Use trunk flare to determine how deep to plant a new tree

“My neighbor told me my tree was planted too deeply. How do I determine the proper planting depth?”

— Nicole Cramer, Glenview

Trees are often planted too deep by homeowners and professionals, so it is a good idea to understand how to position a new tree at the proper depth.

Recently planted trees that resemble telephone poles coming out of the ground are probably planted too deep. Generally, this will not cause problems in the early years, but it can be a major factor in the decline or even death of the tree in the future.

Look at the base of the tree where it meets the ground to determine if your tree is at the proper depth. Mulch should be pulled a couple inches away from the base of the tree. The majority of the roots will develop in the top 12 to 18 inches of soil, though this varies depending on the type of tree and garden soil. The tree’s root system will develop at a shallower depth in heavy clay soils versus better-drained loamy or sandy soils.

If the tree is planted too deep, new roots will have difficulty developing because of a lack of oxygen. A tree that is planted too deep and does not die over time may never reach its full potential.

Use the trunk flare instead of the root ball top to guide you in determining the planting depth of a new tree. The trunk flare is the area at the base of a tree that swells and merges with the roots. Look down the trunk until you see it start to widen at the base — this is the trunk flare.

If the trunk flare is buried in the root ball, plant the tree higher in the hole and carefully remove the excess soil above the root system. Be careful to avoid breaking up the root ball and damaging any roots or the trunk of the tree as you shave off excess soil. Use a sharp, flat spade to do this and push the spade toward the trunk of the tree as you remove the soil. The trunk flare should be visible when planting is completed.

In heavy clay soils, I prefer to plant trees high — with the trunk flare 2 to 3 inches above existing grade. When soils are sandy and very well drained, I generally position the trunk flare 1 to 2 inches above grade. If the root ball appears to be loose, remove the wire basket and burlap after the tree is positioned in the planting hole.

In general, it is best to not amend the soil used to backfill a planting hole. Use the soil that was dug out of the planting hole to fill in around the root ball. I have planted trees this way with good results.

In some situations, replacing the heavy clay backfill soil with new soil or heavily amending it can be detrimental. Water movement through the soil will be impeded when it enters soil with one type of texture (a new or amended backfill) and then meets the existing garden soil. Excessive water can accumulate in the bottom of the planting hole and lead to root suffocation and rotting of the roots. Roots tend to grow in the planting hole where there is better aeration, rather than growing out into the surrounding soil.

It is a good idea to dig a wide planting hole, as it will reduce soil compaction around the root ball and improve soil aeration, making conditions better for roots to grow. When feasible, make the planting hole two to three times the width of the root ball. Make the sides of the hole slanted with a gradual slope upward so it is easier to backfill.

Work the soil in around the root ball so there are no remaining air pockets. If the entire bed has been amended, it is less important to dig a wide planting hole. It is OK to amend the top layer of soil around a newly planted tree.

For more plant advice, contact the Plant Information Service at the Chicago Botanic Garden at plantinfo@chicagobotanic.org. Tim Johnson is senior director of horticulture at the Chicago Botanic Garden.