A tree well is a retaining wall constructed around the base of a tree for aesthetic or protective purposes. This structure creates a turf-free zone around the trunk. The well prevents mechanical damage to the bark from lawnmowers and string trimmers, which can restrict the movement of water and nutrients. It also serves a horticultural function by reducing competition for resources between the tree’s feeder roots and surrounding grass. A properly constructed tree well helps regulate soil temperature and retain moisture, contributing to the tree’s overall health and longevity.
Purpose and Essential Planning
The construction of a tree well is frequently driven by the need to manage grade changes in the landscape. When the ground level around an established tree is significantly increased—often by more than 18 inches—a well is required to prevent the new soil from suffocating the root system. Conversely, a well may be built to retain the original grade when the surrounding soil level is lowered, preventing root exposure. Proper planning determines the ultimate effectiveness and size of the finished well.
The ideal size should extend as far as possible from the trunk, with a minimum radius of three feet considered a starting point for established trees. For maximum benefit, the outer edge should reach the tree’s drip line, the area directly beneath the outer circumference of the branches. This sizing ensures the well encompasses a significant portion of the tree’s fine, absorptive feeder roots, which are concentrated in the top 12 inches of soil.
Selecting the appropriate material affects the well’s lifespan and appearance. Common materials include stacked stone, brick, concrete block, or secured timber. The choice of material dictates the need for specific foundation preparation, such as a concrete footer for a mortared wall or a crushed stone base for dry-stacked blocks. Calculating the required materials is necessary to estimate cost and prevent construction delays.
Step-by-Step Construction Guide
The first step is marking the perimeter and preparing the base. Laying a garden hose or rope helps visualize the size and provides a guide for marking the ground. For a permanent structure, a trench must be excavated for the foundation, or footer, which provides stability and prevents shifting over time. This trench should be dug to a depth below the frost line in cold climates to mitigate the effects of freeze-thaw cycles on the structure.
Once the trench is excavated, a stable foundation must be installed to support the retaining wall. A six-inch deep base of concrete mix or compacted crushed stone is commonly used, screeded level to create a flat, solid working surface. For dry-stacked wells, a layer of compacted gravel provides both a stable base and promotes drainage beneath the structure. This foundational step ensures the retaining wall remains plumb and level after the surrounding soil is backfilled.
The construction of the retaining wall follows the foundation work. If using dry-stacked blocks or timber, the material is laid in courses, ensuring each layer is level. For mortared walls, a mortar bed is spread over the footer, and the courses of bricks or blocks are set. It is important to leave the vertical joints between individual blocks or courses unmortared in dry-stack systems to allow for minor movement and water passage.
Finally, the well’s exterior must be backfilled to secure the structure against the surrounding grade. The soil that was removed during the trench excavation is typically used to fill the area outside the newly constructed wall. This backfilling process compacts the wall into place, locking the structure and preventing it from bowing outward. The construction phase is complete once the physical barrier is in place.
Critical Considerations for Tree Health
Protecting the tree’s health requires attention to the conditions created inside the well structure. The primary horticultural consideration is the exposure of the root flare, the widening base of the trunk where it meets the roots. The bark tissue on the root flare must never be covered with soil or mulch, as this traps moisture and promotes rot and fungal diseases. Covering the flare can also lead to the development of adventitious roots, which circle and ultimately girdle the tree, impeding nutrient flow and causing long-term decline.
Proper drainage must be ensured within the well, especially if the structure is built to accommodate a raised grade or heavy rainfall runoff. If the original soil does not drain well, a system of perforated drain tile may be necessary beneath the new grade to move excess water away from the root zone. In less severe cases, a layer of coarse rock or gravel placed over the root system before adding fill soil can improve internal drainage and aeration.
When adding material inside the well, quality topsoil or a compost-rich soil mix should be used rather than heavy clay, which compacts easily and restricts oxygen to the roots. The fill material should be applied in layers, and the final surface should be covered with two to four inches of organic wood mulch, avoiding the trunk itself. The mulch layer helps retain soil moisture and stabilize soil temperature, but it must be kept several inches away from the root flare to prevent moisture buildup and decay. Long-term maintenance involves monitoring the soil level to ensure the root flare remains exposed and replenishing the mulch layer as it decomposes, while avoiding the creation of harmful “mulch volcanoes”.