So you want to build or grade around a tree

Sunset, Sept, 1988

Here are ways to avoiding damaging the root area

Once trees get established, it's best not to disturb the soil level around them. But if landscaping projects-such as installing a new deck, driveway, house addition, or patio-call for removing soil from a tree's root area (or adding soil to it), you can minimize the risk of damaging the tree by following a few simple guidelines.

Even minor increases or decreases in the

surrounding soil level can endanger a tree. Adding more than a few inches of heavy soil can smother it by reducing available oxygen and water. When soil levels are raised a foot or more, original root systems may die, though new roots may grow into the fill area. Special care can't always keep this from happening, but it can help new absorbing (feeder) roots grow higher up into the fill area.

Removing surface soil-to lay a pathway or driveway, for example can also remove many valuable absorbing roots. Digging deeper can damage or sever large roots and weaken the tree's stability against wind.

The drawings below show three ways of changing the grade while minimizing potential problems.

Where the grade must be raised a foot or more, or if anchor roots must be cut flush with a lower grade, you may need to consult a certified arborist.

When you raise the grade

For pathways or minor leveling (6 inches or less), you can use sandy soil or mulch without harming most tree species. When you raise the grade more, use native soil if you can. Before adding soil, let the ground around the tree dry out so that added soil doesn't compact the ground over roots.

If plans call for significantly raising the soil level around a tree, clear away all debris and plant material surrounding the trunk. To prevent crown rot, keep water and added soil away from the trunk; build a dry well of stone, brick, or wood around the trunk (see center drawing).

Some trees such as native oaks, maple, honey locust, spruce, and pine--don't reroot easily when the grade around them is changed. Raising the grade around these could eventually kill them.

A few trees such as willow, poplar, eucalyptus, and coast redwood- are quicker to develop new roots, and therefore more able to survive until new roots grow up into the fill area. Since these trees are water tolerant (they grow naturally in areas where flooding has changed the grade), you can plant under them soon

after raising the soil level around them.

For these tolerant trees, build a cone of medium-grit sand around the trunk (see drawing at far left). The foot of the cone should extend no farther than 3 to 4 feet from the trunk; leave the cone's top uncovered, Make sure the final grade slopes away from the trunk.

If you're adding more than 6 inches, you may have to install an aeration system. The center drawing shows one: perforated PVC pipes help keep existing roots alive until new absorbing roots grow into the fill area (they let oxygen in and carbon dioxide out). Pipes also prevent root rot by draining excess water. Build a protective dry well of stone or brick, but build it away from the trunk (the deeper the fill, the farther out the well's wall should be).

When you lower the grade

It's best to cut away soil from no more than one side of a tree. If cuts destroy 30 percent or more of a tree's root system (a certified arborist should handle such cuts), you may have to prune the canopy to decrease the tree's wind resistance.

Best are cuts at or beyond the drip line. Using stone, railroad ties, or brick, build a retaining wall (see right-hand drawing). Trim roots so that cut surfaces face downward. Don't paint wounds with pruning seal; it doesn't prevent decay or infection. Keep exposed roots and the embankment moist and covered with a tarp while you build the retaining wall.

If older leaves wilt, or turn yellow or

brown and begin to drop, water the root zone with a soaker hose for 12 hours once or twice a month until the tree shows signs of recovery (take care to keep trunk dry).

To find an arborist . . .

If you need a certified arborist experienced in grade changing, try one of three sources: look in the yellow pages under Tree Service; write or call your cooperative extension agent; or call the headquarters of the International Society of Arboriculture at (217) 328-2032 (the society can give you a number in your area to call for referrals).

Ask any arborist you call for references, proof of insurance, and examples of work similar to what you need.