sign in sign up
<<
Home , Betula nigra, Betula pendula, Populus

Alternatives to Borer- Susceptible B y R o y F a rr o w

n recent years, the population of bronze those standards. Even short periods of heat and borer (BBB), Agrilus anxius, has drought can stress birch , and their shallow been on the rise in our region, leading to roots are easily damaged by soil disruption. Old Ithe demise of many of our beloved birch trees. age, wind breakage and root competition from The hardest hit birches are the non-native nearby trees add to the list of stressors. For , which are “ecologically naïve” to those who are unwilling to submit to a birch-less this native North American -boring existence, choose wisely and manage diligently. beetle. Birch species whose native range over- On the positive side, every loss is an oppor- laps with that of the BBB have evolved some tunity in horticulture. When looking to replace resistance to it. These include Betula papy- a birch with another species, it is important to rifera ( birch), B. lenta (sweet birch), consider the niche that it was occupying, along B. alleghaniensis (yellow birch), B. populi- with any other garden needs of a newly planted folia (grey birch), and B. occidentalis (water tree. When selecting and siting a new tree, I use birch). Betula nigra (river birch) is considered three decision-making criteria. near-immune to colonization by BBB and is CRITERIA FOR CHOOSING AN ALTERNATIVE rarely attacked. The U.S. Service points out that Ecological Depth although all birch species are susceptible to No tree should be an island. As Barry Commoner attack, only stressed and unhealthy native trees put it in his first Law of Ecology: “Everything is are at risk of colonization. Vigorous U.S. native connected to everything else.” Your garden is an trees can produce callus tissue surrounding ecosystem, and all of our individual, personal a BBB larva, causing the insects to die before ecosystems make up the greater urban forest. pupation. Tree vigor depends on a minimum of The degree to which your new benefits its stress. Birch trees require cool, moist soils— local wedge of the world should be maximized. and urban soil conditions often do not meet Overall, Betula species are wonderful wildlife trees. The list of birds and animals that feed 8 v Park Arboretum Bulletin With its white , airy canopy and beautiful fall color, quaking () can be a good birch alternative. (Photo by Famartin/Wikimedia Commons) Aesthetics Your new tree should be something that pleases from birch trees is long: Sapsuckers feed on you, inspires you, calms your mind or makes you the sap, while other woodpeckers feed on any giddy with delight. If you cannot replace the tree grubs living in the wood. Songbirds such as you lost with an exact replica, figure out which Siskins and Chickadees eat the . aspects of the tree you truly enjoyed and would Birches often host small insects, which in turn like to see in your new planting. Also, think about support populations of beneficial insects and what new features you might like to bring in with insect-gleaning birds. Birch foliage provides your next addition to the garden. food for the larvae of lepidopterans, such as Though birch trees have many attractive Mourning Cloaks, Tortoiseshells and Dusky- features—such as pleasant fall color and open, airy wings (at least 80 species!). Due to their canopies—the most prominent attribute that high-nutrient litter, birch trees are known to con- motivates people to plant them is their gorgeous vert mor humus soils, low in microorganisms, to white bark. Unfortunately, this feature seems to mull humus soils, rich in invertebrate life. Any be the defining trait of the BBB-affected birches. tree seeking to replace a birch has a lot to live up to! Betula pendula (European white birch) and Functionality B. utilis var. jacquemontii (Himalayan white birch) This criterion is often summed up as “right are the two most commonly planted amenity plant, right place.” I find that people often do not trees for white bark­—and, as their names suggest, spend enough time considering this basic tenet neither is native to North America. of sustainable gardening. Birch trees generally BEAUTIFUL, BORER-RESISTANT favor plenty of sun on their crown with their roots AMERICAN BIRCHES in cool, moist soil. If you have lost a tree to BBB, (paper birch) is another it is possible that your tree was predisposed to birch displaying beautiful white bark. It grows invasion due to a mismatch with the site conditions. to between 50 and 70 feet tall and produces As gardens grow, site conditions change. good fall color. Paper birch is native to north- Even an originally well-sited tree may eventu- ern North America—including our region—and ally succumb to competition as the neighboring has evolved moderate BBB resistance. If sited grow along with it. Perhaps irrigation was well, with maintained vigor, paper birch should not provided during a droughty spell because the provide years of beauty and ecological benefits. tree was thought to be “established.” Betula nigra (river birch) from the eastern The reality of your planting site must U.S. is a truly phenomenal garden plant. Growing the needs of your replacement plant well. quickly up to 60 feet tall, it takes a more burlesque Though the crown of your 30-year-old birch approach with its bark. Instead of a smooth and was receiving full sun, your five-foot-tall demure white, the river birch’s silvery-white new addition may never thrive in the resulting bark peels gratuitously to pinks, salmons shadowy pocket. Similarly, the rich and roomy and cinnamon-browns underneath. The species rooting zone that the original tree enjoyed may is adaptable to many soil types, except for those now be jealously dominated by the surviving with alkaline pH. River birch also is consid- old-guard trees of the garden. ered to be the most heat-tolerant of the North Also, keep climate change in mind. Though American native birches, a possibly advanta- local effects are difficult to predict, the climate geous trait considering climate change. models for our region indicate a rise in air Dwarf and weeping are available. temperatures, a decrease in soil moisture levels, The B. nigra ‘Cully’, also known as the and a lengthening of the growing season. Trees Heritage® birch, is a Great Plant Picks (great- from warmer climes that were once tricky to plantpicks.org) tree selected for its richly colorful grow here may make better replacements in peeling bark. We recently planted a specimen by years to come. Spring 2020 v 9 Betula nigra ‘Cully’ Acer griseum Alnus rubra (photo by Niall Dunne) (photo by Niall Dunne) (photo by Roy Farrow) the Arboretum Loop Trail, where it intersects to the prevalence of lichens. In spring, the male with the Birch Parking Lot. elongate before the arrive, and ALTERNATIVES TO BIRCHES in fall the cones persist on the twigs. The seeds Following are short profiles of birch alternatives. of are an important food source for Most feature attractive bark, while some also Goldfinches, Chickadees, and other -eating offer other traits associated with birches—such birds in our area. as good fall color; a light, airy canopy; and good For smaller gardens, both Cercis canadensis resources for wildlife. (eastern redbud) and Cercis siliquastrum (Judas Acer griseum (paperbark ) is well tree) provide a lush, airy feel—not to mention a known for its gorgeous, peeling, bark stunning display of brilliant-pink pea and stunning, orange-and-red fall color. A in the spring. Cercis canadensis ‘Forest Pansy’ small tree from China—and actually an endan- is a deep-purple-leafed cultivar. Both species gered species there—it grows to about 25 feet top out at about 25 feet tall and boast good fall tall and produces trifoliate green leaves (divided color. Cercis species prefer moist soils and into three leaflets) with silver undersides. The will require summer irrigation if used in a dry tree does well in full sun, and all but the most landscape. compacted urban soils. As with most , No discussion of beautifully barked trees its flowers, and seeds are eaten by a would be complete without mentioning of birds and small mammals. (gum tree), from Australia. Currently, Alnus rubra (red ) may be the most only a small handful of the genus’s 660 species under-used ecological workhorse in designed can thrive in the cold and wet of the Pacific gardens. Native to the Pacific Northwest, it’s a Northwest, but more may become available to pioneer species that specializes in colonizing us as the climate warms (such as my personal poor and disturbed soils. Alder root nodules host favorite, Eucalyptus nicholii, an almost-hardy, Frankia alni, a nitrogen-fixing bacteria that feeds narrow-leafed, willowy tree with foliage that the tree and enriches the soil. The litter of smells like peppermint when crushed). red alder is particularly nutritious and helps Growing to 20 feet, the beautiful Eucalyptus support an abundance of soil and plant life. pauciflora ssp. niphophila (snow gum) is Alnus rubra grows quickly in its youth and can uncommon around Seattle, even though it’s reach 25 feet tall within 10 years, before slowing exceptionally hardy; only the harshest winters to a more reasonable pace. Its foliage is intolerant will cause damage—and then only to smaller of shade, and this leads to an open, airy canopy, branches. Although pauciflora translates to which in turn provides good viewing of the “few-flowered,” the snow gum can put on a smooth, light-grey bark that ages to white—due wonderful display in spring, providing

