With time remaining to plant trees in Minnesota this fall, John Ball has a suggestion for gardeners: Look at what your neighbors are planting — and choose something different.
The South Dakota State University forestry professor says it’s time to give up easy but unwise tree choices and make a real effort at diversifying the urban forest, lest we face a new tree catastrophe like Dutch elm disease.
“In 30 years, some exotic threat will come over and wipe out the maples, and everyone will say, ‘Why didn’t we plant something else?’ ” he said.
Ball, who once managed a tree-care company in Duluth, recently spoke to Minnesota tree care specialists in the Twin Cities. His argument is that if people really care about trees, in an age when global trade makes it easy for pests to cross oceans and continents, they should plant no more than 5 percent of one type of tree in an area.
That means that just 5 percent of the trees Minnesotans love most in their yards — maples — would be planted. Only 5 percent would be oak or birch. Trees that are now underplanted, like Kentucky coffee tree, buckeyes and shagbark hickory, would become more common.
Ball admits the 5 percent limit is aggressive, perhaps even unrealistic. But he said even trying to reach it would be a worthwhile goal.
“Rules are made to be broken,” he said. “It doesn’t mean that we won’t get exotic threats, but it means the threat is limited.”
Exotic vs. native
While some people advocate planting only native trees, Ball said that will not protect trees from pests. “I have no problems diversifying our urban forests with exotic trees if we avoid these problems,” he said.
Fifty years ago, up to 90 percent of boulevard plantings in Minnesota were elm trees. Most of those trees have been wiped out by Dutch elm disease. Now emerald ash borer is spreading through the state, and is expected to kill millions of trees. Ash makes up an estimated 15 to 20 percent of boulevard trees.
So people are moving to maples, particularly Freeman maples, Ball said. The hybrid of red maples and silver maples is popular because of its fast growth and fiery fall color. But many Freeman maples are already struggling in Minnesota because of improper planting — they are prone to girdling roots if the roots aren’t untangled or cut when removed from the pot or if they are planted too deep — and perhaps because of climate change. Ball thinks that eventually another imported pest could take out these already fragile trees.
One tree that is now underplanted is the Kentucky coffee tree, which has the impressive stature of an elm and is expected to weather climate change well. The University of Minnesota has released male varieties, “True North” and “Stately Manor,” which have no seed pods. The Kentucky coffee tree is slow-growing at first but will reach 50 to 70 feet high and 50 feet wide at maturity.
Ball is a fan of yellowwood, a medium-sized tree with cascades of fragrant white flowers in spring, and leaves that turn bright yellow in fall. Yellowwood needs deep, fertile soil, and is not a good boulevard tree. He also likes amur maackia, a slow-growing small tree that bears white flowers in summer, isn’t picky about soil and is a good substitute for crabapples.
Hop hornbeam, also known as ironwood, is a tough smaller tree that will tolerate boulevards and dry soils. Ussurian pears and buckeyes — the University of Minnesota has introduced a variety called “Autumn Splendor” — have bright fall color. In Minneapolis, buckeyes have been planted recently on boulevards, and in spring, their sprays of yellow orchid-like flowers have spurred interest among passersby.
That’s the kind of city action that’s needed to really diversify urban forests, Ball said. Many of these less common trees can be viewed at the Minnesota Landscape Arboretum, but not all are easy to find at garden centers.
“The first step is to get city foresters to grab the idea, because they recognize the problem,” he said. “I see some cities subbing shagbark hickory in their plantings. Nurseries contract-grow them for cities. Then somebody sees it on a boulevard, likes the tree, and goes and asks if they can buy it.
“That’s when people start growing it for sale. I used to be on the other side, and there’s nothing worse than trying to sell trees nobody wants to buy. You need city foresters and landscape architects to say there’s a market for these trees.”
In the Twin Cities, deciduous trees can be planted into October. Bare-root trees shouldn’t be planted now, but potted trees and trees that have a root ball covered in burlap can be. Trees should be well watered until the ground freezes. Mulch around the tree, but never let mulch touch the base of the trunk, which can promote insect damage and decay.
A cage of hardware cloth, 18 inches to 2 feet high around the trunk, will help protect new trees from rabbit damage over the winter.
Mary Jane Smetanka is a Minneapolis freelance writer, Hennepin County Master Gardener and Tree Care Advisor.