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Plates, Not Mirrors

How Tree Roots Grow and What that Means for Gardeners

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ILLUSTRATION BY LARRY DECKER
  • Illustration by Larry Decker

The Rhizotron Has Spoken

Most of us grew up with—and still often see—illustrations of a tree's root system depicted as a mirror image of the tree's canopy. However tall and wide the canopy, that's how deep and wide the roots grow, right? Turns out, that's not the case. Though they were not the first to examine the issue, more than 20 years ago, researchers at Cornell's Urban Horticulture Institute (UHI) definitely debunked the "mirror image" folklore around tree roots.

The UHI researchers observed root growth with the help of a rhizotron, a simple belowground viewing chamber that gives a literal window into a tree's roots as they grow. (More recently, scientists are using ground-penetrating radar to find out where tree roots are.) The UHI researchers found that the "mirror of the canopy" portrayal of how tree roots grow is incorrect.

For the vast majority of species, tree roots grow close to the surface and spread laterally in a webbed plate; the roots get finer in width the farther out you go. You may have observed this yourself on trees that are growing on, say, the edge of a river bank, where some portion of their root system is exposed.

Have you ever been digging in the garden, nowhere near a tree, and found fine tree roots and been at a loss as to where they came from? Fine "feeder" roots not only grow horizontally beyond the dripline (the canopy edge), there is often a higher percentage of them beyond the dripline than within it. (This is why those orange plastic six-by-six-foot protection zones around trees on construction sites are woefully inadequate.)

Rhizotron observers also found that tree roots are found primarily in the top 12 inches of soil. The reason for this is that oxygen becomes limiting the further down you go through the soil profile. Tiny absorbing roots, responsible for most of the tree's intake of water and nutrients—and therefore critically important—are in the top few inches of soil, right under your feet. The finer the roots, the greater the surface area through which water can enter by osmosis, which is why conservation of fine roots matters.

Another rather amazing observation UHI researchers made has to do with trees that are harvested from the fields of tree nurseries. The most common digging method—with a hydraulic tree spade, with roots then "balled" in burlap—leaves 90 to 95 percent of tree roots behind in the nursery fields! Furthermore, the critical fine absorbing roots that are harvested are easily broken off, damaged, or desiccated.

Water stress, resulting in part from this tremendous reduction in root mass that occurs at the nursery, is the main reason transplanted trees fail. Whether from a nursery field to the city tree lawn, or just from one place in your yard to another, it's the roots that suffer when trees are transplanted.

What can you as a home gardener do to protect your trees' root systems?

Put the tree's health above your garden ambitions. If you value a tree on your property, give it first consideration over other types of landscape plants. Some treasured tree species, like oaks, dogwoods, and sugar maples, have roots that are especially sensitive to disruption. Resist putting in a major new garden underneath or anywhere near these trees. Instead, let them be the beautiful specimens and focal points that they are, and don't make them compete with perennials and shrubs for water and nutrients.

If you must plant, plant small plants. Smaller plants can be fitted among roots with less root disruption than large plants cause—so go for a perennial in a 4-inch pot rather than a 1-gallon pot. These small plants have to be tough species, however, because they will be competing with tree roots for water and nutrients, and sites permeated with tree roots are likely to be chronically dry. Regardless of species, as you're establishing those small plants, you'll need to water them a whole bunch.

Don't wound. Be careful not to wound the major anchoring roots that flare out at the bottom of the trunk, as wounds can lead to decay and they also allow insects and pathogens to penetrate the tree's vascular tissue. But also avoid severing any roots in the tree's root system—including fine roots—as all wounds are stressful to the tree, and root regeneration takes energy away from other important physiological processes.

Don't mulch too deeply. Maintain a depth of about 3 inches of bark mulch, but no more, or else fine feeder roots lose their ability to "breathe"—i.e., exchange gaseous oxygen and carbon dioxide.

Mulch as far out as you can. Because those critical fine feeder roots are so close to the surface, the roots of lawn grasses will be in direct competition for water and nutrients unless you favor the tree. To do this, give your tree as big a mulched bed as possible—at least to the canopy edge and, ideally, beyond. But remember, mulch should be no more than about 3 inches deep.

Avoid transplanting a tree, or if you must move a young tree, take as wide a root system as you can. The more fine feeder roots go with the tree, the more likely it will be a successful relocation. Make sure the tree is well hydrated before transplant day, transplant on a cool day in fall, and water the transplanted tree generously and regularly for the first year at least.

Construction, digging, or staging area nearby? Make sure your tree has a large tree protection zone. The tree protection zone is at least as wide as the tree canopy, but preferably much wider. If you can't accomplish this, work with an ISA Certified Arborist on ways to minimize mechanical damage to the tree and root compaction by equipment. Now that you can visualize the plate of fine roots that grow so close to the surface, you can see why you don't have to dig very deep in order for real damage to occur.

Set your sprinkler farther out than you think. It doesn't make a lot of sense to water close to the trunk of an established tree. Knowing that the majority of critical fine feeder roots on a mature tree can actually be past the dripline, you'll want to set up your sprinkler accordingly—way out there. The bigger your tree's canopy, the farther out the critical fine feeder roots are.

Be mindful of compacting the fine feeder roots with foot or vehicle traffic. Try to avoid traveling heavily across your tree's network of fine feeder roots. This can be hard to do perfectly, but in order to protect your treasured trees, keep that intricate underground network in mind.

The majority of a tree's all-important fine feeder roots are likely to be past the dripline (the imaginary line from the outer edge of the canopy to the ground).
  • The majority of a tree's all-important fine feeder roots are likely to be past the dripline (the imaginary line from the outer edge of the canopy to the ground).

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