The Importance of
Proper Planting Depth
Desert trees, like all plants, are divided, anatomically, into roots and shoots. Roots are all the below ground tissues that are involved in nutrient and water uptake from the soil as well as serving to anchor the plant in the ground. Shoots are all the above ground parts including trunks, branches, stems, and ultimately leaves. Roots extract water and minerals from the soil, leaves produce sugars by photosynthesis and conducting elements that move these products thru the tree are in branches and trunks. The root flair, the base of the trunk, right at or slightly above the soil line, is the divider between these two critical tissue systems. It is an essential and highly vulnerable crossroads for all soil nutrients traveling up and all sugars traveling down the plant. This intersection, between above and below ground plant parts, forms almost immediately following seed germination.
Unlike other tree species, many desert adapted trees (especially all the Palo Verdes) have very little or no bark protecting the tender, essential conducting tissues that lay just beneath the bark. Even relatively minor injuries can cause substantial damage to the root flairs of desert species. For trees to remain healthy and vigorous, care must be taken to insure that the root flair is not injured or damaged. While string trimmers and lawn mowers quickly come to mind, insects, pathogens (diseases), burrowing animals, pedestrian traffic, and vandalism can also be potent sources of root flair damage. Many of these risk factors can be reduced by thoughtful tree placement within the landscape and using proper planting methods.
As they are neither root nor shoot, root flairs must always remain above the level of the soil. Trees planted too deep or that settle after installation (due to insufficiently compacted soils or rootball shrinkage), expose the root flair to excess moisture. Post tree installation re-grading of the landscape site, associated with adding under-story plantings or modifying site drainage, can also serve to bury the root flair. Excessive moisture on the lower trunk and root flair promotes the development of certain root and root flair rotting diseases that weaken the root flair and can lead to wind throw, breakage and death.
A general recommendation: plant 24”/25 gallon at least 1-2” above finished grade; 36”/45 gallon, at least 2-3”; and 48” at least 3-4” above finished grade. These heights offer a margin of error should the rootball shrink over time (the result of decomposition of organic matter in the planting or backfill mix) or soil settling at the bottom of the planting hole.
Trees planted in lawn areas are exposed to the greatest number of risk factors. Risks associated with string trimmers and mowing equipment is multiplied by the fact that they are in the landscape, at least, once a week. The effects of even modest damage, repeated week after week, can be devastating on long term tree health. Creating a turf free area around the root flair can act as an equipment buffer making mowing and trimming near the trunk unnecessary. Planting shrubs, annual color or ground covers around the base will also keep the trimmers at a safe distance. Under-story plantings, beneath desert species, should have watering requirements that are compatible with those of the tree and won’t create over-watering problems.
Another design option is to aggregate desert trees in decomposed granite areas slightly away from turf or trunks can be protected with commercially available, hard plastic guards that wrap around the base of the tree. The problem with these guards is that they may make maintenance workers less careful when working around trees by giving the impression that trunks are “protected” and cannot be damaged. Also, these guards are not suited for large or multiple trunked specimens.
With care and planning, desert adapted trees can be used in a wide range of landscape applications. They can be expected to continue add shade, color and character to landscapes for the long term.
