We've got a problem that required investigative reporting. I've received dozens of questions asking what to plant beneath evergreen and deciduous trees because the lawn grass has grown poorly. Even shade-tolerant grasses weren't thriving. If shade isn't the problem, what is? The answer is surprising.
In past columns, we've discussed alternative plants that will grow beneath trees. But we didn't address the root of the problem causing the feud between turf and trees.
While investigating research about trees versus grass, I located fascinating information from the University of Minnesota compiled by Lorrie Stromme in the university's bulletin "Trees and Turf: Are They Compatible," which is the source from which I've gathered today's details.
As the bulletin indicates, we continue to plant manicured lawns right up to the base of most trees, even though trees and grass are incompatible. They interfere with each other's growth because of two factors: competition and an intriguing phenomenon called allelopathy.
If grass is allowed to compete with a tree, the tree will grow slower in both height and trunk diameter compared to a tree grown in bare soil. If you add mulch over the bare soil, the tree's rate of growth increases compared to both bare soil and grass. This is so important it merits restating. Trees growing in grass have the slowest increase in height and trunk diameter. Trees in bare soil grow next best, and trees with mulch over the root zone have the greatest rate of growth.
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How do grass and trees compete with each other? Aren't tree roots much deeper, giving them the upper hand? No. Most of a tree's feeder roots that absorb water and nutrients are in the upper few inches of soil, trying to occupy the same space as grass roots, while both compete for moisture, fertilizer and oxygen.
Turf roots hoard the majority of fertilizer when sharing soil space with tree roots. This grass competition can reduce the growth, fruit set or flowering of trees. In high-maintenance, heavily fertilized lawns, there is evidence that excess fertilizer makes trees more susceptible to other problems, like insects.
When competing for light, trees get the upper hand. Although some grasses tolerate shade, it doesn't mean they thrive in shade. And growing in the shade of a building is easier than growing under a tree that's not only casting shade, but whose roots are fighting for the same soil space.
The second source of friction between trees and grass is even more surprising. It's allelopathy, which is a seldom talked-about chemical warfare between plants. Allelopathy means one plant inhibits the growth of another plant. Trees and turf both release natural chemicals that act like herbicides to slow the growth of surrounding plants. Allelopathy is used by plants to guard their own space, reduce competition and protect their resources.
For example, one way a tree can protect its root space is to make the roots of other plants die off using allelopathy. The tree can then pull more water from the soil for itself.
How plants engage in this chemical warfare is fascinating. Some allelopathic trees release a chemical gas from the pores in their leaves, causing other plants to absorb the toxic chemical and die. When the leaves of these trees fall and decompose, toxins may be released into the soil.
Some plants release the toxins through their roots, which are absorbed by neighboring plants, which vary in susceptibility. A good example is black walnut, which produces the compound juglone that is a known toxin to birch, basswood, cotoneaster, tomato and potato. Another interesting species is quackgrass. No wonder it's such an invasive weed. It exudes toxins through both its underground rhizomes and leaves, suppressing other plants in its vicinity.
How greatly these toxins affect neighboring plants is influenced by soil type. Heavy clay soils do not drain well, and toxic plant chemicals build up, having greater impact.
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What's the solution? Separate the troublemakers with mulch. Research has shown that trees grow better surrounded by mulch, while keeping grass at a distance. For a just-planted tree, mulch with 4 inches of wood chips in a doughnut-shaped ring (this keeps mulch away from the trunk) extending 12 inches beyond the rootball. Enlarge the radius of the mulch ring 12 to 24 inches per year for at least three years.
Mulching also benefits established trees. Studies showed that after only two months, elimination of grass around 20-year-old trees resulted in a 113 percent increase in fine root density in sugar maples, and increases of over 30 percent in green ash. Hosta, ferns and groundcovers are better companions for trees than lawn grass.
Plants are like people. I guess if they can't get along, it's best to send them to separate corners for a time out.
Don Kinzler, a lifelong gardener, worked as an NDSU Extension horticulturist and owned Kinzler's Greenhouse in Fargo. Tune in to his weekly radio segment from 11 to 11:30 a.m. Fridays on WDAY Radio 970. Readers can reach him at forumgrowingtogether@hotmail.com .
