What trees should we plant when the emerald ash borer takes our green ash trees? Green ash was the tree to plant to replace our American elms, which became the victims of Dutch elm disease that struck in the 1960s and 70s. While it was not as elegant a tree as the splendid arching branches of the elm, green ash was valued for its cold hardiness, ability to grow in poor soil, its durable structure, and for its disease and insect resistance. At one time 80 percent of the street trees in Minneapolis were elms, because it was the perfect shade tree. The predomination of using a single species of tree turned out to be a disaster, when all elms trees died, or were removed to prevent the spread of Dutch elm disease. Unfortunately, many city planners and landscape architects planted ash trees to replace the lost elms. The new theory is to diversify and plant several different species of trees whether as a city plan or as homeowners in your own back yards.
While the emerald ash borer is in Minnesota, don't rush out and cut down you ash trees. Observe your trees carefully, look for dieback in the upper branches and for D-shaped exit holes (which you will probably not find because the woodpeckers have already found them) that could indicate the presence of borers, and call in a trained professional if you think you have ash borer. If you are planning to plant or replace some trees, the following are 10 good choices, according to Nancy Rose, horticulturist, with the University of Minnesota, which are good for street, yard, or park.
Our area is usually considered a zone 3.
- Hackberry (Celtis occidentalis) Zone 3, Height: 40-80 feet.
- Red Maple (Acer rubrum) Zone 3, Height: 40-60 feet
- Freeman maple (Acer x freemanii) Zone 3 Height: 40-60 feet.
- Sugar maple (Acer saccharum) Zone 3 Height: 50-80 feet.
- American linden or basswood (Tilia americana) Zone 3 Height: 40-70 feet.
- Red oak (Quercus Rubra) or Northern pin oak (Quercus ellipsoidalis) Zone 3 Height: 50-80 feet.
The following are a zone 4.
- Ginkgo (Ginkgo biloba) Zone 4 Height: 40-60 feet.
- Kentucky coffeetree (Gymnocladus dioicus) Zone 4 Height: 40-60 feet.
- Elm (Ulmus spp.) Zone 4 Height: 40-80 feet.
- Swamp white oak (Quercus bicolor) Zone 4 Height: 50-60 feet.
Rubber mulch
Rubber mulches are advertised as permanent, effective, and safe materials for use in landscapes and gardens. It may seem that rubber mulches are an environmentally friendly way to recycle used tires, but further research indicates they are neither effective in long-term weed control, nor safe for the environment. While recycling waste tires is an important environmental issue, the solution is not to spread the pollution problem over our landscapes and gardens, according to Linda Chalker-Scott, extension horticulturist at Washington State University.
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- Hazards of rubber mulch:
It's flammable -- a research study comparing several different mulches found that when rubber mulch, which contains petroleum products, is ignited, it is more difficult to extinguish than any other mulch, including wood chips.
It's toxic -- like any other material, tires and rubber mulches are eventually broken down by environmental factors, such as sunlight, or by bacteria and fungi. The chemicals that leach from tires are anything but benign. They include heavy metals, such as aluminum, cadmium, chromium, molybdenum, selenium, and zinc. Two other common rubber leachates are 2-Mercaptobenzothiazole (MBT) and polyaromatic hydrocarbons (PAHs); these chemicals are health hazards for humans and the environment.
It's destructive -- many vegetables and ornamentals mulched with rubber can accumulate high levels of zinc. Other metals found in decomposing rubber can also accumulate in plant roots, leaves, or fruit, depending on the species. Acidic soils are particularly sensitive because heavy metals are more available for plant uptake. Decomposing rubber mulches provide a constant stream of toxic leachates into adjacent aquatic systems. Research has also shown that entire aquatic communities are injured or killed when exposed to these chemicals.
Kyle Schulz is a Wadena County Master Gardener from Sebeka, and the regular gardening columnist for the Wadena Pioneer Journal.
