Using Barrier fabric materials as a liner to make beds.
Barrier materials are materials that trap, deflect, or use a chemical to prevent tree roots from passing through them. There are times when you might want to use a barrier material to line a plant bed. The main use is when your bed is in a area with invasive tree roots.
First when installing a liner always remember to leave at least one inch of material above the soil. Invasive roots can travel along the soil surface. If the liner is at the surface level or in the soil then the roots can travel over the liner. Secondly, when placing a plant bed it is important to know where the tree roots are. Below is a bad placement of a plant bed. This is assuming all tree roots are removed from the bed.
 | Digging out all the tree roots in the square would result in both structural roots and critical transport roots being destroyed. This could lead to permanent damage to the tree. |
If the plant bed had been moved a little farther from the tree the impact would be much different.
 | Moving the plant bed beyond the critical root zone will have much less impact on the tree. |
If a plant bed must be placed inside the Critical Root Zone, then a spoke pattern should be considered. Again this is assuming all tree roots are being removed from the plant bed.
 | Tree roots tend to spread out from the tree like wagon spokes. If you cut a path parallel to the roots the odds are you will cut fewer roots. If you cut a path perpendicular or at a right angle to the roots you will cut across many main roots. |
So if all roots are going to be removed from the bed area a spoke pattern such as the one below should be considered.
 | This pattern of Liners will result in cutting across fewer tree roots because the Liners are placed parallel to the roots. This is similar to the system of trench mulching that now used on trees. |
If a shallow plant bed is required then it is possible to lay the liner over the major transport or structural roots. When doing this it is important to remember two things. First, roots need Oxygen. If you increase the depth that the roots are buried you also can reduce the amount of Oxygen that the roots receive. So maintain the soil at no more than a inch above the original depth. Secondly, tree roots expand as they grow. The expansion can be great if the tree is still young. The root will expand into the space with least resistance. This is almost always the upper surface. If a liner is over the root then the root will expand into the liner. The liner must be placed loose enough so the root can push the liner up or the liner will girdle the root.
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