5 Tips For Watering Trees In Turf

Most lawn irrigation is built for grass. Spray heads and rotors put down a modest amount of water several times a week, enough to wet the top 6 inches or so where turf roots live. Trees work on a different scale. Their absorbing roots spread well past the edge of the canopy and run through the top foot or two of soil, and they do best with a deep soaking followed by a dry spell long enough to let air back into the ground. A tree in the middle of a lawn gets the grass’s schedule by default, and that is rarely the right one.

The two plants also compete. Turf roots form a dense mat in the top few inches of soil and catch most of a light watering before it gets any deeper. A tree in turf can sit in a green lawn and still be short of water at its roots.

1. Give trees their own zone

The cleanest fix is a separate valve and circuit for the trees, so they can be watered deeply and less often while the lawn keeps its own schedule. Drip tubing or bubblers laid in rings around each tree, under the canopy, put the water where the roots are and keep it off the trunk. Trenching a new line across a lawn is a day’s work, and a shade tree can stand for fifty years or more.

Where a separate zone is not practical, a soaker hose coiled under the canopy, or a hose left to trickle at a few spots in turn, does the same job by hand.

2. Water deep, not often

The aim is to wet the soil 12 to 18 inches down at each watering and then let it dry partly before the next. Roots follow water, so deep watering builds a deep root system, and a deeply rooted tree rides out a dry spell or a broken sprinkler far better than one fed a little every day. In practice, that means fewer, much longer runs than the lawn gets. A long screwdriver or soil probe pushed in a day after watering shows how far the water went.

3. Too much water kills too

Roots need air as much as water. Soil kept wet all the time fills its pore spaces with water, the oxygen runs out and fine roots die, and the tree begins to look drought-stressed even though it is standing in damp ground. Light, frequent watering is one of the most common ways trees in irrigated lawns decline, because it keeps the surface wet and the deeper soil either soggy in heavy clay or dry in sand. Make sure the soil drains, check it before watering, and let the top few inches dry between soaks.

4. Build a basin for a new tree

A newly planted tree has all its roots in the small root ball that came from the nursery, and that ball can dry out in a day or two even when the lawn around it is moist. For the first season, water the root ball itself, slowly, until it is soaked through. A low ring of soil or mulch around the edge of the planting hole holds the water there long enough to sink in instead of running off across the grass.

5. Move the water out as the tree grows

Rain falls on the canopy and drips from its outer edge, and a tree’s absorbing roots reach out to that edge and well beyond it. As a young tree grows, move the water outward with it. Widen the basin or remove it after the first year or two, move drip rings farther from the trunk, and add rings as the canopy spreads. Water placed against the trunk does little good, because the roots that absorb water are out in the soil, and bark that stays wet invites rot.

A ring of mulch helps at every stage. A layer 2 to 4 inches deep over the root zone, pulled back a few inches from the trunk, holds moisture in, removes the grass competing for it right where the tree roots are densest, and keeps mowers and string trimmers away from the bark. Mulch piled against the trunk in a volcano does the opposite and should be raked out flat.