Why You Shouldn't Put Soil Over a Tree's Exposed Roots

Have you ever been tempted to cover exposed tree roots with dirt? The temptation is understandable. Here are just a few of the reasons why such roots can drive homeowners crazy:

• They are unsightly.
• If they occur in a lawn area, they are in the way every time that you mow the grass; hitting them by mistake can damage your mower blade.
• Even if they are not in a lawn area, tree roots sticking up above-ground create an unusable space.
• You or other people can trip over them, so they create a health hazard.
• And to add insult to injury, they can push up through walkways, etc., causing damage to your hardscape that costs you money (for repair), time, energy, and consternation.

Maple trees (Acer) are especially apt to cause these issues. So are beech trees (Fagus), and anything in the willow family, such as the quaking aspen (Populus tremuloides). As a result of such problems, you may be wondering if you can put topsoil over the roots to cover them up or create a shade garden under the tree.

It does seem, at first glance, like it would be perfectly safe to have topsoil brought in and spread it on top of the exposed roots. After all, soil can't be a bad thing for roots, right? It is in their nature, is it not, to grow in the dirt? One can readily see why so many homeowners make this landscaping mistake.

Sure, it may seem safe. However, this practice can be bad for the health of your tree, so beware the cover-up! 

Why It Can Be Harmful

If you are thinking of placing dirt on top of exposed roots, at least you are avoiding an even worse mistake: namely, cutting the roots (and yet some beginners mistakenly do just that). Nonetheless, you need to be cautioned against putting soil over tree root. Roots on the surface are actually covered with lenticels, which are gas exchanging structures. Tree roots need oxygen, and dumping a thick layer of dirt on them can suffocate them.

In fact, if you suffer from exposed roots, there is a good chance that lack of oxygen was the reason that the roots came to the surface, in the first place: They were getting insufficient oxygen with which to breathe, perhaps due to their growing in compacted soil.

Here is the good news, though: A small amount of soil can be spread over exposed tree roots, in two stages (if necessary); this incremental approach gives the tree roots time to adjust. But suddenly covering exposed tree roots with enough soil to start a garden could cause serious harm to the tree.

How to Protect Exposed Tree Roots

Exposed tree roots do not need to be covered at all unless they are threatened by mechanical damage from lawn mowers or weed eaters, for example. Mechanical damage on these roots can be rather serious, and it is very difficult for the tree to recover in these areas. If mechanical damage is a threat, the exposed roots should be mulched. Mulch is loose enough to allow for ample gas exchange and will not suffocate the roots. Spread a thin (2-inch) layer of mulch over the tree roots and let them be.

In extreme cases, where exposed roots are sticking up higher than 2 inches above-ground, it is best to just leave them alone and accept that you have a less than ideal situation (unless you are willing to remove the tree).