Shrubs And The Pollinators Who Love Them - Harvard University

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Shrubs and the Pollinators Who Love ThemJames Gagliardi and Holly WalkerPFrom butterflies and bees to beetles and birds,many different kinds of pollinators have evolvedwithin their ecosystems, building unique relationships with plants. When gardeners think ofdesigning landscapes for pollinators, they mayimagine plantings of floriferous herbaceousbeds; however, trees and shrubs are essentialcomponents of the habitat required to supporta wide variety of pollinators.For successful pollination, a pollinator mustfind a flower with a structure that matches itsbody. Consider a butterfly feeding on a daisylike composite flower. The butterfly will gracefully land on the inflorescence and elegantlyunfurl its proboscis, which it precisely insertsthrough the long narrow tube of a central discWILLIAM (NED) FRIEDMANollinators are an essential part of ourgardens, the ecosystem, and the UnitedStates economy. One in three bites offood you eat depends on pollinators. Honeybeepollination adds more than 15 billion to thevalue of agricultural crops in the United Stateseach year, with another 9 billion coming frompollination by other species. Pollinator populations have been declining after decades of stressrelated to loss, degradation, and fragmentationof 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 partof the solution to pollinator loss by creatinglandscapes that support pollinator health.This congregation of honeybees (and one beetle) was spotted on bigleaf magnolia (Magnolia macrophyllassp. macrophylla, 961-89*B).

May 2018DANNY SCHISSLER18 Arnoldia 75/4Pollen loads the hind legs of this honeybee, which was noticed on the early spring flowers of an Ozark witch-hazel(Hamamelis vernalis, 6099*D).flower to drink nectar hidden inside. Duringthis process, a cleverly positioned anther (maleflower part) rubs against the butterfly depositing pollen. After drinking nectar, the butterflyflutters away to the next bloom where the pollen will be brushed against the stigma (femaleflower part).Now consider beetles, which are sometimesreferred to as “mess and soil” pollinatorsbecause of how they blunder their way throughblossoms searching for food. Beetles are important pollinators for flowers like those of magnolia species, often arriving early in the seasonwhen temperatures are still too cool for mostother pollinators. Since beetles did not originally evolve as pollinators, plants had to adaptto find a way to lure these insects. Most earlyseason beetles are attracted to rotting materi-als, as many beetle grubs are decomposers ofdecaying wood and plant tissue. The stronglyfruited or slightly fetid smells associated withmagnolias play on these preferences.Beetles don’t possess special pollen-collecting features. Instead, with magnolias, they getcovered in pollen while chewing on anthersand tepals (the term for undifferentiated petalsand sepals). They then carry the pollen to thenext flower. The process may not be as refinedas that of a butterfly, but it is just as necessary for certain plants. Beetles are ancient andrank among the earliest evolving pollinators.Therefore, their correlation as the pollinatorof ancient plants like magnolias makes evolutionary sense. Beetles and magnolias existedbefore bees and butterflies, and though beepollination has been observed on several mag-

JAMES GAGLIARDIPollinators 19At Smithsonian Gardens, the Urban Bird Habitat is located on the south and west sides of the National Museum of NaturalHistory, not far from the Pollinator Garden.JAMES GAGLIARDIoccupies a relatively small footprint in the urbanlandscape. In a 400-by-40-foot space, we cultivate more than two hundred types of plants,including many woody species. Diversity andseasonality are among the most importantfactors when choosing plants for pollinators.Plants with high wildlife value and great aesthetics help our public landscape achieve ourgoals of creating a healthy ecosystem, whileboth attracting and educating our visitors.Selecting Shrubs for PollinatorsVirginia sweetspire (Itea virginica ‘Henry’s Garnet’) is anexceptionally adaptable shrub in the Smithsonian’s landscape.nolia species at the Arnold Arboretum (see JuanLosada’s 2014 article in Arnoldia), the uniqueconnection between these beetles and plantswas developed well before other players arrivedto the pollination game.At the Smithsonian Pollinator Garden inWashington, DC, we showcase trees, shrubs,and herbaceous plants that support a wide arrayof pollinators and other wildlife. The gardenThe evolution and lifecycle of shrubs makethem a particularly important part of a wildlife garden. All of the shrubs on this list aretough and adaptable in Mid-Atlantic gardensand often beyond. In addition to producingbeautiful pollinator-attracting blooms, manyhave multi-season horticultural impact. Whenchoosing the best combination of plants forthe landscape, be sure that you plan for agarden that serves pollinators throughout theseasons. Include shrubs that flower early inspring, as well as others that will attract heavypollinator activity in summer or during the

