Forest Green® Oak (Quercus frainetto ‘Schmidt’) is an in-demand street tree. It has caught on in California and is a favorite in cities in British Columbia.Photo courtesy of J. frank schmidt & son co.

By Elizabeth PetersenThe benefits of trees in urban areas

are well known. They improve air and water quality; they provide shade, shel-ter and wildlife habitat; they improve real estate values, provide significant human health benefits and generally enhance the quality of life.

But less-than-ideal growing con-ditions make selecting trees that will thrive in cities a significant challenge.

The challengesUrban foresters, growers and land-

scape architects, whose work involves

adding trees to the urban environment, are focused on a common goal: to find and match the right tree to the right place.

Paul D. Ries, an instructor and extension specialist of urban forestry at Oregon State University, has more than 25 years of urban forestry experi-ence. He also manages the Urban and Community Forestry Program for the Oregon Department of Forestry. He shared his perspective on selecting the best trees to take on and survive the challenges of city living.

“There is no such thing as a perfect tree for an urban environment,” Ries

Urban development drives demand for new tree varieties that can handle challenging growing conditions in the city

Street tough

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said. “Most trees we plant in cities are installed in less-than-native environ-ments and less-than-ideal conditions.

“Urban trees must compete with challenges, including hardscapes, turf conflicts and lack of maintenance and care. Often, urban foresters find that space is the biggest challenge. Urban trees need overhead and underground space to thrive. Without it, they may survive, but not thrive.”

For lining city streets, planners need to select trees that tolerate the heat island effect, automobile exhaust and compacted, poor soils. Site conditions dictate the species selection options.

Matt Krueger directs the Urban Tree Canopy Program for the City of Portland’s Environmental Services. He noted that desirable urban trees must be hardy, low maintenance, drought or semi-drought tolerant and not messy.

The need for diversityKrueger’s number one concern for

trees in urban areas is species diversity.“We want to have balance, so that

if a new disease arrives that attacks a certain genus, we don’t lose as high a percentage of our trees,” he said.

To find these specially adapted trees, growers and researchers have searched for and evaluated many possibilities, choosing the best from thousands of seedlings that perform well in trials.

There is “a demand for greater vari-ety from urban foresters who understand the important role species and cultivar diversity plays in protecting the health of urban forests,” said Nancy Buley, com-munications director for J. Frank Schmidt & Son Co. (JFS), a wholesale grower of shade, flowering and specialty ornamen-tal trees in Boring, Ore.

“Growers need to pay attention to urban forestry trends and offer a broad-er plant palette,” Buley said. “We need innovative, diverse solutions.”

In his role as director of product development at JFS, Keith Warren is intent on breeding and selecting new varieties that will survive and thrive in urban environments. Over the course

of his 39-year career at JFS, Warren has evaluated hundreds of thousands of seedlings from a wealth of genetic material. He also oversees extensive field trials of trees that are sent to JFS by plant breeders, arbo-retums, research institutions, growers and individuals from around the world.

Promising selections are tested in more than 40 sites around the country to establish the strengths and limitations of new trees and to deter-mine their regional suitability. Special attention is given to developing trees that can thrive in urban sites despite soil com-paction, air pollution, insect attack, disease, bitter cold, intense heat and many other environmental factors.

Fitting inFitting beneath utility

wires is one important consid-eration. JFS offers a substan-tial list of UtiliTrees®, which mature at heights that are unlikely to tangle with overhead lines. The list is a good start-ing place for foresters and landscape architects, Buley said, but some are more broadly adapted and others are more regionally appropriate.

A recent addition to the UtiliTrees® line, City Sprite® Zelkova (Zelkova ser-rata ‘JFS-KW1’ PPAF), has been well received in regions including the Mid-Atlantic, Northeast, Southeast, California and Oregon.

Urban foresters and utility compa-nies favor the compact growth habit of this somewhat broad, dwarf (25–30

feet) city tree. Short internodes, gener-ally upright branching, a good central leader and a great oval or vase shape have made it a popular selection. The finely textured foliage radiates a brighter green in summer than typical Zelkova and produces yellow fall color. Hardiness is USDA Zone 5.

Likewise, Wireless® Zelkova (Zelkova serrata ‘Schmidtlow’) stays low with a broad spreading shape and red fall color.

Relatively small size is only one concern for urban foresters. They also strive to increase the shade canopy, which helps moderate the heat island effect, increase air filtration and reduce

Afterburner® Tupelo (Nyssa sylvatica ‘David Odom’)

is a new cultivar featuring beautiful, high-gloss summer foliage that turns bright red

in fall, putting on a show even after most seedling tupelos have defoliated.

Photo courtesy of J. frank schmidt & son co.

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storm water runoff. For these purposes, big, leafy trees are needed, and ones that can tolerate heat and drought are especially useful.

Emerald Avenue® Hornbeam (Carpinus betulus ‘JFS-KW1CB’ PP 22814), which came from a seed source in the Mediterranean, can withstand both heat and drought. This JFS introduction main-tains healthy, deep green foliage when other cultivars of this species look tired and heat stressed in late summer.

Vigorous and easy to grow, it boasts a stout trunk, a strong central leader and sturdy branching. Strongly textured leaves, which are durable, pest and disease resistant, turn buttery yellow in autumn. About 40 feet tall with a 28-foot spread and Zone 5 hardi-ness, Emerald Avenue is a good choice in the South and arid landscapes across the country, Buley said.

Native trees for urban sitesPeople like the idea of planting

native trees, but pollution and compact-ed soils can make doing so problematic. Many native trees fail to thrive in urban settings. After all, there is no tree that is technically native to any 5-by-5-foot concrete cutout.

