This spring will mark my 15th year as an arborist. Having spent more time at home this season due to the coronavirus, I’ve had time to reflect on and express some gratitude on my career choice. I’ve seen some amazing things over the years, from planting 35 foot tall Japanese maples and saving specimen trees from insects and disease to working on relief efforts after Hurricane Matthew by helping residents and businesses clear trees from their living rooms. I’ve watched nature’s beauty and power unfold daily right before my very own eyes. Despite all that I’ve seen, it’s the parts that I haven’t seen that make my daily work so interesting and challenging. One of my mentors, a salty old New England arborist who started his career a good 20 years before I was even born, once told me “Nick, most arborists look up. But the best arborists look down.”

What he was referring to was the roots! It’s easy to forget that nearly half the organism is BELOW ground. And often, the health, condition, safety and appearance of what’s above ground has more to do with the half of the tree that’s below ground. Sometimes as an arborist we’ll get the opportunity to inspect and excavate trees’ roots systems. They are unbelievably complex and beautiful. It often amazes me how far they’ll grow; how resilient they can be; and how they interact with the complex biological community within the soil that’s often even more diverse than the biological community above ground.

Despite this resilience, trees can often run into challenges below grade, especially in the urban and developed landscape. Being aware of and taking proactive steps to minimize the effects of these challenges can often mean the difference between a tree’s success and failure in the landscape.

Looking down: The exposed roots of a fallen tree on the Arboretum’s property.

Buried Root Flares and Girdling Roots

I’m often asked what is the most common tree issue in the landscape. People usually expect a certain insect or disease as the answer. Overwhelmingly, the most common tree related problem in the landscape is buried root flares. This is probably because this issue transcends across the species. In nature (pay attention next time you take a hike), a tree’s growth is pronounced by its root flares and exposed buttress roots. This part of the tree is simply not intended (or biologically designed) to be below grade. Proper tree planting calls for this part of the tree to be either at grade or even slightly above. Unfortunately, trees almost always get planted too deep by landscapers and homeowners alike.

Girdling root on a holly, courtesy of Cinthia Milner.

For proper tree planting guidance, try looking at the Arboretum’s trees! I’ll never forget one of my first meetings with Mac Franklin, director of horticulture at The North Carolina Arboretum. I just had to comment to him how all the trees were planted right! And it shows – the Arboretum’s trees have significantly fewer health issues than those I routinely encounter in the landscape. When trees are buried for extended periods of time, excess moisture along the stem promotes decay and infection and can disrupt the normal flow of water and nutrients in the tree. It’s also the perfect environment for stem girdling roots to develop. These are opportunistic roots that grow in the soil around the stem and as they expand, they essentially girdle and “choke” the tree.


My Soil Science class at Penn State was certainly a hard one to stay awake through, but one thing stuck with me – ideal soil composition. Ideally, soils should consist of 45% mineral content, 25% water, 25% air and pore space, and 5% organic matter.  Unfortunately, in the urban landscape, soil compaction (think foot traffic, cars and most often construction equipment) destroys this composition by compacting the soil and drastically eliminating air and pore space. The effects of this damage are complex and long lasting. With less air and pore space it can be challenging for roots to grow, breathe and most importantly, it severely damages the living conditions for the symbiotic organisms that make up the soil food web that is so important for a tree’s health. While measures to fix compaction are available, they are often costly and intensive. Preventing compaction in the first place is the best practice through proper planning and landscape design.

A row of healthy Chinese elms (Ulmus parvifolia ‘Allee’) planted at The North Carolina Arboretum along Frederick Law Olmsted Way .


The debate on whether trees need fertilization is certainly older than I am and will likely continue long after I’m gone. The truth? Well, it depends. Nutrient deficiencies in the forest (and the need for fertilization) are quite rare. Why? Because most native trees grow and are adapted to the soils of their native region. And even more importantly, in undamaged forest soils, natural nutrient cycling in uncompacted soils with complex biological communities often meet the nutritional needs of trees. For the urban landscape, a whole different story unfolds. During development, surface soils (top soils with appropriate amounts of organic matter) are often removed to provide sturdy and compacted base layers for homes, streets, sidewalks and other buildings. What’s left?  Not much. Repairing and rebuilding soils is possible but often takes decades or more to reach some semblance of a natural forest soil. And that’s without competition from turf or the removal of leaves and other organic matter that are so integral to nutrient cycling.  The answer? Test, test, test. Different tree species have different nutritional needs. Performing species-specific soil tests can help create a prescription for what treatment may or may not be necessary. We can all imagine how unproductive (and possibly dangerous) it would be if we all went to the same doctor and got the same medicine regardless of our ailment, condition, etc. Soil treatments and fertilization aren’t any different. Identifying deficiencies or imbalances in macro and micro nutrients, pH levels and organic matter levels, and then treating accordingly will give a tree exactly what it needs…and nothing else! Applying unnecessary products and nutrients can cause significant tree health problems and raise environmental runoff and pollution concerns.

Caring for trees’ roots systems and their soil homes is often the easiest and most practical way to improve and maintain tree health. Trees with healthy root systems and soils are much less prone to other diseases, insect and environmental stress issues – thus reducing the need for pesticides and other costly measures.

Remember, “Don’t look up… look down!”

About the Author

Nick Duffy is an Aborist Representative with Bartlett Tree Experts, the world’s leading scientific tree and shrub care company and a proud Community Partner of The North Carolina Arboretum. With more than 15 years of industry experience, Nick is an ISA Board Certified Master arborist, the highest professional certification in the industry. In his free time, he enjoys fishing, camping, gardening and spending time with his two young daughters.