10 v Washington Park Arboretum Bulletin Snow gum (photo by Acabashi/ Lagerstroemia ‘Musk0gee’ in the Stewartia monadelpha flower and Wikimedia Commons) Arboretum. (Photo by Niall Dunne) bark. (Photo by Niall Dunne) resources to early pollinators. The bark of the mature specimens—exfoliates to reveal plates snow gum is smooth, typically white to light grey, of green, tan and white beneath. In spring, the and exfoliates in patches to give a mottled tree is covered with interesting, red, apetalous appearance. The evergreen leaves are blue-grey flowers. And in fall, the foliage color runs the and concentrated with the compound cineole gamut from orange to purple. Being native to (a.k.a. eucalyptol), which gives them their northern Iran and the Caucasus region, it is quite signature scent. drought tolerant when established. Lagerstroemia (crapemyrtles) are known Another regionally native tree worth consid- for beautiful, exfoliating bark and amazing ering is Populus tremuloides (quaking aspen). fall color. Depending on the species or culti- Growing up to 50 feet tall, it offers beautiful var, bark displays come in a mix of pinks, white trunks, small leaves that shimmy in the tans and browns. Native to Southeast Asia and breeze, and wonderful yellow fall color. As with northern Australia, crapemyrtles generally red alder and our native birches, its wildlife prefer hot, sunny climates, however more and value is high. The foliage hosts Tiger Swallowtail more hybrids are appearing that perform well and butterflies, as well as sphinx moths. in our region. The showy flowers arrive in late Many birds feed on the aspen seeds and insects summer to early fall, depending of the warmth attracted by the tree. Quaking aspen is not toler- of the summer and the microclimate of the tree. ant of urban pollutants, however, so this is not a Flower color ranges from white to pink to laven- tree for city centers. der all the way to deep red, and ranges Another genus known for outstanding bark from wide-spreading shrubs to 25-foot-tall, is Stewartia (stewartia). The bark of Stewartia upright trees. Crapemyrtle seeds are voraciously pseudocamellia peels off in thin plates revealing a consumed by House Finches, Dark-Eyed reddish-brown and buff-colored mottle pattern Juncos, and other birds. akin to sycamore. Stewartia monadelpha displays Parrotia persica (Persian ironwood) is fine-peeling reddish-cinnamon bark. Both another tree species that deserves to be planted species exhibit stunning fall color, typically a more often in our gardens. Growing between 20 mixture of oranges to deep purples. Both prefer and 30 feet tall, it features silvery bark that—on moist, well-draining soil and mature at about 40 feet. And both display large, camellia-like white Close up of Persian ironwood bark. (Photo by Niall Dunne) flowers in early summer, a source of pollen and nectar for bees. m

Roy Farrow is the UW Botanic Gardens horticulture supervisor at Washington Park Arboretum. Spring 2020 v 11