Gardening and Landscaping under Large Trees
Anyone who has tried to landscape or plant a garden under a mature tree knows that it is not as simple as choosing shade-tolerant plants and then treating the space like any other in your yard. In addition to casting shade, the trees themselves are a major source of competition for water and nutrients. New perennials and shrubs need some extra nurturing if they are to become established and thrive.
If you are planning on building a deck, walkway or patio under a tree you also need to understand the structure of a tree’s root system to avoid damaging the tree or having the tree damage your landscaping.
Despite the challenges of working under or around mature trees, some of the most beautiful and peaceful outdoor spaces that I have experienced include, or are defined, by large trees.
As with any endeavour, the place to start is with an evaluation of where you are. The type of tree that you are working under is important. If it is a smaller leaved variety, such as a locust or ash, you should have dappled light under the canopy and only a moderately competitive root system to deal with.
If the tree is a maple, especially a Norway maple, you are likely to have a very dense shade and the soil will be thick with large, woody roots and finer fibrous roots. If the tree is a conifer it will tend to cast dense shade and shed water toward the end of its branches, which will keep the soil beneath it particularly dry. You are also likely to have more acidic soil here than elsewhere in your yard.
The soil is also very important, it will likely be densely filled with roots, but other features may vary. If you have spent years raking leaves or needles up from under these trees the soil is likely depleted in organic matter and nutrients. The microorganism content is probably also quite low.
You should also have a basic understanding of the structure of a tree’s root system. The first thing is that the persistent image of the taproot is not accurate. The root system of a mature tree resembles a shallow dish.
Right under the trunk, the main structural and anchoring roots might be as deep as three or four feet, but as you move away from the trunk this quickly changes to little more than a foot in depth for the bulk of the roots.
The heavy, woody roots extend to approximately the same distance as the branches. The fine, fibrous feeder roots extend at least as far again.
The roots perform several critical functions. They absorb water and nutrients from the soil, they exude very mild solvents which help break down the minerals in the soil so that they can absorb them. They also anchor the tree structurally during storms.
In addition to water and nutrients, roots need oxygen to function. The root systems of plants burn sugars, which are produced by the foliage through the process of photosynthesis. To burn these sugars roots need oxygen, the same as we do. If you smother or waterlog the root system through a change in a grade you can significantly damage or even kill a tree.
You should also keep in mind the relatively shallow depth of a tree’s roots when you are planning on installing any hard surface. When a base for a patio, walkway or driveway has installed the soil is excavated to an absolute minimum depth of 12”. This will sever all of the major roots which run through this area. Keep in mind that this will also sever the connection to all of the roots beyond this area if you are working close to the trunk. Trees can survive the loss of some of their roots, but severing a significant percentage can severely stress or kill a tree, and can leave it dangerously unstable during a storm.
Tree heavily damaged by paving over the roots
If you do need to sever a root, much of the standard advice for removing a branch applies. Cut the roots smoothly with a sharp pruning saw or pruners (but not your best pair because the soil will dull them) making sure not to leave ragged or split ends, which may become infected with fungus or insects. Don’t treat the cut with anything, the tree will naturally develop a seal behind the wound and anything that you add will only interfere with this process.
With this information in mind, take a look at how you want to use the space. If you are planning on simply creating a shade garden you can skip ahead to the soil preparation and planting information. If you are planning on creating pathways, sitting areas or other features there are a few other points you should consider.
Compacted soil can be tough for roots to grow in, so pathway areas should ideally be covered with a thick layer of mulch or wood chips to reduce the impact on the soil from foot traffic. You can also use stepping stones with space between them to guide people through the garden without installing a complete gravel base and walk.
If you are adding a sitting area the material that you use will have an impact on how close to the trunk of the tree you can be. If you are installing a patio of flagstone or paving stones you will need a compacted granular base and the installation will remove the root system from this area. This type of construction should be kept well back from the tree.
If you are installing a permeable gravel area over geotextile and building the area up rather than excavating, you can get away with working closer to the trunk, but you should still limit the area that you are covering as much as possible. This type of cover probably won’t cause much harm to the larger roots but any feeder roots directly below the gravel will likely die out.
If you must work near the base of the tree, consider constructing raised wood decking. You will still need to install some foundations, but a few carefully placed holes for foundation piers are less damaging than removing or smothering the entire root area that you are covering.