Rejuvenating Tree Roots for Compacted Soils and Soils Covered by Fill
Root Flare Excavation * Vertical Mulching * Radial Trenching
There are four types of mechanical root rejuvenation for compacted soils and soils covered by fill soil:
And to remedy the all-too-common buried root flare, there is
root flare excavation (see below)
Often I use more than one root rejuvenation technique and sometimes all four techniques on the same tree.
Radial trenching involves excavating spokes outward from the trunk. I like to start at a distance from the trunk equal to three times the trunk diameter. So if the tree diameter is 24 inches, then I would start the radial trench 72 inches (6 ft.) from the trunk.
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Danny LeBlanc is doing radial trenching to stimulate fine root growth on the Maltby Oak
The radial trench should extend out to the dripline of the tree. The depth of the trench depends upon soil conditions, tree health and desired effect. Some researchers recommend cutting small roots that will quickly regenerate and grow into the compost-filled trench. Other experts only recommend making the trench but leaving all the roots intact.
Figure 2 The radial trench is made with an Air Spade. Notice the roots are exposed but not damaged.
Figure 3 Only small roots are cut to stimulate new growth into the radial trench where a soil compost mixture has been added.In Figure 1 above, Danny LeBlanc of Taylor Tree Services is assisting me with a more extreme method of radial trenching which uses a trenching machine that makes a 3-inch wide trench that is about 18 inches deep. We are working on the Maltby Oak. The trench is refilled with a mixture of soil and compost. If the soil quality is good, I use the same soil to mix with the compost. If the soil quality is poor as is sometimes found on construction sites, I add new better-quality soil and combine radial trenching with a compost stirring technique described below.
The trenching machine will sever smaller roots that cross the path of the trench. When the treated tree has reasonably good health, the severed roots should quickly sprout and grow into the compost-soil mixture in the trench where nutrients, organic matter and general conditions are better than in the surrounding soil. Cutting larger roots over 1-inch in diameter should be avoided.
Another radial trenching technique utilizes the Air Spade (Figure 2 above) if there are utilities such as electrical or water lines that could be damaged by the trenching machine. The small pencil-thick roots are cut with a chain saw or hand saw (Figure 3). Then a compost mixture is added to the trench and raked and mixed with the existing soil (Figure 4 and 5).
Figure 4 The radial trench extending outward from the tree trunk is filled with compost which is mixed with the existing soil.
Figure 5 The compost and soil are raked and mixed in the trench.
Vertical Mulching is the process of making
vertical holes in the soil usually where fill soil has been added over roots or where compaction with heavy equipment has occurred over the roots. Some use heavy duty hand drills with small augers to dig the vertical holes. I prefer the Air Spade which tends to lift and fluff the soil.
In the photo above I am inserting the Air Spade into the fill soil down to the native soil level. I am trying to make 2 to 3 feet of fill soil more porous and hospitable to roots that have been covered and smothered by the fill soil.
Figure 6 When conditions are dry, a large amount of dirt is blown into the air by the Air Spade.
Figure 7 The Air Spade is used to make vertical holes about 18 to 24 inches deep. The idea is to loosen and aerate the soil allowing the roots covered by fill soil to grow upwards into the fill. The fill soil should also be removed from the base of the tree in what is called a root crown excavation.
Compost and Soil Stirring Another method that I use to improve soil conditions around an established tree is to spread compost over the soil
surface beneath the dripline of the tree. Then with the Air Spade I stir the soil mixing in the compost down to a depth of 4 to 6 inches. as in the photo (right). Then following the compost stirring and mixing, 2 to 3 inches organic mulch is spread over the soil. The organic mulch is not shown.
Both vertical mulching and radial trenching have been criticized by some researchers because these processes only affect a small area of soil and roots that are adjacent to the trench or the vertical holes. Some critics of vertical mulching and radial trenching generally prefer the stirring of the compost into the upper few inches of soil. I try to combine all three techniques. The extent of work, however, depends upon the health of the tree. Too much root work can stress the tree.