May 2018KYLE PORTLarge fothergilla (Fothergilla major, 968-88*A) attracts pollinators from April to May, much like its smaller-staturedrelative (F. gardenii).Virginia sweetspire (Itea virginica, Zone 5–9)is another versatile spring-blooming shrub inthe Smithsonian’s landscape. It is prominentlyfeatured at the entrance to the Pollinator Garden in the shade of a black gum tree (Nyssasylvatica, Zone 3–9). We have also used it in afull-sun planting, where it retains a slope alonga parking lot. It thrives in both locations, butthe blooms are best in full sun. Its spires of fragrant white blossoms appear in early summerand draw nectar-loving insects like butterfliesand bees, including native bumblebees andsweat bees. Best planted in a mass, the plant’srich red to purple fall color will persist into thewinter in southern areas as a semi-evergreenplant. If some of the foliage has been damagedby insects, it might be a sign of more pollinatoractivity, as this plant is a host to the Americanholly azure butterfly (Celastrina idella).Dwarf fothergilla (Fothergilla gardenii, Zone4–8) has showy bottlebrush inflorescences thatemerge from March to May before their leaves.They attract bees with their white color andpleasant scent, and then offer their pollinators asweet nectar reward for those that get past theirdense tangle of anthers. Ornamentally, it is agreat three-season plant with nice foliage thatbecomes particularly attractive in fall, whenit turns brilliant shades of yellow, orange, andred. Consider pairing fothergilla with oakleafANSEL OOMMEN, BUGWOOD.ORG DANNY SCHISSLER20 Arnoldia 75/4Japanese spicebush (Lindera obtusiloba, 376-86*A) producesearly spring flowers like its American counterpart (L. benzoin).bustling fall when pollinators are building theirwinter reserves.Witch-hazel species bookend the pollinationseason in our gardens. Common witch-hazel(Hamamelis virginiana, USDA Hardiness Zone3–8) is one of the last plants that will bloomeach year. When the days grow short and littleelse is flowering, the strap-like petals and strongfragrance of witch-hazel flowers draw pollinators like owlet moths, and potentially gnatsand late-season bees scavenging for food. At thestart of the year, Ozark witch-hazel (Hamamelis vernalis, Zone 4–8) is one of the first bloomsto greet pollinators.The caterpillars of spicebush swallowtail (Papilio troilus)feed predominantly on the foliage of northern spicebush(Lindera benzoin).

Pollinators 21JAMES GAGLIARDIIn 1995, Smithsonian Gardens openedthe Butterfly Habitat Garden, alongthe east side of the National Museumof Natural History in the heart ofWashington, DC. After twenty-oneyears, this popular landscape wasrededicated as the Pollinator Garden.The new theme helps visitors discoverthe who, what, when, where, why,and how of pollination by interpreting the unique relationship betweenpollinators and flowers.The garden’s title change and ourextended educational efforts reflectthe growing importance of supporting pollinator health (not just butterflies alone), as championed with atask force formed by President BarackObama in 2014. Furthermore, the garden is part of the Million PollinatorGarden Challenge, launched by TheNational Pollinator Garden Network.This effort is a partnership betweenconservation organizations, gardeninggroups, volunteer civic associations,and participating federal agencies. Itaims to inspire people and organizations to create more pollinator habitats by registering a million publicand private gardens and landscapesthat support pollinators.As a key advocate for pollinators,the Smithsonian’s reinterpretation ofthe Pollinator Garden on the NationalMall educates millions of visitors onthe wide diversity of pollinators andthe types of plants that support them.JAMES GAGLIARDIThe Pollinator GardenJames Gagliardi, below, helped with redeveloping the Pollinator Garden toshowcase an evolving national awareness of the importance of all pollinators.The garden was officially dedicated in June of 2016.hydrangea (Hydrangea quercifolia, Zone 5–9),which likes similar growing conditions along awoodland edge. Its flowers appear from May toJuly, after fothergillas have finished, and drawlater-emerging wasps and flower flies, alongwith the aforementioned bees.Not all plants will contribute as much to theaesthetics of your landscape as they will to pollinators. Spicebush (Lindera benzoin, Zone 5–9)is not often the focal point in a garden, but itoffers a full package of horticultural benefits.It is tolerant of deer, drought, heavy shade, andclay soil. Green-yellow flowers appear in earlyspring before leaves emerge, and while the flowers are small, they have garnered enough attention for this native woodland understory shrubto be called the “forsythia of the wilds.” Theplants are dioecious, requiring small bees and

