by Steve Turner
So now what? We are losing thousands of ash trees to the emerald ash borer (EAB) on our city streets and in our own yards. The big question now is what do we plant to replace them? Everyone wants to know what the best tree is to replace an ash.
When all the elms started dying from Dutch elm disease, the replacement tree of choice was the ash and cities and residents planted them by the thousands to replace all the lost elms. And now three decades later we are once again in same position and everyone is searching for the magic tree that will take the place of the ash.
So what is it? Is it an oak or a maple or maybe a more exotic species like a ginkgo or a Zelkova? Well the answer is easy if we look to the past for a solution. History tells us we are bound to repeat this process over and over again if we seek just one tree. The real answer is all of the above, plus many more types of trees if we want to break this cycle we have been repeating over and over since the demise of the American chestnut in the early 1900’s.
This is a heavily discussed topic among arborists, who all agree that we need diversity in our urban landscapes. We can no longer afford to plant monocultures (single species) on a city block or even one yard.
Another discussion point is the continued heavy use of single cultivars as replacements, as opposed to naturally occurring species. Some in the industry feel that this could lead to problems in the future. One example is the use of ‘Red Sunset’ maples instead of just random native red maples. Their concern is that since all trees of the same cultivar are genetically identical, if one is vulnerable, so are the rest. There is no natural diversity that might make some better able to defend themselves better than others.
This was the case with the ‘Marshall’s Seedless’ green ash. It is by far the most susceptible to the EAB infestation, but on the contrary the ‘Autumn Purple’ white ash seems more resilient than others. So there are pros and cons to each side, but it still is a very valid point that not only do we need to be conscious of species diversity, we also need to be careful about cultivar diversity within the species we plant.
There seems to be a trend among some growers toward collecting native seeds and growing native plants for resale. Hopefully in time there will be more and more of these types of trees available in retail nurseries. We need to educate consumers on the benefits of these native plants and as the demand increases so will our choices. The push to use native species is based on the fact that these trees have evolved in our climate and soil conditions for thousands of years and they will require less care and will be better able to adapt than other species.
In most cases this is true, but we still cannot discount the diversity that the proven introduced species provide us. Trees like gingko, dawn redwood, zelkova, katsura, kousa dogwood, European beech, Chinese lilac and Norway spruce have proven to be as good or better for our urban landscape than some of our native trees.
Others, however, like Norway maple, European birch, purple leaf plum, Colorado blue spruce and Siberian elms, have chronic problems that make them undesirable for southeast Michigan. These trees are either prone to develop girdling roots or are more susceptible to insects or diseases than their native cousins. So while there should be an emphasis on native species (in many cases they will perform better in the long run), we should not ignore using proven introduced species as good alternatives to ash.
Some of the native species I would suggest looking at include red and white oaks. Swamp white oak is very tolerable of poor, compacted soils. Sugar maples are good, but not for street trees where they will be exposed to salt. River birch is a better choice than white birches and is more resistant to borers. Hackberries are very tolerant of poor, compacted soils and will provide a great food source for wildlife without the mess—the birds eat them so fast the fruit will never touch the ground.
Any of the nut-producing trees like walnut, butternut, hickory and beech are very solid trees with few problems and are seldom prone to storm damage. Catalpa, yellowwood, and tulip poplars are all large trees that flower and are very showy when in bloom. Medium-sized trees like hawthorn, redbud, and serviceberry are good choices for smaller lots where space is a concern.
Native trees that are more difficult to find but well worth the effort include paw paw, elder, sassafras, and buckeye. Linden, red maple and honey locust are also native, but have been used so frequently in recent years I would be reluctant to use them in areas where they are already planted in large numbers, for fear that one of them could become the next ash tree. The same thing applies to ornamental pears and crab trees, there are already too many of these trees planted, and trying to find something a little more diverse would be a wise decision for these types of trees.
The best advice I can give you is to do your homework first before you visit the nursery. Know your soil type (clay, loam, or sandy) and whether or not it is compacted (most new subdivisions will have compacted soil). Look at the exposure and sunlight of the planting area, as well as its drainage. And last but not least, drive around your neighborhood to see what types of trees are growing and try to avoid planting the same kind if many of the same species are already there. This way you can help diversify your own area and help avoid the overpopulation of tree species in your neighborhood.
Ash tree replacements
If you need to replace an ash tree or are simply looking for good trees for our region, an excellent guide can be found here: www.hrt.msu.edu/ash.alt.
Steve Turner is a Certified Arborist from Arboricultural Services in Oakland County, Michigan.