Garden Variety: Soil depth at base of trees key for long-term health
Trees with soil or mulch piled around the bases of their trunks exhibit symptoms of stress and have shorter life spans than trees that are planted and maintained at the proper depth. Well-meaning gardeners who are planting trees, building tree rings, grading soil around trees, adding mulch and completing other garden tasks should take care to find appropriate depth for their trees and protect that level.
The problem of trees being planted too deep or having soil piled around their trunks later in life has been going on for decades. The nursery and landscape industry recognized that the practices were problems and have worked to make change, but progress has been slow. There are also many opportunities for trees to end up too deep in the soil, from the tree nursery to the installer to the landscaper, and to builders and property developers.
How do you know if your tree is too deep in the ground? Look for what industry professionals call the root flare. The root flare is the place where a tree naturally starts to widen as the tree transitions from vertical trunk growth to the horizontal growth of the root system. The curve of the first horizontally growing root should be level or nearly level with the surrounding soil surface when a tree is growing at the proper depth.
If the bottom of the tree goes straight down into the soil without a flare, it is too deep. If the tree widens out and the top of the root system is exposed, the tree is too shallow.
For existing trees that are too deep, removing soil from the base can reduce stress and lengthen the life of the tree if the tree is still mostly healthy. Use a shovel to carefully scrape away layers of soil until the root flare is found. You are likely to find fibrous roots growing above the root flare as they have extended up looking for air. Sometimes the fibrous roots encircle the trunk, a condition horticulturists and arborists call tree girdling roots or TGRs.
Mulch piled against the trunk suffocates roots the same way that soil does and makes a favorable environment for pests. Pull mulch away from the trunk to create a mulch pancake or donut shape over the tree’s root system. Most resources recommend roughly 4 inches of mulch or less.
New trees can arrive from the nursery or garden center too deep in the container or mass of soil surrounding the roots (root ball). When planting, take care to find the root flare and adjust planting depth so the root flare ends up level or nearly level with the surrounding soil surface.
Trees sold in containers may have an excess of fibrous roots above the root flare from inattention during transplanting. Be courageous with the shovel or with a serrated knife and remove enough roots to find the flare and plant the tree at the right depth.
When building new landscape beds, grading lawns and landscapes, adding mulch, etc., avoid putting any mulch or soil against the trunk of an existing tree. Avoid adding excess amounts of soil over the root system in general, which extends out like a giant pancake under the tree (tap roots disappear when trees are still young). Remember that tree roots closest to the surface need air, and thick layers of soil suffocate them.
For trees that are planted too shallow, some soil can be added to protect the roots and provide moisture. Avoid adding large amounts at one time. Mulch is a good option also to cover and protect exposed roots.
— Jennifer Smith is a former horticulture extension agent for K-State Research and Extension and horticulturist for Lawrence Parks and Recreation.