Question
My husband wants to regrade around an existing mature oak tree here in Eastern MA, north of Boston.  Question: Will removing up to 6 inches of soil under part of the tree damage it?  How much digging can we safely do?

He is digging within 5-6 feet of the trunk.  The total girth of the branches is near 30-40'.  Is it true that the feeder roots are farther our from the trunk than he is digging?... therefore no damage is being done?

The tree sits in soil on a rock slope ON OUR HIGHEST terrace within 2 feet of a long rock retaining wall.

Answer
Yes removing 6 inches of soil within 5-6 feet of the trunk will damage the roots of the tree and will result in dieback of foliage or death to the tree. The roots of a tree are simular to the limbs they are large near the trunk and feather out as they go away from the trunk. Most are within 6-12 inches of the surface of the ground.
If you cut the roots off around the tree trunk within 5-6 feet you will be cutting all the feeder roots from that large root and this is not good.

Trees depend on the feeder roots in the upper 6 to 12 inches of soil for air, water and minerals. Most feeder roots are in an area under the tree crown in open-grown trees. If you extend perpendicular lines from the widest portions of the tree crown to the ground and connect these points, you抣l enclose an area that includes most of the root system.   

The extensive shallow (feeder) roots usually extend 1/3to ?again the dripline-to-trunk distance beyond the drip line of the tree. Grading, cutting, or trenching around oak trees is often detrimental due mainly to the shallow feeder roots being cut or damaged by machinery, or exposed by scraping away the topsoil. This may weaken the tree by reducing its ability to take up water and nutrients from the soil.


Because most of the roots are in the upper 6 to 12 inches of soil, lowering the grade and consequently removing those roots under a tree crown can be harmful

How much can you dig--I would say very little if any without dmaging the root system. It would be better add a medium on top of the area and plant your flowers etc. in this medium instead of diggin up soil.

The first step to this above- ground planting technique is to remove all debris from the planting area ( i.e., leaves, sticks, old mulch). Next, lay down old newspaper in the planting area. This will help reduce weeds and prevent the old tree roots from invading your planting area until the new plants are established. It is best to water the newspaper to prevent the wind from blowing it away.

Now add pinebark mulch on top of the newspaper to the depth of the pots. If you抮e using one gallon plants this would be about 6". The reason to use pinebark mulch is that it is not toxic to plant roots. Hardwood mulch or any mulch containing woody material will rob your plants of nutrients as it breaks down. Be careful not to pile the pine bark on the existing tree trunks. That would be an invitation for insects and diseases.

If your planting area is large, the pinebark can be acquired in bulk. It is sold by the cubic yard at landscape supply companies. A cubic yard is 27 cubic feet. If the area will be 6" deep, you can expect to cover 54 square feet per cubic yard. That same area would take 9 (3) cubic foot bags from your local garden center.

After the area is covered in bark the holes can be dug by hand. Remove the plants from the pots and pull at the roots if they are wrapped in circles. If the plants are very root bound cut from top to bottom with a knife or shovel. Place your new plant in the hole and backfill to the top of the root ball. Don抰 put any mulch on the top of the root ball. Everything that was above the roots in the container should still be seen.
Water the bark well to settle in the plants. A drip hose should be used for the first few weeks while the plants become established. It is hard to say how often to water because it depends on the time of the year and the amount of wind the area will receive. I would place a pan under the tree and turn the sprinkler on and when the pan has 1 inch of water in it stop. In a couple of days, check the soil under the pinebark and if it is dry water this way again.

The pinebark has very little nutrient content and needs additional fertilizer. A slow release fertilizer is best. Slow release fertilizers have a coating that prevents all the nutrients from being released at once. This prevents the plants from being injured and your fertilizer money from being washed away. All garden centers have many different brands of fertilizers, so ask a sales person for a good slow release brand and follow the directions on the bag.

The pine bark weighs a lot less than top soil and is more porous letting water and air get to the trees roots.