May 2018JAMES GAGLIARDI22 Arnoldia 75/4The drupes of northern spicebush (Lindera benzoin) are popular with bird species, including gray catbird (Dumetellacarolinensis), snacking here even before the fruits were completely ripe.various flies to move pollen from the larger,showier flowers on male shrubs to those on theseparate females, providing a critical resourcefor native pollinators when many food sourcesare not available on the landscape. Once pollinated, the female shrubs produce red drupesthat are a good food source for birds and a possible nutmeg substitute for bakers. The plant alsofeatures aromatic leaves that turn an attractiveyellow in the fall.Spicebush is one of the few host plants usedby the spicebush swallowtail (Papilio troilus), awell-recognized visitor of gardens and naturallandscapes in the eastern United States. Theadult female spicebush swallowtail has evolvedto recognize specific compounds on the surfaceof its host plant before laying eggs, to ensure asuitable food source for maturing larvae. Otherrelated hosts to spicebush that can support thenative spicebush swallowtail include sassafras (Sassafras albidum, Zone 5–9) and redbay(Persea borbonia, Zone 7–11).Red chokeberry (Aronia arbutifolia, Zone4–9) supplies a great deal of value to wildlife inour gardens. Butterflies and native bees, such asmason bees, mining bees, and bumblebees, visitits clusters of white flowers from March to May.Chokeberry foliage turns stunning shades in falland provides a food source to some hairstreakbutterflies and moths, including bluish springmoths (Lomographa semiclarata) and praeclaraunderwings (Catocala praeclara). This herbivore activity can be observed as typical chewingdamage along leaf margins. True to their name,red chokeberry fruits (though actually pomesand not berries) have a dry, astringent taste forbirds and humans alike. The fruits persist fromsummer into the winter, and after a long periodof exposure to cold weather, the fruit becomesmore palatable. This makes chokeberries animportant late-season native food for birds afterother food sources are exhausted. The fruit’spersistence through late winter also makesit a beautiful ornamental plant in the winter

JAMES GAGLIARDISUSAN HARDY BROWNPollinators 23KYLE PORTAttractive throughout the seasons, red chokeberries (Aronia arbutifolia) flower in mid-spring and bear lustrous red fruits well into winter.Flowers of inkberry holly (Ilex glabra ‘Compacta’, 745-69*D) aretucked within dense evergreen foliage.garden. Both red chokeberry and blackchokeberry (Aronia melanocarpa, Zone3–8) are useful for mass plantings or formixing into a naturalized perennial border. Running serviceberry (Amelanchierstolonifera, Zone 4–8) is another plantthat provides showy white blooms forpollinators in May, edible berries in summer, and striking fall foliage.Hollies (Ilex spp.) are a strong drawfor pollinators in late spring and earlysummer. Their small scented flowersoften go unnoticed by garden designers, but they effectively draw the attention of bees and flies. The prolific andshowy fruits of winterberry holly (Ilexverticillata, Zone 3–9) are another creditto good cross-pollination required forthese dioecious plants. At the PollinatorGarden, 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. Otherhollies like inkberry (Ilex glabra, Zone4–9) may not have showy flowers orfruits, but you’ll still find them coveredin bees and later with birds looking for asnack. The same is true for the commonwax myrtle (Morella cerifera, Zone 7–10).Shrubs can also fill a flowering lagin the summer landscape. Buttonbush(Cephalanthus occidentalis, Zone 5–9)

