Out of sight, out of mind is the attitude most people have when it comes to tree roots in the landscape. That is unless they are in your way. However, carelessly disturbing a tree’s roots is not a wise course of action unless you are willing to lose your tree.
Healthy roots are essential to a tree’s health, serving several important functions including; the uptake of water and nutrients; providing a stable anchor for a large tree; and preventing soil erosion.
What are Tree Roots
Tree roots extend much further than one would think. While the initial roots that anchor the tree may go down several feet, the majority of roots grow outward in the upper foot to foot and a half of soil. In fact, most roots extend far beyond the tree’s drip line, and for some trees may go out three to seven times as far as the canopy. Roots at this point are further enhanced by an association with symbiotic fungi, which expands the effective surface area and root function.
There are two types of underground roots – woody and non-woody. Woody roots play a role in anchoring the tree and in providing a water and mineral transport system. The non-woody roots, called feeder roots, tend to be smaller and have fine root hairs that are necessary to absorb water and nutrients from the soil. The water from these roots will feed into the larger roots for transport into the tree trunk for distribution to the branches and leaves.
It is not advisable to plant grass directly under trees. It is rarely successful and trying to establish grass in the tree’s root zone will result in over fertilization and watering of the tree. It is usually not necessary to dig down to fertilize a tree. Typically, broadcasting fertilizer on your lawn and around the base of the tree will allow sufficient nutrients to reach your tree. However, broadcasting herbicides such as dicamba on lawns to kill broadleaved weeds can cause damage to trees where the roots extend into the lawn, so use care with herbicides.
Tree Root Problems
Tree roots require water and nutrients, but also need oxygen. They will grow wherever they can find a suitable environment. Trees growing in an urban or suburban landscape likely have their root systems compromised by streets and driveways, poor topsoil, and damage from compaction during construction. Soil compaction is the largest single killer of trees. Compaction reduces or eliminates pore spaces that hold oxygen and water necessary for root survival. Compaction can also harden the soil such that roots can no longer penetrate through it.
Cutting roots that are two inches or larger can have severe consequence, as can cutting roots that are too close to the trunk. If you have a construction project that will impact large roots or a large percentage of roots, you should consider taking down the tree, as you will eventually lose it anyway. However, if you do want to landscape under a tree, there are ways to be successful without causing significant damage.
Some trees are more sensitive to root damage than others. Species such as black and scarlet oaks, red maple, magnolias, and dogwoods are particularly sensitive to root disturbances. This is because the “plumbing system” forms a direct connection between roots and branches. Thus, if a root is killed, the corresponding branch will also die.
Other tree species have different systems where roots serve all branches and thus can compensate for some root injury. Redbuds, red maple, river birch and hickory can abide some disturbance, while white oak, poplars, silver maple and crab apple are even more tolerant.
If you choose to plant beneath a tree, turf is not recommended. Trying to establish grass under a tree will cause great stress to both grass and tree and will result in over fertilization and over watering of the tree to get grass established.
Care in planting annuals and perennials can be successful if you choose small starter plants, don’t disturb large roots, and select plants that can grow in low light and moisture. Make sure there is adequate good topsoil, but don’t add more than two inches on top of roots. Additional soil will decrease both moisture and oxygen availability leading to root suffocation and death. Water the plants to get them to settle, and then as needed if rainfall is not adequate. Finally add an inch or two of organic mulch or compost, but keep it at least a foot or so away from the trunk. Mulch will hold too much moisture against the trunk and could favor disease development, but mulch is beneficial to hold in moisture around the roots and newly established plants. With these steps you can have your flowers and enjoy your trees.
Cynthia Sollod has always loved plants, hence a B.S. in botany and a PhD in Plant Pathology. She has volunteered with the Wake County Master Gardener program since 1995. She enjoys painting, illustrating plants and writing about gardening.