Tree Care: The Planting Hole
By William M. Fountain
The Planting Hole: The Key to Establishment of Trees and Shrubs in Landscapes
The species has been selected. You made sure that it s adapted to the site’s cultural conditions and will not grow larger than the soil volume or above-ground space. The plant is superior quality with well-spaced branches and a solid root ball. All that remains is to get it in the ground.
Planting is not as simple as digging a hole and filling it in. Failure to properly install your new tree or shrub can result in a slow decline and death. The majority of transplant failures are related to water and/or failure of the plant to establish a new root system.
Balled & Burlapped (B&B), Container, Bare Root
Image courtesy Buffalo Green Fund
Trees and shrubs are moved from production nurseries to landscapes in one of three forms; balled and burlapped (B&B), bare root, or as container-grown specimens. The characteristics associated with the planting hole are essential for the reestablishment of these plants.
Water, either too much or not enough is the most common reason plants die in the first one to two years after transplanting. It takes a little longer for transplanting errors to result in death. Sometimes water does not get into the soilball where roots are. Burlap left on the rootball sheds water preventing the water from moving to where roots can absorb it. Plantable containers also prevent water from reaching roots. This results in surrounding soil being wet while soil around the roots remains dry.
Container-grown plants are produced in soil-less mixes that are better drained than natural soil. If these materials are not partially removed so that existing roots are surrounding by real soil, they can remain dry out even if the soil in the backfill is wet.
It is always easier to add water than to get rid of it in poorly drained soils. This is why a perk test is important in determining if the soil will drain sufficiently to grow plants that are not tolerant of wet feet.
Transplanting trees and shrubs is not like pulling a cork out of one bottle and inserting it into another. New roots must grow away from the soilball and into the surrounding soil. When this cannot occur because of hard, compacted soil, roots grow in a circle. This is especially common in container-grown plants. Roots in container-grown plants must be loosened and directed out into the backfill. Failure to do so results in confinement of the roots and ultimately girdling of the trunk.
Roots grow through the path of least resistance. In natural systems (forests) we see new roots growing through worm holes, holes where the roots of a now dead plants once grew, and cracks in the soil that develop as it expands and contracts. This is the type of environment we want to create in plant’s new location.
How Big Should a Planting Hole Be?
Digging large planting holes is one of the best things that you can do for a newly transplanted tree or shrub. Two or three times the diameter of the soilball is usually large enough. However, the harder it is to dig the planting hole, the more important it is to dig it even wider than the minimum of two or three times the diameter of the soilball.
Digging a large planting hole allows you to break up the soil so that there is an abundance of cracks between clods for new roots to grow. This helps oxygen and water infiltrate and move through the soil for uptake by roots. This results in more roots and ultimately more shoot growth than anything else you can do.
Digging wide planting holes allows you to not only break up the soil so that roots can grow away from the trunk, it allows you to find and discard rocks and building materials that may have been buried during construction. These materials obstruct root growth in the backfill and reduce the ability of roots to anchor the tree in wind.
While the planting hole should be dug much wider than the soilball, it should never be any deeper than the soilball is tall. The plant should be installed so that the upper most roots in the soilball are 1 to 2-inches below the soil surface. The only way to know this is to locate them in the soilball. Gently remove the upper layer of soil until you find the first large roots coming out of the trunk. These first order roots must be close to the soil surface.
If the hole is dug too deep for the first order roots to be near the surface, you must add enough soil to raise the first order roots to the proper grade. This soil must be firmed in the bottom of the planting hole to prevent the soilball from settling. Soilballs that settle allow surface water to collect resulting in smothering and death of roots.
Planting too shallow results in roots dying out and dying. If the top of the soilball is allowed to dry, it then becomes hard to re-wet.
Adding amendments like compost or peat moss to the backfill will not make much of a difference for plants in very good soils. If organic matter is added to compacted soils or soils that do not drain well, this organic matter can decompose anaerobically (without oxygen) and release chemicals that are toxic to young roots. Sandy soils typically drain well but adding sand to soils high in clay will reduce drainage. It is best to have the same type of soil inside and outside the planting hole.
It is hard to dig soils high in clay that are extremely dry. While it may be easier to dig when the soil is wet, it can result in glazing. Glazing is the smearing of the clay into a thin sheet that resembles the glaze on a piece of pottery. When roots reach this glazed sidewall, they are unable to penetrate it and turn, growing back into the loose backfill. This results in the backfill becoming filled with roots and the failure of the plant to become established. The plant becomes stressed, growth slows, the plant declines, and finally dies several years after installation. If you suspect that the sidewall is glazed, scratch the sidewall until the soil is rough and crumbly.
