Wednesday, May 20, 2015

Trees: Understanding Roots And Root Care

PaulHetzlerTreeRootAngelaPerry4002.5April showers bring May flowers, the calls of spring peeper frogs, and of course, backhoes. Yes, it’s construction season, which for arborists and trees is also root-damage season.

As far as trees are concerned, root injury is the source of all evil. Well, most of it, anyway; chainsaws and forest fires aren’t so kind to trees, either. But regardless of the worrisome signs a tree may develop, whether early fall leaf color, tip dieback, slow growth, or even some diseases and insect infestations, the problem is below ground in the majority of cases.

Part of the issue stems from a flawed understanding of tree biology. There is a lot of tree-root apocrypha floating around the public consciousness. One myth – let’s call it the Legend of the Big Taproot – maintains that trees make enormous deep taproots. While the legend allows that a few side roots may branch off, the key element is the Big Taproot.

It’s true that trees such as oaks and walnuts have a significant taproot when they’re young, but in maturity their root systems look like a pancake, not a carrot, the same as other tree species. Most of us have seen trees that have blown down, but that monster taproot has yet to be spotted. It’s no coincidence that the flat root system one sees on a windthrown tree is referred to as a root plate.

About 90% of tree roots are in the top 10 inches of soil, and 98% are in the top 18 inches. A tree’s roots extend, unless there is an obstacle like a road or building, at least twice the length of its branches. This is a tree’s root zone; a broad, shallow, vulnerable mass of roots.

Sadly, the Big Taproot Legend has dreadful health implications. For trees, at least – who knows what it portends for our well-being. If we believe tree roots like it deep, we won’t think twice about adding soil or fill, or even paving some of the root zone.

What’s wrong with that? To survive, roots need oxygen, which they get directly from soil pores. Even though they make oxygen when they photosynthesize, trees cannot transport oxygen through their vascular tissues that work so nicely for carrying water, sugars, and nutrients.

Soil compaction from operating vehicles or equipment within the root zone causes the same problems. In wet soil conditions, even excessive foot traffic can cause enough compaction to mash soil pores shut and exclude oxygen. In these cases, roots slowly suffocate, and the tree will eventually show symptoms of decline.

Excavating or trenching within a root zone severs some tree roots and usually compacts the rest. Sometimes root damage will kill a tree outright within a few years, but more commonly there will be a prolonged decline over five to 10 years or more. Because of the time lag, secondary opportunistic agents often get the blame.

As with relationships, where trees are concerned the problem at hand is often not the real issue. Imagine glancing out the window one day to see wood chips the size of baseballs raining down from your favorite white pine. You rush outside with your Kevlar umbrella and discover an army of Jig Sawflies, their carbide blades freshly sharpened, power-sawing their way down the trunk. As they smirk at you atop the mound of pine chips, you search the Internet for an exterminator, knowing you’ll miss sitting under the pine’s yellow foliage.

Wait a minute! Yellow foliage? How long was it like that? Maybe there is something else going on here. A strong, happy tree will be able to respond to insect feeding by manufacturing chemicals known to scientists as Bad-Tasting Stuff to repel them (bugs, not scientists). It will endure some loss due to insect feeding, but it will be able to keep the balance in its favor.

Let’s think back on your white pine. Wasn’t that the one that you worked so hard not to hit with the backhoe when the septic went in six years ago? Or was that the one the gas company trenched nearly 10 years ago? It doesn’t matter. Human activity compromised the root system, resulting in the demise of the tree years later. Sawflies or no, that pine was doomed.

By now you may be thinking, I could sure use another coffee, or, how do trees in those little concrete squares (tree pits) in the sidewalk survive? The difference is that they are put there as little tykes and never come to depend on a normal root system. They have adapted to available root space and are considered “unhappy.” Mature trees that have a large root system suddenly cut or damaged to the size of tree pits are considered “dead.”

You can preserve trees in a construction site by cordoning off the root zone at least to the tree’s drip line (branch length) with snow fence before the project begins. Keep in mind that even stockpiling material under trees causes root damage. If driving near trees is unavoidable, adding four to six inches of wood chips or gravel (two-inch or larger) to the traffic pathway will help.

If excavation within the root zone is unavoidable, cut roots cleanly, flush with the trench wall. If possible, lay wet burlap over the root ends until backfilling is done. If over 50% of a tree’s root system needs to be cut, it is probably best to remove the tree. Any significant root damage, including compaction, can lead to future instability of the tree.

To repair damage already done, act quickly. Once symptoms show up years later it is usually too late. Hire a tree care company to loosen the soil with high-pressure water or air injection. Soil injections of beneficial microbes in a solution of dilute sugars and various natural compounds have been shown to be valuable. If this isn’t in your budget, aerating on a 2-foot grid using a soil auger (1-2” diameter by 18-24” long) will help.

Don’t create raised bed gardens around trees or otherwise add soil to the root zone, and try not to drive or park within it.

Photo courtesy Angela Perry.

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Paul Hetzler has been an ISA Certified Arborist since 1996. His work has appeared in the medical journal The Lancet, as well as Highlights for Children Magazine.You can read more of his work at or by picking up a copy of his book Shady Characters: Plant Vampires, Caterpillar Soup, Leprechaun Trees and Other Hilarities of the Natural World

One Response

  1. Wally Elton says:

    Took me a while to get to reading this, but I’m glad I did. Interesting. Is it harmful to throw compost around the bases of trees? Usually it is smaller ones that I’m trying to encourage to grow.

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