As humans, we protect that which is valuable. Trees have to be near the top of the list — or, at least, some trees. Volunteer alleyway trees that grow into the power lines may be worth very little protection, while well-placed sturdy specimens that add value to the landscape in the form of all-season appeal, shade, disease resistance and desirable branch angle attachments deserve ramped up measures to ensure their longevity.
At a minimum
In order to succeed in the long term, most trees need a certain minimum surface and rooting area. This minimum space should be of the soil type suitable for the species in terms of drainage, pH, depth, porosity and fertility. In this regard, the question commonly arises, “So, how much room do we need to allow for these trees?” The answer most certainly isn’t “whatever or wherever the drip line is.” It largely depends on the individual species.
When in doubt, follow the 1,000-square foot rule as suggested by forestry consultant Phil Pierce. The idea here is that, in order for a tree to thrive, most species will perform well if they simply have access to 1,000 square feet of unlimited surface area to absorb water and nutrients and to establish roots for stability.
The unfortunate part of this premise is that it only works well in the landscape design process, the first few years of establishment or in the replanting phase of a renovation. Far too often, clients inquire about what steps can be taken to improve the health of trees that are already growing in too little space or in poor soils, where there are few options.
In most situations, homeowners focus their concept of tree health on the visible, therefore aboveground portions of the tree. It’s quite easy to forget about the root system, which provides absorption of water and nutrients, nutrient storage and anchorage in the soil. The common misconception is that roots stay within the circumference of the branch spread (or drip line). As tree care professionals, it’s our job to inform the public that roots won’t stop at the drip line unless a physical or environmental barrier prevents them from continuing their expansion.
Tree roots are easily damaged. The roots of most ornamental trees, for example, grow in the first 1 to 2 feet of soil. The majority of the fine roots — the ones that absorb water and nutrients — are located in the upper foot.
Roots in colonies
When trees grow in native stands in forests, along river banks or at least in groves and masses, they tend to establish in colonies. In these settings, they are naturally close to one another and are interdependent, a sort of strength-in-numbers phenomenon. As such, their roots intertwine, their leaves and branches shade out competitors as they grow taller, with their canopies reaching for the sun.
In urban or suburban settings, it’s quite different. We’ve all seen the common developer/builder special “one of four possible species” plopped in the sea of turf in the front yard. Dealing with this unfortunate occurrence isn’t easy. Fostering the development of trees in these settings is difficult due to many factors, including the competition of turf plants, the tendency to overwater and fertilize the rooting space in favor of the turf and damage to the trunk from string trimmers and lawn mowers.
The best remedy or opportunity to change this unnatural urban environment is to redesign the landscape, separating turf from ornamentals, as least as best as can be done.
When trees are situated in the midst of turfgrasses, it’s wise to work collaboratively with a landscape designer or architect to reform the elements of the landscape to replicate Mother Nature instead of perpetuating the “turf and tree together” arrangement.
Unfortunately, tree roots are easily damaged. For the most part, the injuries are due to:
- the roots’ proximity to the soil surface, within 18 to 24 inches in most cases, and;
- a knowledge gap among most property owners in terms of root location, the effects of actions taken on root health, function and viability, long-term impact of root loss and possible alternatives to proposed actions.
A good rule of thumb for root loss for arborists and tree care workers is that, once 40 percent of a tree’s root mass is lost, the eventual death of the tree is likely. Possible preventive actions for various root zone injuries include:
Construction soil being piled on and over roots: Insist that construction soil not be placed on top of the soil surface inside and outside of drip line. Doing so is of key importance in protecting the root zone. Temporary soil placement of a month or less may be of lesser impact than longer-term projects.
Soil placed by homeowners in planter boxes or tree surrounds: Remove added soil and treated lumber configurations at the base of tree trunks, if possible.
Actual root removal: Many property owners feel that aboveground roots are ugly and annoying and want to cut them out for many reasons. For example: to improve the look by planting perennials and groundcovers underneath the tree, to avoid difficulty in mowing across them, to avoid a trip hazard (especially for older persons), etc. Plant trees with lesser surface-rooting tendencies, plant at the correct depth, replace turf with groundcovers that are planted between surface roots and discourage homeowners from removing visible roots.
Tearing and trenching: Utility crews often need to create trenches through root systems to install new gas, water and electric lines, severing roots in the process. Negotiations in the exact location of the lines can help to avoid damage, in some cases.
Herbicide damage: Lawn care operators and homeowners alike can damage roots, shoots and crowns of trees through the misapplication of weed control products for turf. Certain herbicides, such as Banvel, are known to be more likely to be absorbed by roots than others. If done with tact, consulting with persons with turfgrass weed control responsibility can be fruitful.