At JFS, Warren has focused selec-tion efforts on the most promising native species. Seedlings of Nyssa syl-vatica, for instance, are quite variable. They often display a droopy branching habit and weak leaders, making them unsuitable for planting on city streets. Lately, though, Warren has singled out seedlings that demonstrate a strong cen-tral leader and upward branching, both characteristics appropriate for use as a street tree.

Some 15 years of trial and selec-tion have resulted in the introduction of several new cultivars, including Afterburner® Tupelo (Nyssa sylvatica ‘David Odom’). A great shape formed by symmetrical, uniform branches and a straight central leader plus excep-tional vigor make this cultivar easier to grow and more beautiful. Its high-gloss summer foliage turns bright red in

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Urban foresters and utility companies favor the compact growth habit of City Sprite® Zelkova (Zelkova serrata ‘JFS-KW1’ PPAF); plus, the foliage is a brighter green in summer than typical zelkova and radiates yellow fall color. Photo courtesy of J. frank schmidt & son co.

fall, putting on a show even after most seedling tupelos have defoliated.

The broadly pyramidal Red Rage® Tupelo (Nyssa sylvatica ‘Haymanred’) was discovered in southern Indiana and brought to JFS for trial and evaluation by plantsman Mike Hayman. A co-intro-duction with Hayman, it resists leaf spot and has great fall color.

Gum Drop® Tupelo (Nyssa syl-vatica ‘JFS-PN Legacy1’) was chosen at Princeton Nursery in New Jersey for its upright, oval shape, good density, somewhat compact form and bright red fall color. After trial and evaluation at JFS, it was co-introduced in 2012.

All three cultivars handle compacted or wet soils well, so they have the poten-tial to be good urban street trees. All are rated hardy through USDA Zone 5.

Another native tree that promises predictable performance and appear-

ance plus adaptability is Emerald City® Tulip Tree (Liriodendron tulipifera ‘JFS-Oz’). A strong central leader, predictably uniform growth habit and relatively compact form distinguish it from the species and recommend it for landscape use. Glossy, emerald green foliage turns bright yellow in autumn.

The not-so-new“New” is a relative term, especially

when talking trees. “Tree years are even longer than dog years,” quipped Buley. It often takes 20 years from planting for a tree cultivar to catch on.

One example is Forest Green® Oak (Quercus frainetto ‘Schmidt’), which is suddenly in high demand. It has caught on in California, where it is being used in a number of high-profile projects, and it is a favorite street tree in cities in British Columbia, Buley said.

Landscape architect Marianne Zarkin, who designed plantings for the west-side Portland-Milwaukie light rail, speci-fied 22 Forest Green oaks in her plan. Commonly called the Italian or Hungarian oak, Q. frainetto is great for the West Coast, where Mediterranean-like condi-tions reign: wet winters and dry sum-mers. An upright-growing, vigorous oak with a strong central leader, it displays good drought resistance and adaptability. Glossy, deep green foliage and an attrac-tive symmetrical shape make it one of the best-looking trees of summer.

Environmental and aesthetic ben-efits are hard to quantify, but an infor-mative sign created by Portland Parks and Recreation for an Arbor Day event predicts that Forest Green® Oak will contribute benefits valued at $18,314 over its lifetime.

Krueger reports that oaks are

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showing promise in adding to the diver-sity of Portland trees.

“Although not native to Portland, oaks are tough, long-lived and drought tolerant in general. We are having great success with some West Coast natives from southern Oregon and California,” Krueger said. These include Q. kelloggii, Q. wislizeni and Q. douglasii.

Carpinus caroliniana has a repu-tation for droopy limbs and variable color, both of which make it less useful as a street tree. Fifteen years of search-ing for a better selection, though, have produced the Native Flame® American Hornbeam (Carpinus caroliniana ‘JFS-KW6’), which was selected for good upright growth and form, and bright red fall color. There is more potential from this versatile native

species, Buley said. “Hornbeams are ripe for cultivar development.”

Less messThe Urban Pinnacle® Oak (Quercus

macrocarpa ‘JFS-KW3’ PP 22815) is a large, adaptable tree with acorns that are much smaller than those of other burr oaks, which cuts down on litter. It has a narrow, upright habit, grow-ing to about 55 feet tall with a 25-foot spread and a strong central leader. In addition, this cultivar features attractive, glossy foliage and resistance to both mildew and anthracnose. Hardiness is USDA Zone 3.

The Espresso™ Kentucky Coffee Tree (Gymnocladus dioicus ‘Espresso-JFS’) is an eastern native widely adapted to areas including California,

New Mexico and Utah. It has huge, doubly compound leaves that give it a tropical feel and arching branches reminiscent of an elm. It produces no seed litter and has good tolerance to heat and drought. Cold-hardy to USDA Zone 4, it gets about 50 feet tall with a 35-foot spread.

“We are interested in trying species not commonly known in Portland—ones that work well in our climate and add to our diversity,” Krueger said. “So we are looking to areas with similar cli-mates to see what is growing there.”

Elizabeth Petersen writes for the garden industry and teaches SAT/ACT test prep at www.satpreppdx.com. She can be reached at gardenwrite@comcast.net or satpreppdx@comcast.net.

Selected for its low height and broad spreading shape, Wireless® Zelkova (Zelkova serrata ‘Schmidtlow’) has an ideal shape for street plantings under utility lines. Its red fall color is an attractive extra feature. Photo courtesy of J. frank schmidt & son co.

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