flower in search of more food, unknowingly ensuring cross-pollination.
Summersweet (Clethra alnifolia, Zone 3–9) attracts a diverse group of pollinators, including butterflies, bees, and hummingbirds, which have evolved to take advantage of narrow, tubular flowers. It is one of the few blooms you can find in late-summer shade in our Pollinator Garden. Similarly, bottlebrush buckeye (Aesculus parviflora, Zone 4–8) draws butterflies, bumblebees, and hummingbirds from July to August with its big showy panicles of flowers that occur in part to full shade. Note that the ruby-throated hummingbird (Archilochus colubris) is the only breeding species of hummingbird on the East Coast each summer. It usually arrives in Washington, DC, in April, after migrating north from Mexico and Central America.
Summer pollinators also love Chenault coralberry (Symphoricarpos × chenaultii ‘Hancock’, Zone 4–7). We planted a large grouping of it at the National Museum of Natural History to retain a steep slope, and it may attract the greatest density of pollinators of any shrub in the collection. The plants are thriving and often need to be trimmed, but because they are so popular with honeybees from a hive in the museum’s insect zoo, located a few hundred feet away, our gardeners refrain from working with the plants during the summer months, preserving our record of being sting free.
Both native and non-native viburnums (Viburnum spp.) work as powerhouses in the landscape, as they attract an exceptionally wide range of pollinators with strong scents that promise either a nectar or pollen reward. Scarab beetles of the genus Cetonia are particularly interesting viburnum pollinators, possessing branched hairs on their bodies that are similar to pollen-collecting hairs found on bees. These hairs ensure a better chance of cross-pollination for self-sterile viburnum species. Beetles, however, are only one of myriad pollinators that are necessary for the successful reproduction of viburnums. As Michael Donoghue reported in Arnoldia in 1980, viburnums with long corolla tubes and sweet scents are more often pollinated by species belonging to the order Lepidoptera, while viburnums with shorter corolla tubes and muskier odors receive frequent visits from flies and small bees. This relationship corresponds to the size of the insect mouthparts. It is important to note that most viburnums produce very little nectar despite the wide range of pollinators associated with the genus. It is thought that the primary reward, at least for bees, is not nectar but pollen.
More than Flowers
Flowers are not the only consideration when creating a garden for pollinators. We must consider the needs of pollinators throughout their entire lifecycle. Creating a habitat means maintaining gardens that provide shelter and food. At the Pollinator Garden, we wait to cut back plants and remove dead foliage until spring, if at all.
To accommodate the full lifecycle of pollinators, we must cater to caterpillars and other immature insects. In Eric Carle’s book The Very Hungry Caterpillar, generations of schoolchildren have learned that we will not have beautiful butterflies without munching caterpillars. Caterpillars can be picky eaters, so we plant a wide variety of host plants in the Pollinator Garden. Pollinators often rely on specific trees, shrubs, perennials, and annuals as food sources. Some plants, like spicebush (Lindera benzoin), the host plant of the spicebush swallowtail butterfly, have pollinator-friendly flowers. But even wind-pollinated species can be important for pollinators. The foliage of smooth alder (Alnus serrulata, Zone 4–9), for instance, provides a significant food resource for beetles, aphids, moth caterpillars, and other insects.
When planting a garden for pollinators, we need to be okay with leaves being eaten. It is also best to acquire plants from nurseries that have not treated their plants with systemic insecticides. In the 180 acres maintained by Smithsonian Gardens, we do use insecticides, but only as a last resort. Our preferred methods of control are mechanical, cultural, and biological. The plants in the Pollinator Garden are in good health, in part because maintaining a diverse plant inventory supports a balanced garden ecosystem. During our tenures at Smithsonian Garden, neither of us can recall spraying insecticides in the Pollinator Garden or in the preceding Butterfly Garden. In the extreme case that we ever need to apply an insecticide in the future, we would certainly make sure that the product would not affect beneficial insects and pollinators.
In the end, pollination is all about survival and sex. The insect and the plant both require something. The pollinator is often drawn to a plant with an offer of food. In turn, the plant uses the pollinator as a vector to move its pollen to the stigma of another flower. Plants have evolved with particular traits, and pollinators select blooms for their preference for color, odor, nectar, nectar guides, pollen, and flower shape. These traits, combined with bloom period and location, make for a variable matrix of pollinator and plant interactions. Therefore, it is important to grow a large selection of plants, including shrubs, to support the needs of a great variety of pollinators.
James Gagliardi is a supervisory horticulturist with Smithsonian Gardens in Washington, DC. After President Obama released a memorandum to promote pollinators in 2014, he worked on a task force with the Council on Environmental Quality to draft Supporting the Health of Honey Bees and Other Pollinators. He is honored to be the editor of the Smithsonian’s first gardening book, Encyclopedia of Garden Plants for Every Location.
Holly Walker is the Plant Health Specialist at Smithsonian Gardens in Washington, DC. With a diverse background in integrated pest management (IPM), biological control, and native pollinator conservation, she works to educate the public in environmentally responsible pest management in both urban and rural landscapes. She recently completed her PhD in entomology at the University of Delaware.