Garden Talk: November 1, 2012
From NGA Editors
2012 Urban Tree of the Year
Generations ago, stately American elms lined the streets in many of our cities and towns, their arching branches creating a canopy of leafy shade. Then imported Dutch elm disease (DED) struck. First identified in Ohio in 1930, it moved across the country, killing off most of the elms in urban areas, leaving streets bare of greenery. Many of these streets have been replanted with a variety of trees, as the loss of the elms taught a lesson in the danger of planting a monoculture of just one kind of tree. But few other trees display the elegant, vase-shaped growth habit of the American elm, and researchers have been on a quest to find an American elm cultivar or another, similarly-shaped species or hybrid to take its place in our landscapes and along our streets.
One such tree is the Accolade™ elm (Ulmus 'Morton'), which was named the 2012 Urban Tree of the Year by the Society of Municipal Arborists. This elm is a hybrid, a cross between two Asian species, Ulmus japonica and Ulmus wilsoniana. Its mature habit is upright and arching, similar to that of the American elm, but more compact, with an expected height of 40-60 feet and spread of 35-40 feet at maturity. Reliably hardy to Zone 4, it is resistant to DED (but not immune), as well as elm yellows, and does not appear to be overly susceptible elm leaf beetle infestations. A vigorous, fast grower, it transplants easily, tolerates a wide range of soil conditions, including alkaline soil, and displays nice yellow fall color.
The original elm from which Accolade™ is propagated (pictured) was planted at the Morton Arboretum in Ohio in 1924 from seed distributed by Boston's Arnold Arboretum, where it grows to this day, having survived three epidemics of DED at the arboretum over its more than 80 year life span so far.
For more information about the Accolade™ elm, go to: Tree of the Year.
Tough Trees for Tough Sites
Trees growing in urban landscapes often have it tough. Compacted soils, reflected heat, planting sites with little soil, lack of water, interference from overhead wires, and lack of maintenance are all common urban conditions that keep trees planted along streets, in parking lots, and other city spaces from thriving.
To help those interested in greening the urban environment to be as successful as possible, Cornell University's Urban Horticulture Institute (UHI) offers on-line information, including a series of videos, that covers many of the issues that need to be considered when deciding what, how, and where to plant trees in a city landscape.
The 23-minute video "Tough Trees for Tough Sites" follows an actual urban tree planting project in Ithaca, NY to explain how and why to assess a site and prepare it for planting, how to make an appropriate plant selection, and the importance of proper planting and aftercare. Narrated by two Cornell professors, horticulturist Nina Bassuk and landscape architect Peter Trowbridge, along with Ithaca, N.Y. City Forester Andy Hillman, the video takes you through the basic steps involved in assessing the conditions of an urban planting site, improving them if needed, selecting the trees best adapted to the site, and caring for the trees after planting.
For more detailed information on trees most likely to do well under tough urban conditions, check out another offering on the website, "Recommended Urban Trees: Site Assessment and Tree Selection for Stress Tolerance," which profiles over 90 trees adapted to Zone 6 and colder that can take what a city has to dish out. Trees are grouped both by size and by their tolerance to different sites or planting condition, such as trees that tolerate partial shade, have some salt tolerance or are sensitive to salt, or those that are easy to transplant bareroot.
The information and resources from UHI will be helpful to municipal officials responsible for community street plantings, members of the public interested in advocating for or participating in municipal greening projects, as well as urban home gardeners who want to make the best choices in their own landscapes.
To explore all the on-line resources from UHI, go to: Urban Horticulture Institute.
Urban Heat Island Effect
Cities are hot places; in fact, they can be islands of heat in the midst of a cooler regional environment. "Urban heat island" is a term that many of us may have heard of, but not fully understood. The term comes from the fact that urban areas, with their many buildings, expanses of pavement, and other impermeable surfaces, are warmer than surrounding rural and suburban areas, forming "islands" of higher temperatures, both in the atmosphere and at the surface.
Surface heat islands tend to be warmest during the day when the sun is shining and surfaces are reflecting heat, with exposed surfaces often as much as 50 to 90 degrees warmer than the air! Atmospheric heat islands are generally hotter during the night as buildings and roads radiate the heat they absorbed during the day back into the air. On a calm, clear night, all this stored heat can make it as much as 22 degrees warmer in a city than in surrounding, non-urban areas.
Why do we care if our cities are warmer? Higher temperatures increase the demand for electricity for cooling, leading to increased emissions of pollutants from power plants. They also promote the formation of harmful ground-level ozone. The heat and increased pollution make life more unpleasant and sometimes more dangerous for city dwellers. And when the heat from impermeable surfaces is transferred to water running off it, this raises the temperatures of the waterways receiving the runoff, causing harm to aquatic ecosystems.
What is one of the best ways to combat the urban heat island effect? Plant trees, of course! Increasing the area covered by trees and other kinds of vegetation helps to keep cities cooler. Trees in the urban landscape provide many benefits. They offer city dwellers beauty and a connection to the natural world and improve the urban environment by reducing storm runoff and cleaning the air. Just as important, city trees also help to reduce the negative impacts of the urban heat island effects both through the cooling shade they provide to surfaces and the evaporative cooling that occurs as the trees transpire moisture into the air.
To find out more about the heat island effect and its impact on energy use, water and air quality, and human health, and learn more about strategies for reducing urban temperatures and heat island mitigation activities in your state or community, check out the informative resources from the U.S. Environmental Protection Agency at Heat Island Effect.
Trees aren't the only plants to grow in the city. As the interest in healthy eating, local foods, and sustainable agriculture grows, we see not only more and more home food gardening in city backyards, rooftops, and community gardens, but an explosion of interest in urban farming as well. From rooftop farms to aquaponics (raising fish and plants together in a mutually beneficial system) to reclaimed vacant lots, innovative urban farmers are pushing the boundaries of agriculture in new and exciting ways, enhancing the urban environment, and providing sustainably produced food in the process.
If the possibilities of city-grown food excite you, find out about what's on the cutting edge of urban farming at the Sustainable Cities Collective website. Billed as ″the world's best thinkers on the urban future,″ this site aggregates posts from a number of sources on a wide range of issues relating to cities, including a section on urban farming. Read about the exciting things happening in urban agriculture in Grand Rapids, Michigan, the unused Chicago warehouse that's become a net-zero urban farm, or the plans for the world's largest rooftop farm in the Bronx.
From front yard food gardens to large scale commercial ventures, growing food in cities is the wave of the future. To keep up with its latest developments, go to: Sustainable Cities Collective.