Unless you want a dead tree on your hands, take special care if you plan to add fill dirt around one of your mature trees. Adding fill dirt to your lawn can help improve the layout and overall structure of your property, but it may also permanently harm a mature tree's root system.
According to the University of Florida Cooperative Extension, mature tree roots need soil that contains roughly 50 percent air; adding fill dirt too close to your mature tree's trunk and established root system can reduce the air in the soil. The key step in adding fill dirt around your mature tree is leaving enough undisturbed soil directly under the tree to avoid soil compaction and root suffocation.
Decide if it's essential for you to add the fill dirt around your mature tree. According to the University of Florida Cooperative Extension, adding fill dirt around your tree incorrectly can easily kill your tree, a slow process that may take up to 7 years before your tree actually dies.
Consider your reasons for adding the fill dirt; if you're doing it just for aesthetic purposes and the tree has sentimental value, your best option may be to just leave the tree alone rather than risk killing it.
Locate your tree's drip line. The drip line is the outermost circle of leaves on your tree's branches. According to the College of Agriculture and Life Sciences at Mississippi State University, the area of ground within this circle is where your tree gets most of its moisture to stay alive; covering this area with fill dirt will reduce the availability of oxygen and moisture for your tree's roots, which will most likely result in death for your tree.
Mark the drip line on the ground with a can of spray paint. You should have a circular pattern around your tree. You can add fill dirt in the entire area outside this circle if you have to, but your tree's health may still be negatively affected by the compacted soil.
To be safe, the University of Florida Cooperative Extension suggests that you imagine a circle that is 2 to 3 times as big as the drip line and add fill dirt outside that area.
Add the fill dirt around your mature tree. Make sure you don't put any extra soil inside the tree's drip line. Aim for the soil to angle gradually, so you're not drastically shifting the amount of water that drains naturally toward your tree. If you use a backhoe or bulldozer, avoid driving the equipment across the soil inside your tree's drip line to reduce chances of soil compaction over the tree roots.