24 Arnoldia 75/4 May 2018Pollinator SyndromesThe combinations of floral characteristics associated with particular types of pollinatorsare known as pollinator syndromes. Other than bat pollination, which most often occurs intropical and desert ecosystems, all of these syndromes can be observed in the SmithsonianPollinator Garden. The Smithsonian team has adapted this information to create seven “pollinator profiles” for bees, beetles, butterflies, hummingbirds, flies, moths, and wind (alongwith special references to bats and water).Using a field-journal theme, each profile in our Pollination Investigation describes thepollinators’ favorite flowers based on floral characteristics. The panels teach pollination on ageneral level and are not designed for our garden alone. On May 20, 2018—the United Nation’sfirst World Bee Day—the panels were unveiled at University of Ljubljana Botanical Garden inthe Republic of Slovenia, in recognition of Slovenia’s leadership proposing the new celebration. Additionally, our Pollination Investigation panels are available to educators in gardensaround the world free of charge through the Smithsonian Gardens website.USDA FOREST SERVICEPOLLINATOR SYNDROME TRAITSTRAITBATSBEESCOLORDull white,green, orpurpleBrightwhite,yellow,blue, or UVNECTARGUIDESAbsentBEETLESBIRDSBUTTERFLIESDull whiteor greenScarlet,orange, red,or whiteBright,includingred ;emitted atnightFresh, mild,pleasantNone tostronglyfruity Sometimespresent;not hiddenPOLLENAmpleLimited;often stickyand sedduring dayShallow;have efunnel-like;cups,strong perchsupportFLIESMOTHSPale anddull to darkPale andbrown ordull red,purple;purple, pink,flecked withor whitetranslucentpatchesWINDDull green,brown, orcolorless;petalsabsent orreducedAbsentAbsentAbsentNoneFaint butfreshPutridStrongsweet;emitted allyabsentAmple;deeplyhiddenNoneLimitedModest inamountLimitedAbundant;small,smooth, andnot stickyRegular;tubularwithouta lipRegular;small andstigmasexertedShallow;Narrow tubefunnel-likewith spur;orwidecomplex andlanding padtrap-like

Pollinators 25

is a captivating and attention-grabbing plantfor sun to part shade. Planted at the entranceto our Pollinator Garden, buttonbush drawsthe interest of visitors with its unique roundflower heads. The flowers are also a magnetto bees and butterflies in June, just as our hotDC summers begin to peak. Additionally, theflowers of American beautyberry (Callicarpaamericana, Zone 6–10) often go unnoticed bygardeners in June and July but draw bees andbutterflies for pollination, enabling the glossypurple fruit that gardeners and birds adore. Infull sun, bluebeard (Caryopteris clandonensis,Zone 6–9) and leadplant (Amorpha canescens,Zone 2–9) are other great summer pollinatorplants 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 fragrancecombines hints of pineapple, strawberry, andbanana. Similar to magnolias, Carolina allspicehas 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 scentof sweet fermentation, and they work their wayinto the shade of the overlapping tepals to findfood from April to July. The flowers are easyto enter but difficult to depart. Once trappedinside, the beetle picks up pollen. After theflower further matures, the inner parts of theflower fold back to release the beetle. By thatpoint, the stigmas will have already withered,and the beetle will move on to another flowerin search of more food, unknowingly ensuringcross-pollination.Summersweet (Clethra alnifolia, Zone 3–9)attracts a diverse group of pollinators, including butterflies, bees, and hummingbirds, whichhave evolved to take advantage of narrow,tubular flowers. It is one of the few bloomsyou can find in late-summer shade in ourPollinator Garden. Similarly, bottlebrushbuckeye (Aesculus parviflora, Zone 4–8)draws butterflies, bumblebees, and hummingbirds from July to August with its bigshowy panicles of flowers that occur in partto full shade. Note that the ruby-throatedDANNY SCHISSLERMay 2018JAMES GAGLIARDI DANNY SCHISSLER26 Arnoldia 75/4Summer shrubs for pollinators include, top to bottom, buttonbush (Cephalanthus occidentalis, 123-78*A), Carolina allspice(Calycanthus floridus), and summersweet (Clethra alnifoliavar. alnifolia ‘Rosea’, 239-47*MASS).