Placing the Plant in the Planting Hole
Balled and burlapped plants can be heavy. Rough handling of the plant while getting it out of a vehicle, moving it to the site, or dropping it into the hole can result in root damage if the soilball cracks or falls apart. If the plant is too heavy to move by yourself, get assistance.
After the soilball has been placed in the planting hole and you are sure that it is at the correct level, it is time to remove any remaining materials that protected the soilball in transit. This includes all twine, burlap, wire baskets, and any other materials around the soilball or trunk.
Synthetic burlap does not decay while natural jute burlap will. It is very difficult to tell the difference between these two materials. Even though jute burlap will decay, this can take a couple of months. Until it is thoroughly decayed, it will shed water allowing the soilball to dry out even though the surround soil is moist. As a result, it is recommended that all burlap always be removed. Cutting slits in the burlap does not allow roots to escape or water to enter the soilball. At a minimum, all burlap should be completely removed from the top and sides of the soilball. Like synthetic burlap, plastic twine does not decay. Cutting the twine and allowing it to remain in the hole is also a bad practice and can still result in girdling of roots.
Wire baskets are beneficial in keeping large soilballs from breaking while in transit. Even though the baskets have large holes for root growth, the wire still needs to be removed. The burlap cannot be removed without also removing these wire baskets. In the decades that it takes the wire to rust, the metal often becomes embedded in large scaffolding roots limiting the transport of water and essential elements.
One advantage of digging an extra large planning hole is that it is easier to get bolt cutters into the planting hole to cut and remove the wire basket. If the baskets are not removed the wire on the top of the baskets will protrude above the soil surface. If mowing equipment hits these metal loops, there is the potential for these metal shards to become a lethal missile.
Backfilling the Hole
The best soil to fill the hole with is the same soil that came out of the planting hole; less any rock or foreign materials that you find. This material is the same as the surrounding soil and will allow roots and water to move easily through these similar materials.
While oxygen is essential for root growth, roots do not grow through air pockets. Air pockets can also prevent the natural movement of water through soil. Stomping on the soil removes these air pockets but it also compacts the soil. This makes it more difficult for roots to grow into the surrounding soil. The best way to settle the soil and remove air pockets is to use water to gently settle the soil. After the planting hole has been partially filled with soil, water it thoroughly. Fill the hole with soil and water again to settle the soil. Repeat this until the soil in the planting hole is level with the surrounding soil.
Mulching is one of the best things that we can do for a plant or one of the worst things we can do to a plant. Mulch should be 2 to 3-inches deep. It should be coarse and organic. Mulch should be about 3 or 4-inches away from the trunk and extend out at least past the edge of the planting hole. You can find more information on the practice of mulching in Mulch Myths.
Most bare root trees will need to be staked after planting. Most balled and burlapped and container grown trees and shrubs do NOT need to be staked. Plants should be staked only if there is a real potential for the plant to blow over.
Trees that are staked grow taller, faster; increase in caliper (trunk diameter) more slowly; and regenerate new roots more slowly than trees that are allowed to sway gently in the wind. Stakes that remain on the plant for longer than one growing season can girdle the trunk. This disfigures the trunk and reduces the ability of the trunk to conduct water and mineral elements.
If there is a real possibility plants will not remain upright, use three stakes to stabilize the plant. These stakes should be located outside the planting hole and be driven deep enough into the soil so that they are stable. Wire with a piece of water hose is not flexible and will rub and damage the trunk. The ties from the stake to the trunk should protect the tree from extreme winds but allow it to move in normal winds. This will help the plant to grow properly and regenerate roots.
Inspect and Monitor
Regenerating roots and producing new shoots and leaves after transplanting requires plants to use a lot of their stored energy reserves. Extremes in temperature and moisture can further exhaust these reserves making recently transplanted trees and shrubs more likely to be attacked by opportunistic diseases and insects. It is essentially that your new plantings be provided with every opportunity to thrive. Proper watering is much more important than fertilizing in the first year.
During the first year you should inspect to ensure that:
- the plant is watered once a week during the growing season when the plant does not receive at least 1-inch of rain
- depending on the species, protective sprays should be applied at the proper time to prevent opportunistic diseases and insect pests
- all staking and ties are removed at the end of the first growing season after transplanting
- mulch is replenished so 2 to 3 inches cover where the roots are growing
- all mowers and string trimmers are kept away from the base of the plant
- the trunk is protected from sunscald the first winter after transplanting.
Bare root, container and balled and burlapped tree (Buffalo Green Fund)
About the Author
Dr. William M. Fountain is an Extension Professor Emeritus of the University of Kentucky Department of Horticulture. Dr. Fountain is ISA Board Certified Master Arborist and is ISA Tree Risk Assessor Qualified. He is also a Register Consulting Arborist with the American Society of Consulting Arborists. Email at Bill.Fountain@uky.edu