PRINEVILLE, Ore. - When people think of trees to plant for fall color, they are no doubt influenced by countless calendars whose October page features the blazing colors of New England's sugar and red maples (Acer saccharum and rubra). Oregon Department of Forestry officials want people to know there are good reasons to not be swayed to plant maples or any other tree that is found abundantly in town.
Many less common trees have brilliant autumn color and work equally well as yard or street trees. Planting these can reduce a community's vulnerability to catastrophic tree loss.
In many Oregon cities and towns, a few tree species are being planted at levels considered risky. In fact, the consensus among urban foresters is that communities need to avoid having an urban forest with more than one in 10 trees belonging to the same genus, whether it be a maple, an ash, an elm, or a birch. In other words, to have a healthy, long-living urban forest, it should be made up of a wide collection of tree species.
Having a mixture of different trees lessons the risk from introduced pests finding an abundant food supply in a neighborhood's streets and yards, according to ODF Community Assistance Forester Katie Lompa.
"The Asian longhorned beetle is one reason over-reliance on one genus of tree is so risky," said Lompa. "This beetle, while not found in Oregon yet, has entered the U.S. and attacks and kills a variety of trees, including maple, elm and birch. It threatens to change the landscape we all know and love because once a tree is infested there is no cure."
Yet another invasive pest, the emerald ash borer, has rapidly spread throughout parts of the U.S. since it was found in Michigan in 2002. Lompa said ash (Fraxinus spp.), with amazing reddish-purple fall color, used to be a recommended alternative for maples as well as for American elms (Ulmus americana) wiped out by Dutch elm disease. But with this borer fatal to all ash growing in the U.S., the continued planting of this tree needs to be considered carefully.
Lompa said it is especially important for central and eastern Oregonians to consider if their soil is alkaline, neutral or acid and choose a tree that tolerates that type. Soil pH can greatly affect tree health, as well as fall color. Early frosts and drought stress during the growing season can also destroy fall color. "Know that all newly planted trees will require at least some irrigation," said Lompa.
To help Central and Eastern Oregon communities develop a wider collection of trees, she offers residents the following tree species as options for fall color:
American hornbeam (Carpinus caroliniana)
Native to eastern North America, fall color varies on American hornbeam from a ho-hum yellow to brilliant orange and red. For reliable reddish or orange fall color, pick one of the new cultivars, such as 'Native Flame', 'Firespire' or 'Palisade' (although 'Palisade' often has some gold, too). Besides the autumn fireworks, homeowners will enjoy improved form, strong wood, few pests or diseases and manageable size (usually 20-35 feet tall). Hardy down to USDA zone 3 or 4, American hornbeam prefers acid soil but is moderately tolerant of alkaline conditions, according to the Morton Arboretum.
Chinese pistache (Pistacia chinensis)
For residents of warmer parts of eastern Oregon, such as the Columbia Gorge, this USDA Zone 6 tree might be worth experimenting with. Heat and drought tolerant, Chinese pistache takes full sun and poor soils in stride, accepting both alkaline and acid conditions. The tallest pistache can reach 60 feet tall, but the trees are usually closer to 30-40 feet. Their compound leaves turn a respectable gold-orange to scarlet in fall. Female trees have pea-sized nutlets that ripen from red to blue and attract birds. If nutlets aren't wanted, select a male clone, such as 'Keith Davey'.
Ginkgo (Ginkgo biloba)
Fossils of ginkgo leaves are abundant in eastern Oregon, proving the trees grew here millions of years ago. Ginkgo trees are hardy to USDA zone 4. Somewhat gawky when young, they mature into fuller, rounder trees. Fall color is a bright, butter yellow. Unfussy as to soil, they will survive in acidic or alkaline conditions. Give them ample water their first decade to help them become well established. Virtually free of pests and diseases, ginkgo trees can live for hundreds of years. Choose male clones to avoid the fruit, which has an unpleasant smell.
Persian ironwood (Parrotia persica)
Can't decide on a favorite fall color? Have them all by planting a Persian ironwood. These tough, drought-tolerant trees can sport yellow, orange, red and purple all at once. One color or two colors may dominant in a particular tree, so select in autumn to ensure getting the desired color mix. The cultivar 'Vanessa' has more uniform orange to reddish-orange fall color. Years after planting this strong-wooded tree, the flaking cream-and-gray bark becomes an added ornamental feature. Few pests or diseases bother this native of Iran. Able to survive Chicago's Zone 5 winters, Persian ironwood prefers acid soils but reportedly tolerates mildly alkaline conditions. Mature size is 20 to 40 feet.
Oaks (various species)
Northern red oak (Quercus rubra) has dominated oak planting in Oregon since the early 20th century. But fall color varies dramatically in that species. Despite the name, red oaks can turn a dull yellow to yellow-brown. More reliable for fall color is scarlet oak (Quercus coccinea). This species genuinely lives up to its name, turning a gratifying deep red in fall before the leaves fade to brown. They may hold on like that until the end of winter. Hardy to USDA zone 5, a scarlet oak will not be happy in alkaline soil, preferring acid conditions. These long-lived trees can grow 40 to 75 feet tall and provide welcome shade under their broad-spreading branches. Shumard oak (Quercus shumardii) is similar in appearance and hardiness to scarlet oak (being hardy to USDA zone 5B), and also has good red to reddish-orange fall color. These stately trees grow 50 to 80 feet tall with a 40 to 60 foot spread, making them good shade providers. They also prefer acid soil.