Surface roots present common issues, including creating tripping hazards, damaging sidewalks and foundations, and stressing the surrounding turfgrass. Trees often develop shallow root systems in response to environmental conditions rather than an inherent biological need. Understanding the factors that drive this upward growth allows for targeted interventions. This article explores practical methods to encourage tree roots to seek resources deeper in the soil profile.
Understanding Why Roots Stay Near the Surface
Tree roots require oxygen for respiration, a process generally found in the upper few feet of soil. When soil becomes heavily compacted, the small pore spaces that hold air are eliminated, effectively suffocating the deeper root structures. Waterlogged conditions also fill these spaces with water, creating an anaerobic environment that forces roots to grow toward the surface where gas exchange is possible.
The widespread practice of watering lawns frequently for short durations teaches the tree’s root system to remain high in the soil. Roots grow where moisture is consistently available, and if the water source never penetrates deeply, neither will the roots.
Physical obstructions in the soil, such as a dense layer of hardpan clay or underlying bedrock, also limit downward growth. Roots are opportunistic and will follow the path of least resistance, spreading horizontally just above these impenetrable layers.
Proper Planting Depth and Initial Care
Setting the tree at the correct initial depth is the primary preventative measure for promoting deep root development. The root flare, where the trunk widens at the base, must be fully visible above the finished soil grade after planting. Planting the tree even a few inches too deep restricts oxygen supply and can lead to girdling root formation.
When preparing the site, the planting hole should be dug two to three times wider than the root ball, but no deeper than the height of the root ball itself. This ensures roots can easily penetrate the surrounding, loosened soil horizontally. Digging a deep, narrow hole often leads to poor drainage and a “bathtub” effect, which encourages shallow growth.
Before placing the tree, inspect and correct any circling roots on the outside of the root ball. These spiraling roots should be shaved off or scored deeply to encourage them to grow straight out into the native soil. This practice prevents the roots from eventually strangling the trunk.
Applying a layer of organic mulch, such as wood chips, over the planting area helps regulate soil temperature and moisture. The mulch layer should be kept several inches away from the trunk itself to prevent moisture buildup and disease.
Cultivating Deep Root Growth Through Watering
The most effective way to drive established tree roots downward is by intentionally altering irrigation habits to favor infrequent, deep watering. Roots exhibit hydrotropism, meaning they naturally grow toward a consistent source of moisture. If water is only applied to the top few inches of soil, the roots will remain in that zone.
To train the root system, the goal is to saturate the soil profile down to a depth of 18 to 24 inches during each watering event. This forces the roots to extend vertically to reach the receding moisture as the upper layers dry out. A slow, steady application of water over a period of several hours is far more beneficial than a quick, heavy burst.
The frequency of this deep watering should be reduced compared to typical lawn irrigation schedules. A mature tree may only require deep watering every two to four weeks, depending on the species and local climate conditions. Young trees should still be watered deeply to establish a strong downward trajectory.
Homeowners can measure the depth of water penetration using a long probe pushed into the soil 24 hours after watering. The probe will slide easily through moist soil but will stop abruptly when it hits dry ground, indicating the saturation line. Adjusting the watering duration based on this measurement ensures the target depth is consistently met.
This practice anchors the tree better and makes it significantly more resilient to drought conditions over time.
Soil Environment and Remediation Techniques
Addressing severe soil compaction around established trees requires remediation techniques. Compaction limits oxygen and physically impedes root expansion, necessitating the use of pneumatic tools like an air spade. This device uses high-pressure air to fracture dense soil without damaging existing roots.
Deep-root fertilization tools can inject soil amendments or create vertical channels. Creating these openings involves drilling holes 6 to 12 inches deep and filling them with coarse sand or organic compost. These columns serve as conduits for water and oxygen to reach lower soil depths.
Strategically installed root barriers can redirect horizontal root growth away from vulnerable structures. These barriers are placed vertically in the soil to divert the roots downward.
Ensuring appropriate drainage prevents waterlogging. Maintaining a soil pH and nutrient content suitable for the specific tree species supports healthy growth and the utilization of deeper soil resources.