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Start from the bottom with tree care

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The Garden Guru: Start from the bottom with tree care

By Neil Sperry

Special to the Star-Telegram

There’s a lot of odd information out there in our big world of shade trees. A lot of trees are hurt by uninformed well-wishers, and gardeners often don’t realize that they’ve been the culprits until it’s too late — if at all.

Fact: 90 percent of any tree’s roots are in the top foot of soil. That’s obviously true for a mangled old pine hanging onto the barren top of a mountain, but it’s just as true for a 75-foot pecan growing in the deep, fertile soils along the Mississippi River. Sure, that big pecan has a tap root to anchor it, but the roots that do all the work of pulling out water and pumping up nutrients are near the soil’s surface, because that’s where nutrients accumulate, and that’s where the rain falls.

Your take-away from all of that: Don’t do major soil work in and around shade trees if it’s going to have serious consequences for those critical roots. Most of all, don’t add more than an inch of fresh topsoil per year, and preferably none at all. Adding fill soil beneath a large tree is one of the best ways to guarantee that a tree will die a slow (three-five years) death. Oxygen will gradually be compacted out of its root zone.

Rolling with it

If you’re installing new sprinkler lines or other utilities, remember the visual of an old-fashioned wooden wagon wheel. It has a rim around its boundary, and that rim is the equivalent to the imaginary drip line of a tree — that is, where the water would fall if the tree were an umbrella. That’s where the most active roots will always be found, but leading out to that drip line will be a network of larger roots (the spokes of the wheel), and it’s critical that you not dig trenches across them as you’re getting ready to lay new pipes or wiring.

Spend the few extra dollars on more pipe and cable. Dig your trenches roughly parallel to those bigger roots, coming in from outside the wagon wheel and heading straight toward the trunk.

If you’ve planted a new shade tree this spring, your care will need to ramp up considerably for the rest of this growing season. The tree’s roots don’t yet extend out into the rest of the native soil, and that means it will dry out more quickly than the established plants around it. You’ll need to water it by hand every two or three days for the first summer. Let a hose run slowly and soak its soil deeply. Hopefully, you built a shallow water-retention berm with the soil left over from the tree’s planting. Fill the basin, and let it soak in completely. (That basin is the poor person’s more efficient equivalent of the water bags you see hanging around trees’ trunks.)

I’m also confused by the “mulch volcanoes” you see pulled up around new trees after planting. Mulches certainly do allow water to soak into the soil instead of running off, and they certainly lessen the soil-to-air contact, and therefore the fast drying, but enough, already! A couple inches is plenty. You’ll see mounds that are 8 and 10 inches deep. They’re soaking up half the rainfall and irrigation, and they’re not letting it soak into the soil.

Up and away

It’s normal to see roots expanding out of the soil as the tree grows. Remember that those roots are in that top foot of soil, so when roots grow and enlarge to 7 or 8 inches (or more) in diameter, it’s only natural that part of that growth will push them up and out of the ground.

So do you cover them or cut them? No and no! You figure ways to adapt them into your plantings, because by the time you have large roots, you probably also have heavy shade from the trees’ canopies, and that means you’ll be looking for shade-tolerant ground cover that will take the place of failing turfgrass.

As you’re making that choice, you can also find ground cover that will help conceal all those roots. Mondograss and liriope both grow tall enough to hide roots. Asian jasmine and purple wintercreeper euonymus vine and sprawl, so they’ll grow up and over the roots.

Finally, what do you do if those big roots are heading straight toward your foundation? Roots do, after all, suck water out of the soil, and dry soils lead to foundation problems. Foundation-repair people tell scary tree stories. Begin by hiring a certified arborist to act on behalf of your trees — their advocate, as it were.

As one simple example, you may be able to trench and install a root barrier, to keep tree roots from getting near your house and its slab. Such a barrier could be vinyl pond liner, and it would need to be inserted 15 to 18 inches into the ground. Since roots might be cut as the trenching is being done, this kind of work is best left for September or October. Certified arborists know all of that, and they’ll know whether it’s a good option for your tree.

Our urban forests are critical parts of our Texas lifestyle. They shelter us, they cool us, and they add beauty to our surroundings. And it all starts with their roots. Protect trees’ roots, and you’ll protect your trees.

Neil Sperry publishes Gardens magazine and hosts Texas Gardening 8-11 a.m. Sundays on WBAP AM/FM. Reach him during those hours at 800-288-9227 or 214-787-1820.


Written by vaphc

June 14, 2012 at 11:22 am

Posted in Uncategorized

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