Submitted by Dave Meadows
Pioneer Columnist
According to a study in the USA over 20 years ago (Smiley & Booth 2000), 93 per cent of professionally planted trees were planted too deep. The depth of root systems in landscape trees has become a major topic of concern and discussion in the green industry and has resulted in the leading cause of tree decline and deaths in Canada and the USA.
The accumulation of soil around the base of the tree resulting from soil cultivation in the nursery was blamed for roots being too deep. Trees can survive, and even thrive, in the nursery with deep roots because of the high quality, forgiving soils. When transplanted to a lesser?quality landscape site, the same tree will struggle and may not survive. Site quality is a major factor in the survival and performance of trees with deep roots.
The term most often used to describe deep roots is “planting too deep”, but this only describes one of several causes. It is not just a problem of planting in the landscape. Trees subject to construction involving grade changes can, and do, suffer from soil piled up onto the existing trunks resulting in a similar, slow decline and ultimate death years later. Deep structural roots are a better description. Structural roots are the large woody roots giving characteristic form and shape to the root system, and the depth of these roots is the real concern.
In 2004, the American Standards for Nursery Stock addressed root depth for the first time. It stated that for B&B (Balled & Burlap) trees, “soil above the root collar or flare … shall not be included in the ball depth measurement and should be removed.” Nursery production methods are only beginning to be recognized as a possible major contributor to deep structural roots, stem girdling roots (SGR’s) and poor root architecture…the three leading causes of tree decline and death.
Verifying standard nursery stock specs or determining the correct root ball depth is difficult and time consuming for professionals, never mind casual gardeners. Working with the District of Invermere, Public Works department, we planted over 100 trees, all supplied from reputable growers over the past two years. In almost every case, structural roots were found to be between three and eight inches deep in the root balls. In every case, evergreen trees were the most affected exhibiting poor root architecture and suffering from stem girdling roots (SGR’s).
‘Getting the roots right’ is a problem that involves every aspect of the green industry and everyone has to do their part to keep roots at the right depth. i.e., nursery growers, landscape architects, designers, and contractors, arborists and gardeners.
First, locate the structural roots in the root ball before digging. This can be done by probing with a screwdriver. Then, remove the excess soil covering the structural roots; measure from the root collar to the bottom of the root ball or container. That should be the “true” depth of the planting hole. Dig the hole; plant the structural roots just below grade and root collar above grade, remembering to remove plastic or peat containers, any burlap fabric, strapping and wire basket, if included.
For more information, you can download a free Tree Owner’s Manual pdf guide from the US Forest Service at https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5368392.pdf
Dave Meadows has been an ISA, Certified Arborist since 1996. Dave owned and operated Invermere Tree Care until his retirement this year. Dave also works part time for the District of Invermere, Public Works, helping with municipal tree care operations, and planting new trees for the Urban Forest.
