We have a huge pin oak (also called water oak) tree behind the house that covers the entire back yard on the left side. Due to the neighbor’s draining lawn flowing into our yard and a gutter at the back of the house, we have standing water on rainy days around much of this wonderful oak. This summer there were two cinnamon-colored armillaria fungi, indicating root rot. The fungus likes soils that stay continually damp. Due to the tree’s location by the back door and frequent use by the family, we made the decision to try a French drain. However, when searching the internet, articles did not consider the trees root system in their “how to’s”.
With two goals, drain the water from runoff and save tree roots, we came up with eight steps to build the French drain.
1. Locate and mark the straightest direction for the drain from the standing water area to the end (rain drain, ditch, etc).
2. Measure the depth of the drain at the end, to calculate the angle of the slope from the beginning. Keep checking during the digging to avoid disturbing root system or digging too deep.
3. Dig out soil from the trench for the drain from the side furthest from the tree trunk to save as much root system as possible. Be careful not to damage roots, especially the ones larger than the size of a pencil. Also, be gentle with the smaller roots not to bend or break them.
4. Measure and cut the landscape weed control fabric into long strips. To get the width, measure the trench bottom and one side. You will need x2 for the depth and x3 for the width. For example, the bottom is 3 inches and the side is 2 inches. Therefore, it is 3+3+3=9 and 2+2=4 or 9+4=13 inches width.
5. Place fabric into the bottom of the trench and up both sides. The excess is to be pulled onto the side, held tight with small nails to keep it flat.
6. As you are placing the fabric, cut the fabric to go around roots. Fill the trench with clean, large gravel. Avoid any soil coming into the drain.
7. Pull the fabric that has been on the sides of the trench down over the top of the gravel, sealing it snug like a baby.
8. Small roots that had to be cut should be layered over the top of the fabric/gravel. When roots have been replaced as carefully as possible, cover the French drain with small, clean gravel to ground level.
Now the French drain is installed, the roots protected as much as possible, the top surface is ready for grass or ground cover.
Sandy Pannett, Smith County Master Gardener
Texas A&M AgriLife Extension Service