Sadly, it has been confirmed that the bronze birch borer (Agrilus anxius) is present in the Arboretum. The boring insect, a birch specialist endemic to North America, is prone to periodic outbreaks and has now been implicated in the demise of at least one birch, and the decline of several others in the Arboretum.
All North American native Betula species share a co-evolutionary history with the bronze birch borer. This means that they possess natural defenses to the borer and will resist colonization unless stressed. All Eurasian species are considered “evolutionarily naïve” and are much more susceptible to colonization. Culturally, there are two practices proven to improve a tree’s resistance to bronze birch borer. For existing trees, do not allow a birch to experience drought. For new plantings, choose resistant varieties, e.g. North American native species.
Fortunately, it has been found that one of the most striking of species of birch is also one of most borer resistant, with some sources reporting possible immunity. Betula nigra, most commonly known as river birch, is best appreciated for its wonderful exfoliating bark. The light-colored outer bark peals exuberantly to reveal cream, salmon, gray-brown, salmon-brown, cinnamon-brown to red-brown inner bark. With age, the bark darkens and becomes ridged and furrowed.
In order to maximize the effect of its wonderful bark, river birch is often grown as a multi-trunk tree. This also has the benefit of forcing the tree to remain toward the lower end of its potentially imposing height range. Additionally, there are many cultivars of river birch which might make it a more suitable tree for a smaller lot. Betula nigra ‘Little King’ [Fox Valley tm] is a dwarf cultivar reaching around 15-20’ in 15 years. B. nigra ‘Graceful Arms’ is a more spreading, semi-weeping selection. B. nigra ‘Cully’ [Heritage tm] is known to be a superior form of the species exhibiting glossier deep green leaves, an excellent yellow in the fall and reportedly up to a 50% increase in vigor over seedlings. Michael Dirr describes the bark of the last as “in every way superior to row-run seedlings… I have marveled at the fine coloration.”
In addition to the showy bark, I find the fall color on my own river birch to be a warm lemon yellow, a shade cooler than butter. Though pretty, birch leaves fall soon after turning, leaving you with a pleasant yellow carpet about the tree.
Dirr also describes Betula nigra as “probably the most trouble-free birch” with regards to pests and diseases. Bronze birch borer resistance is due to a lack of Rhododendrol, a chemical boring insect attractant present in the bark of stressed and dying cambial and phloem tissue. Aphids are commonly present, though not troublesome, ensuring visits from bushtits, kinglets, chickadees and other small insect-gleaning birds.
River birch is easily grown in medium to wet soils. As the common name suggests, river birch tolerate periodic flooding. As with other birch species, river birch is well adapted for holding the banks of water ways in place. Naturally wet soil is not an absolute requirement, though mulch underfoot and occasional irrigation in the drier months will keep river birch in top form. One final note on care: avoid pruning river birch in the spring as this species is known to bleed sap profusely.
Scientific name: Betula nigra
Common name: River Birch, Black Birch, Red Birch
Origin: Massachusetts to Florida, west to Minnesota and Kansas. Naturally restricted to river banks and moist sites.
Height and spread: 40-70 feet. In 2013 the National Champion was recorded at 117 feet in Kentucky.
Location: In the Washington Park Arboretum, an excellent four-trunked specimen planted in 1952 stands along Azalea Way, just at parking lot #19, the Birch lot. Three more single-trunk specimens planted in 1964 are located just to the north along Arboretum creek.
(Click to see large photos)
Dirr, Michael A., Manual of Woody Landscape Plants, Stipes Publishing L.L.C., Champaign, Ill, pp.127-129.
Muilenburg, Vanessa and Herms, Daniel, A review of bronze birch borer (Coleoptera: Buprestidae) life history, ecology and management, Environmental Entomology 2012 Vol 41 No. 6, pp.1373-1385.
Santamour, F.S. Jr., Rhododendrol and susceptibility to the bronze birch borer, Journal of Arboriculture 1990 Vol. 16 No. 10, pp.260-263.