When I do radial trenching, I stir in compost between the radial spokes. I also do vertical trenching between the spokes of the radial trenches if the soil is compacted or if the natural grade has fill soil added. I also follow up with a layer of organic mulch placed directly over the turf and natural grade. Do not install a weed mat between the organic mulch and soil because the weed mat creates a barrier that inhibits the breakdown of organic matter into the soil. We want the organic matter to decompose and become soil. A weed mat does not permit this mixing and improvement of the soil. For more information on proper mulching, go to Mulching Trees on this web site.
Although I own and use an Air Spade, there is a similar competitive product called the Air Knife. I have used both types of air excavation equipment and, in my opinion, both are equal.
Root Flare Excavation
One of the most common tree problems I encounter is a root flare that has been covered with soil.
When a tree's root flare is not visible, that means the root flare has been covered by fill soil. This can cause a series of problems for the tree.This problem can be caused by planting a tree too deeply or by adding fill soil over the natural grade and root flares at the base of the trunk. Adding fill soil over the tree roots within the dripline is often detrimental to tree health. But fill soil added to the base of a tree is especially damaging over the long term. The fill soil over the root flares can cause the bark and cambium layer in the trunk and root flares to decay. The trunk cannot tolerate constantly moist soil like root tissue can. No root flares are evident in the photo above indicating fill soil has been added. In the photo fill soil along with the restricted root space and the very probable construction damage do not provide a good prognosis for the long-term survival of this tree.
Figure 9 A stem girdling root grows around the base of the trunk and over a root flare gradually strangling the tree. Notice also the black strap over the girdling root. The black strap was carelessly left by the tree installers years ago and can also strangle the trunk and roots.Another problem caused by fill soil over the root flare is the formation of girdling roots that can cut off circulation in the root flares and portions of the trunk (Figure 9). For more information on stem girdling roots and circling roots, go to the Transplanting Section of this web site.
Damage Caused by Fill Soil at the Base of the trunk
The Maltby Oak on the courthouse grounds in Palatka, Florida is a good example of what fill soil left against the trunk can do over time. Around 1985 someone in maintenance or administration decided there should be turf underneath the canopy of the Maltby Oak. About 12 inches of fill soil was added and sod placed over the fill right up to the trunk. Irrigation was added for the turf. Then 20 years later in January 2005 the Maltby Oak was condemned and scheduled for removal by the County Commission because large branches were dying and endangering the public who passed by the tree every day on their way to the courthouse. (For more details of the Maltby Oak history and treatment, click here.)
Figure 11 Once the root crown had been excavated in late 2010, the large number of adventitious roots that have arisen just below the fill soil line became apparent. These roots emerged because fill soil had suffocated much of the original root system and these roots are now sustaining much of the tree. It appears that the large lateral roots (not visible in the photo) emerging from the root flares are still intact and providing support and stability for the Maltby Oak.
Figure 10 In 2005 this is the way the base of the Maltby Oak looked. There was at least 14 inches of fill soil piled against the base of the trunk over the root flares. This soil against the trunk kept the trunk constantly moist and caused the bark, cambium and sapwood to decay.
Figure 11 shows the exposed circling and girdling roots and the decay at the base of the trunk where the fill soil was. The fill soil caused about one third of the 14.5 foot circumference of the trunk to decay. I also discovered that a sprinkler was hitting the tree trunk where most of the decay was located. Irrigation spray should never be hitting the trunk of any tree for an extended period of time.
Figure 12 A ganoderma fungal conk appears annually at the base of the trunk that was previously covered with fill soil.In spite of the extensive damage to the trunk, the removal of the fill soil allowed the lower trunk to dry and the decay has slowed on the healthy trunk tissue. The irrigation spray head has been redirected. The decayed area of the trunk will not become whole again. And the internal decay will continue slowly inside the trunk. The fungal conk, Ganoderma applanatum, is an indication of internal decay. However, the tree, if healthy, will continue to add new solid, strong wood outside the decayed area. And tests with the Resistograph indicate the decayed areas at the base of the trunk are mostly near the surface and the interior heartwood is still very solid.
