Tree Myths and Misconceptions

A myth is a belief or notion based on tradition or convenience rather than on science or facts. Years ago, people believed things like the human heart was the center of emotions and draining someone’s blood could cure a disease. As scientific research progressed, people better understood the earth, their bodies, and diseases, and old common myths were discarded and replaced with science-based knowledge.

Likewise, a lot of myths and misconceptions about trees and tree care persist because we heard them from a friend or neighbor. However, scientific research on trees continues to improve our understanding of how trees function and, in turn, our knowledge of how to keep trees healthy and safe. Following are some common myths about trees, and why science shows us those myths are inaccurate.

Myth 1 – Planting a tree deep will keep the roots growing deep.

If a tree is planted deep in soil or mulch, its roots will become starved for oxygen, and it may not survive. Planting a tree deep can lead to girdling roots, root death, and death of the entire tree. The trunk and stems of trees are not adapted to the constantly moist conditions when buried in soil. As a result, opportunistic decay organisms can gain access, causing crown and stem rots. To explore more on this subject, read Are My Trees Buried Too Deep?

Myth 2 – At planting, prune off branches to balance the tree’s crown with the roots.

Old horticultural practices focused on pruning over a third of a young tree’s branches and foliage to compensate for the roots lost or removed during the transplanting process. In theory, it might sound logical to balance the roots with the canopy, but understanding where a tree’s food or energy comes from and how hormones regulate root and top growth changed that practice. Removing live branches and buds during transplanting robs the tree of food production (photosynthates) in the leaves needed to grow new roots. Additionally, removing buds on branches reduces the production of auxin, a growth hormone produced in those buds, which then moves to the root system to initiate root growth in the spring.

Young transplants need as many leaves as possible to produce energy that will help them overcome transplant shock and lost roots to become established in a new landscape. Any pruning should be focused on removing dead, broken, or rubbing branches only. Newly planted trees need regular watering to support the growth and functioning of leaves and compensate for roots lost during transplanting. It is best to provide an inch of water over the root zone each week (rain or shine) or 10 gallons per inch of tree diameter.  View Planting Landscape Trees for more advice.

Myth 3 – Tree topping doesn’t hurt the tree because it produces lots of leaves.

Tree topping is actually one of the worst things you can do to your tree. It can remove as much as 100% of the tree’s food-making foliage (removing buds when not in leaf has the same impact). Tree topping not only starves the tree and its root system, but it also opens the tree up to major decay and rot, caused by improperly healing cuts that cannot close (trees don’t heal, wounds close). Topped trees deceptively green up, producing very long water sprouts by dipping into stored energy, leaving the tree vulnerable to diseases and insects. Trees that have been topped will soon become dangerous trees weakened by internal decay, leading to more storm damage and tree failure. It is best to prune your tree properly and work with its natural defense mechanisms. Avoid tree topping because it will shorten the life of your tree, cause potential property damage from tree failure, and be more costly in the long run. Learn how to properly prune by reading Pruning Landscape Trees.

Myth 4 – Painting Wounds Stops Rot.

If you are trying to prevent wood rot from impacting your house, deck, or wooden boat, paint it. That doesn’t work on living trees. When a living tree is wounded, it leaves the tree exposed to decay-causing fungi to enter. Paint (or other substances) doesn’t effectively seal out decay fungi. Rather, it can hold moisture in, creating an ideal environment for fungus to take hold.

The best way to reduce internal decay is to prevent wounding and understand how trees evolved to wall off the spread of decay within themselves. Research by Dr. Alex Shigo, former USDA Forest Service plant pathologist, led to the concept of CODIT (Compartmentalization of Decay in Trees), where trees create barriers or walls internally to slow the spread of decay. His initial research for the forest service was to discover what could be painted on the trunks of trees wounded during logging operations. He discovered how branches are compartmentalized from the trunk, preventing decay from entering when branches die. Further, he learned how protected branch collars form callus tissue to close wounds. Painting wounds does not help the tree, and some oil-based paints or tar will inhibit and slow down wound closure.

Myth 5 – Prune off branches as close to the trunk as possible.

Making flush cuts does more harm than removing the branch. Flush cuts injure the trunk by removing the branch collar and branch protection zone (critical areas in CODIT). Flush cuts remove the branch collars, allowing major internal decay to form in the trunk that spreads up and down the tree. Callus and wound wood will eventually form and close a flush cut, but the decay will continue to spread internally. Dr. Shigo used to say that flush cuts did not happen in tree care until the light chainsaw came into use. No one using a handsaw would make a larger cut than they needed to.

Cutting at the proper location just outside the branch collar is a lesser amount of wood to cut through and results in a smaller wound than when a flush cut is made. It is best to learn to identify branch collars (a swollen, raised area at the base of the branch that is mainly trunk wood), and remove branches back to the collar without cutting into it. When shortening branches, make proper reduction cuts back to healthy lateral branches that are one-half to one-third the diameter of the branch being removed.

Myth 6 – Don’t prune trees in the winter because they will bleed.

When some species of trees are pruned in the winter (out of leaf), sap will flow from the wound. It is not "tree blood!" Maples, birch, beech, and elm were called the “bleeders,” and people were concerned that they would be harmed when pruned without leaves on them. Sap flow, the so-called “bleeding,” is a natural defense mechanism. Pruning trees during the dormant season (winter) is better for tree health because insects and diseases are dormant, and the tree will put energy into closing pruning wounds come spring. It is also a good time to view branch structure and make improvements to the tree, such as removing co-dominant leaders or rubbing and crossing branches, along with deadwood. If pruned properly, a little sap flow will not harm the tree.

Myth 7 – You should feed your tree with fertilizer.

Fertilizers are not tree food. Tree food is made in the leaves through the process of photosynthesis. Fertilizers are nutrients replacing or supplementing those normally supplied by healthy soils. Trees growing in natural forested settings do not need fertilizers because nutrient-containing organic matter is recycled back into the soil each year as leaves fall and decompose. Fertilizers such as nitrogen stimulate growth of plant parts. Fertilizers are also salts, and can burn roots if too much is applied. Much fertilizer doesn’t even reach the tree, as it leaches out through the soil and can also be a pollutant to our waterways. Before fertilizing trees, it is best to perform soil tests and follow the recommendations provided by the lab tests. Avoid using weed and feed turf fertilizers around trees because the broadleaf herbicide in the fertilizer can injure the tree. 

Myth 8 – It is good to apply more mulch each year around your tree.

Thick layers of mulch create several problems for trees and their roots. Roots require oxygen and will grow up into a thick mulch layer where they can begin circling the tree trunk. With time, those circling roots will become girdling roots that will slowly kill the tree by choking off the flow of water and nutrients through the trunk. Mulch piled up on the trunk creates a moist environment for crown rots to form and injure the tree. Overly mulched trees also do not harden off properly and prepare for winter. Mulch, when properly applied, is very beneficial and mimics the way trees grow in a forest setting with leaf litter.

Mulch should be no deeper than 3–4 inches and spread from the edge of the trunk (not piled on the trunk) out to the drip line (the edge to which the tree’s canopy extends). New mulch should only be applied when needed, every few years when the mulch layer has broken down and become thin. In most years, when new mulch is not needed, lightly rake the top surface of any mulch that has become matted down. For more information about mulching, read Mulching Landscape Trees.

Healthy and safe trees require management practices based on science, on the biology of how trees grow and protect themselves. Arboricultural and horticultural research continues to expand our understanding of how trees grow, what impacts their health, and how best to care for them.