Tree Myths

By Marilyn Loser

The more I learn about trees, the more I realize the science of arboriculture --the branch of horticulture concerned with the care, planting and maintenance of trees -- has come a long way in the last 40 years. Yet, myths still abound. 

Myth:Large, established trees do not need to be watered.
Fact: All trees need water, especially during drought. It is important to keep the upper 8-12 inches of soil moist at least as far as the branches spread (drip line) and preferably further out. In our dry, windy climate, the very top of the soil may be dry while the soil underneath is still damp; don’t wait until your tree shows signs of drought stress, like wilting or yellowing, before watering. On the other hand, avoid over watering your trees. Tree roots don’t like mud as they can’t absorb oxygen, just as we can’t breathe underwater.

Myth: Branches grow upward as trees gain height.
Fact: A low limb on a small tree will remain at the same height as the tree grows. Trees don’t grow proportionally like human beings, yet many of us envision tree growth as being similar to human growth.

Myth: There is no such thing as too much mulch.
Fact: Mulch is extremely important, especially in our dry, windy climate. It helps retain moisture in the soil and protects the trunk from lawnmower damage.  However, too thick a layer deprives the roots of vital oxygen and may prevent moisture from reaching the soil.  Keep the mulch a few inches away from the trunk to dissuade insects and disease from settling happily in damp bark. Two – four inches of mulch is the best.

Myth: Root systems are a deep, mirror image of the tree’s trunk and limbs.
Fact: I used to think if I watered a tree deeply and infrequently, the roots would grow down to the water table, which is frequently 3 – 4 feet below the surface in our yard. In fact, most tree roots are in the top 18 inches of soil and spread laterally far beyond the width of the tree canopy.

Myth: Trees do well in a small 3 ft. by 3 ft. area surrounded by cement or heavy plastic covered by mulch.
Fact: This is closely associated with the root system myth above. In reality these small growth spaces are tree prisons.  I made the mistake of surrounding trees with thick plastic and a layer of rock 12 years ago and am now paying for my folly. With our wet spring the soil underneath the plastic is very moist, but the roots are deprived of air and one tree looks like some of the ash trees on Main Street with more deadwood than beautiful green canopy. I’m getting a workout moving the gravel and replacing the plastic with good quality yard fabric that discourages weeds from growing up but allows water to seep down.

Myth: It’s better to plant a big tree than a small one.
Fact: Studies show it takes longer for larger trees to adjust after transplanting. A small tree re-establishes roots faster and puts on top growth sooner than a large tree. Small trees are also less expensive.

Myth: Tree roots break sewer lines.
Fact: Tree systems don’t send out scout roots that seek and destroy sewer lines.  Nor do roots nose out water and head for it.  Rather, trees produce roots that grow in all directions.  Roots that encounter good conditions succeed.  If there is a leak or break in a sewer line, especially a line that is not very deep, roots will take advantage of the situation.  Arborists point out that rarely do they receive an irate call from a home owner bemoaning roots harming incoming water pipes.

Myth: Roots surface and damage lawn mowers.
Fact: This makes me think of a possible sci-fi movie title – “Return of the Killer Roots.” Actually, under good conditions, roots grow under the soil surface. When roots cannot penetrate downward because of soil compaction, they grow in the top few inches, creating above-ground roots as the tree matures. Construction equipment or heavy foot traffic can compact soil. When people plant a tree, cover the surrounding compacted, neglected dirt with a few inches of topsoil, then start a lawn, above-ground roots are almost inevitable.

A future column will present tree-pruning myths.  For information sources, visit

“I am the Lorax, I speak for the trees, for the trees have no tongues.” Dr. Seuss