Pollinators 27DANNY SCHISSLERhummingbird (Archilochus colubris) is theonly breeding species of hummingbird on theEast Coast each summer. It usually arrives inWashington, DC, in April, after migrating northfrom Mexico and Central America.Summer pollinators also love Chenaultcoralberry (Symphoricarpos chenaultii‘Hancock’, Zone 4–7). We planted a large grouping of it at the National Museum of NaturalHistory to retain a steep slope, and it mayattract the greatest density of pollinators of anyshrub in the collection. The plants are thrivingand often need to be trimmed, but because theyare so popular with honeybees from a hive inthe museum’s insect zoo, located a few hundredfeet away, our gardeners refrain from workingwith 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 thelandscape, as they attract an exceptionally widerange of pollinators with strong scents thatpromise either a nectar or pollen reward. Scarabbeetles of the genus Cetonia are particularlyinteresting viburnum pollinators, possessingbranched hairs on their bodies that are similarto pollen-collecting hairs found on bees. Thesehairs ensure a better chance of cross-pollinationfor self-sterile viburnum species. Beetles, however, are only one of myriad pollinators thatare necessary for the successful reproduction ofviburnums. As Michael Donoghue reported inArnoldia in 1980, viburnums with long corollatubes and sweet scents are more often pollinatedby species belonging to the order Lepidoptera,while viburnums with shorter corolla tubes andmuskier odors receive frequent visitsfrom flies and small bees. This relationship corresponds to the size of the insectmouthparts. It is important to note thatmost viburnums produce very littlenectar despite the wide range of pollinators associated with the genus. Itis thought that the primary reward, atleast for bees, is not nectar but pollen.More than FlowersBottlebrush buckeye (Aesculus parviflora, 558-2003*C) produces featherypanicles from July to August.Flowers are not the only considerationwhen creating a garden for pollinators.We must consider the needs of pollinators throughout their entire lifecycle.Creating a habitat means maintaininggardens that provide shelter and food.At the Pollinator Garden, we wait tocut back plants and remove dead foliageuntil spring, if at all.To accommodate the full lifecycleof pollinators, we must cater to caterpillars and other immature insects.In Eric Carle’s book The Very HungryCaterpillar, generations of schoolchildren have learned that we will not havebeautiful butterflies without munching caterpillars. Caterpillars can bepicky eaters, so we plant a wide variety of host plants in the Pollinator Garden. Pollinators often rely on specifictrees, shrubs, perennials, and annuals as

May 2018food sources. Some plants, like spicebush (Lindera benzoin), the hostplant of the spicebush swallowtailbutterfly, have pollinator-friendlyflowers. But even wind-pollinatedspecies can be important for pollinators. The foliage of smooth alder(Alnus serrulata, Zone 4–9), forinstance, provides a significant foodresource for beetles, aphids, mothcaterpillars, and other insects.When planting a garden for pollinators, we need to be okay with leavesbeing eaten. It is also best to acquireplants from nurseries that have nottreated their plants with systemicinsecticides. In the 180 acres maintained by Smithsonian Gardens, wedo use insecticides, but only as alast resort. Our preferred methodsof control are mechanical, cultural,and biological. The plants in the Pollinator Garden are in good health, inpart because maintaining a diverseplant inventory supports a balancedgarden ecosystem. During our tenures at Smithsonian Garden, neitherof us can recall spraying insecticidesin the Pollinator Garden or in thepreceding Butterfly Garden. In theextreme case that we ever need toapply an insecticide in the future, wewould certainly make sure that the Horticultural care is an important factor when gardening for pollinators. Atthe Pollinator Garden, horticulturists wait to remove winter foliage, which isproduct would not affect beneficial necessary for insect habitat.insects and pollinators.In the end, pollination is all about survivalJames Gagliardi is a supervisory horticulturist withand sex. The insect and the plant both requireSmithsonian Gardens in Washington, DC. After Presidentsomething. The pollinator is often drawn to aObama released a memorandum to promote pollinatorsplant with an offer of food. In turn, the plantin 2014, he worked on a task force with the Council onuses the pollinator as a vector to move itsEnvironmental Quality to draft Supporting the Healthof Honey Bees and Other Pollinators. He is honored topollen to the stigma of another flower. Plantsbe the editor of the Smithsonian’s first gardening book,have evolved with particular traits, and polEncyclopedia of Garden Plants for Every Location.linators select blooms for their preference forHolly Walker is the Plant Health Specialist atcolor, odor, nectar, nectar guides, pollen, andSmithsonian Gardens in Washington, DC. With a diverseflower shape. These traits, combined withbackground in integrated pest management (IPM),bloom period and location, make for a variablebiological control, and native pollinator conservation,matrix of pollinator and plant interactions.she works to educate the public in environmentallyTherefore, it is important to grow a large selecresponsible pest management in both urban andtion of plants, including shrubs, to support therural landscapes. She recently completed her PhD inentomology at the University of Delaware.needs of a great variety of pollinators.JAMES GAGLIARDI JAMES GAGLIARDI28 Arnoldia 75/4

Selecting Shrubs for Pollinators The evolution and lifecycle of shrubs make them a particularly important part of a wild-life 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

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