Planting a tree: depth, width, and the first three years
Most young trees that die in their first few years were planted too deep, watered wrong, or both. The hole and the water are the two things a planter controls. This guide covers both, with a cross-section to try depths on and a watering calculator that uses the same rule as the Tree Planting Tool.
Find the root flare first
The root flare (or trunk flare) is where the trunk widens into the main roots. On a tree in a forest, it sits right at the soil surface. On nursery stock it is often buried 2 to 4 inches down, under soil added during potting or balling.
Before digging, scrape the soil off the top of the root ball until the flare and the first main root show. Measure the root ball's height from there, not from the top of the soil it came in. That measurement sets the hole depth.
- Container trees: slide the tree out and look for roots circling the outside of the root ball. Tease them outward, or slice through the outer inch in three or four places. A circling root keeps circling in the ground and can strangle the trunk years later.
- Balled and burlapped: set the ball in the hole, then cut away the twine, the top of the wire basket and as much burlap as comes off without breaking the ball.
- Bare root: soak the roots for a few hours before planting, and spread them out over a cone of soil in the hole.
Try it
Set the depth and the hole width
A tree planted too deep rarely dies that year. Bark that stays wet under soil or mulch decays, roots grow upward toward air and start to circle the trunk, and the tree declines over five to ten years. By the time the canopy thins, the cause is long buried.
Quick check
Where should the root flare end up once the tree is planted?
Answer: At grade or slightly above. The ball settles after planting, so setting it a little high leaves the flare at grade a year later. Nursery soil level is often 2 to 4 inches above the flare.
The hole: wide, not deep
Dig the hole two to three times as wide as the root ball and no deeper than the ball's height from flare to bottom. Leave the bottom undisturbed so the ball sits on firm soil and does not sink. Slope the sides outward, and roughen them with the shovel where they are glazed smooth, as happens in wet clay.
Most new roots grow outward into the top foot of soil. A wide hole of loosened soil gives them an easy first year. Depth adds nothing, and soil loosened under the ball settles and takes the tree down with it.
Backfill with the soil that came out of the hole. Breaking up clods is fine. Compost, peat and planting mix in the backfill do harm in clay soil: water drains quickly into the loose fill and then sits there against the clay walls, a "bathtub" that drowns roots. Roots that do grow tend to stay in the rich fill instead of crossing into the native soil. Add soil in layers, water each layer to settle it, and skip the stomping.
Dig a test hole about 12 inches deep, fill it with water, let it drain, then fill it again and measure how far the water drops in an hour. Under half an inch an hour is poor drainage: plant with a third of the root ball above grade and slope soil up to it, or choose a tree that tolerates wet soil. The Tree Planting Tool reads a perc result and adjusts the plan.
Mulch and stakes
Mulch holds moisture, keeps grass and mowers back, and moderates soil temperature. Spread 2 to 4 inches of wood chips or shredded bark in a ring at least 3 feet across, wider for larger trees. Keep it pulled back a few inches from the trunk so the flare stays dry and visible.
Stakes are for trees that cannot stand on their own, such as a top-heavy tree, one with a small root ball, or one on a windy site. Most trees do not need them. A trunk that sways grows stronger and thicker. Where staking is needed, tie low and loose with wide, soft straps, and remove everything after one growing season. Wire left on for years cuts into the bark.
Quick check
A new tree has a mound of mulch 8 inches deep piled against its trunk. What is the problem?
Answer: A mulch volcano works like planting too deep: wet bark decays, and roots grow up into the mulch and circle the trunk. Pull it back to a flat ring, 2 to 4 inches deep, clear of the trunk.
Watering through establishment
At planting, every root a tree has is inside the root ball, which can dry out in a day or two of warm weather even when the soil around it is moist. A tree is established once its roots have grown far enough into the surrounding soil to support it in a dry spell. Until then, it needs water from a hose.
The University of Florida work by Gilman and colleagues gives two rules that hold up well:
- About 1.5 gallons per inch of trunk caliper per watering, applied over the root ball, slowly enough to soak in.
- Establishment takes about 3 months per inch of caliper in hot climates (USDA zones 9 to 11), about 6 months in zones 7 and 8, and about 12 months in colder zones.
Try it
Water for a new tree
Frequency for loam in a warm season. Water sandy soil more often, and clay less often: a finger pushed two inches into the root ball says more than any schedule.
Summary
- Find the root flare before digging, and set it at grade or an inch or two above.
- Dig two to three times as wide as the root ball and no deeper than it.
- Backfill with the native soil. Amended backfill in clay holds water like a bathtub.
- Mulch 2 to 4 inches deep in a wide ring, clear of the trunk. Stake only if needed, and for one season.
- About 1.5 gallons per caliper inch per watering, until established: roughly a year per inch in cold climates.
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