Tree Talk: Got roots?
March 21, 2008
The most misunderstood and overlooked part of a tree is the root system that hides beneath the ground.
Most people think trees have giant tap roots that grow deep into the ground to hold the tree up. They think they “saved” a tree just because they didn’t cut it down. They call leaves “green waste” and haul them to the dump or burn them. These beliefs could not be further from the truth.
If people could see the roots, or understand them, they would make better choices for healthier and safer trees.
A tree’s roots are essential for support, intake of water and minerals, storage of sugars and synthesis of compounds. When the seed of most trees germinates, the first thing to grow is the tap root. In the first year, the tap root may grow 3 to 5 feet down or to the side in search of water and minerals, while the above-ground portion of the tree is less than a foot tall.
As the tree grows, new roots emerge near ground level and grow vigorously outwards. As the tree matures, these lateral roots outgrow the tap root, making it indistinguishable. A few of these, the heart roots, enlarge and grow as deep as the tap root to provide support before branching laterally.
The lateral roots provide the majority of the support through tension. Some trees also have sinker roots that grow down from the lateral roots for additional support. As all the roots grow and branch out, the lateral roots remain the majority.
Soil conditions affect root growth. In fertile, moist, well-aerated soil, roots branch more, are smaller and closer to the trunk. In poor and dry soils, fewer large roots spread greater distances.
Where the soil is compacted, or remains wet, roots grow very shallow. Roots only grow deep in loose, dry soils.
In our area, 90% of tree roots are in the top 1 to 3 feet of soil. Most of the feeder roots doing the absorption are in the top 6 to 12 inches. The root system extends laterally in all directions 1 to 3 times the height of the tree. The root system is only about 20% of the mass of the tree.
Most of the root mass is within the tree’s drip line (the area under the canopy), but most of the root systems length and surface area is outside the drip line. The trees in our area have few, shallow roots spreading great distances to absorb limited resources near the surface.
The root systems of most trees “saved” during construction are not protected and suffer significant damage. Equipment cuts and crushes roots, fill dirt and impermeable surfaces suffocate roots, and soil is compacted making it unfavorable for new root growth.
Landscapers damage more roots by installing irrigation and plants close to the base of the trees.
Frequent irrigation slowly rots the roots of trees that are adapted to dry summers. Leaf litter is often raked and blown away, leaving bare ground to erode and become compacted. These forms of root damage cause the tree to slowly decline in health and safety over years, or even decades, before it falls or dies.
To truly preserve a tree during construction, the root system must be protected and soil conditions must be favorable for new root growth.
Mulch is the best way to promote healthy roots. Organic mulch, such as leaves, needles, woodchips, straw, and bark, improve the soil conditions. Mulch improves water absorption and retention, prevents compaction and erosion, insulates the soil, slowly releases nutrients, and reduces weeds. A layer of mulch 2 to 4 inches deep is recommended over the entire root system.
The best thing you can do to promote and maintain root health is leave the leaves!