Watering a new tree through its first two summers

A tree bought from a nursery comes with a root system cut down to fit a pot or a burlap wrap. Once it is in the ground, nearly all the roots that can take up water are still inside that ball, a cylinder perhaps two feet across, while the leaves above lose water as fast as any established tree’s. Until new roots push out into the native soil, the tree can only drink what is in the ball.

Why the sprinkler misses

A lawn sprinkler sets out to wet a shallow layer of soil over a wide area. That suits grass and does little for a root ball. The ball of a container-grown tree is mostly potting mix, which is far more porous than garden soil, and water in the surrounding ground does not move sideways into it. University of New Hampshire Extension’s planting and mulching fact sheet makes the point directly: water right over the original root ball as well as the backfill, and do not count on lawn sprinklers to give a new tree the right amount. Container-grown trees dry out faster than balled and burlapped ones and need water more often for their first several weeks.

The first weeks

Planting day calls for a long, slow soak. UNH suggests letting a hose trickle until the whole ball and the backfill are saturated, which can take a couple of hours. After that, water often for the first few weeks, every other day if there is no rain and the soil drains well. Skip a turn only if more than an inch of rain has fallen or the soil under the mulch is plainly moist.

Each watering should be generous. UNH puts it at about an inch of water each time, a little over half a gallon for every square foot of soil, which for a tree with a two-inch trunk can come to ten gallons. A rule of thumb from the same extension service is a gallon a day per inch of trunk width, the trunk measured six inches above the ground. Since few people water daily, it suggests twice a week instead, at around ten gallons each time for a root ball 20 to 24 inches across. In Florida, University of Florida Extension’s guide to establishing trees recommends watering trees planted in spring or summer two to three times a week, with 2 to 3 gallons per inch of trunk diameter each time, then weekly once the first few months have passed.

A bucket with a few small holes drilled in the bottom, set next to the trunk and filled, delivers water at about the right pace. A hose left at a slow trickle on top of the ball does the same.

The second summer

Roots grow out of the ball over the first year, and the area that needs water grows with them. As they spread, water less often but over a wider circle. UNH’s guide to using water efficiently gives one gallon per square foot of root zone as a working figure once roots have left the original ball.

How long the tree depends on this help is set by its size. UNH counts the establishment period as about a year per inch of trunk diameter, so a tree planted with a two-inch trunk needs supplemental water through two growing seasons, and a four-inch tree for about four. The second summer is the one most easily skipped. By then the tree has leafed out once and looks settled, while its roots still reach only a little way past the old ball.

When it can manage alone

An established tree, by the UNH definition, has enough roots to support normal yearly growth for its species and can generally go two to three weeks in summer without rain. A longer drought calls for deep, occasional soaks toward the canopy’s edge, and a dry, snowless winter for a soak once or twice a month. Until it gets there, the best check is the soil itself. Push a finger or a trowel into the ball, not just into the lawn beside it, and water when the ball feels dry below the surface. A two to three inch layer of mulch over the ball and out toward the edge of the branches, kept a few inches back from the trunk, slows drying between waterings and keeps grass from competing for the same water.