Trees perform best with infrequent, deep watering, and can suffer from the frequent short-duration irrigation that is commonly applied to turf. Frequent irrigation encourages shallow rooting patterns in trees. Shallow roots will not compete well with turf for the applied water, while irrigating deep into the soil profile will cause tree roots to grow deeper, below the level where turf will outcompete trees for water and nutrients. Competition from turf due to shallow watering will lead to nutrient deficiencies and even moisture stress, despite regular watering. Competition from turf is also one of the most common reasons newly planted trees fail to establish in a landscape.
Shallow rooted trees are also less firmly anchored. Many whole-tree failures are the result of shallow root systems that cannot support the tree under windy conditions, particularly in saturated soils during or following heavy rain events. Irrigation patterns that encourage deeper root growth lead to a more stable root system, where roots are growing in, and anchored into, a larger volume of soil.
Another common cause of tree decline is over-watering, which harms trees directly and can lead to root disease or decay. Oxygen is necessary for root growth, and when soils remain saturated for long periods due to overwatering or poor drainage, soils become anoxic and roots die. Dead roots are a common entry point for decay organisms, which can progress into the structural roots and lead to tree failure due to root and basal trunk decay. This is one of the most common modes of tree failure, and is one of the most dangerous due to the difficulty in assessing the structural integrity of roots growing below the soil.
In addition to harmful anaerobic conditions and decay, saturated soils are ideal for the development of one of the most common diseases of plants, Phytophthora root disease. There are many species of pathogens in the genus Phytophthora, and dozens of these species cause root disease in woody plants. Phytophthora is a fungus-like organism, but is not a true fungus. One major difference between Phytophthora pathogens and true fungi is that Phytophthora species are very dependent on saturated soils in order to cause disease. Referred to as “water-moulds” in the older texts, Phytophthora species produce spores when soils are saturated that literally swim through the soil water before infecting roots. In addition, roots that are stressed from saturated conditions will “leak” carbohydrates and amino acids that help attract the spores of Phytophthora to them. If soils are not saturated, this type of disease is rarely a problem.
Another irrigation issue that can cause problems for trees relates to emitter type and direction of spray. It is very common in landscapes to see irrigation water spraying directly against the trunks of trees, and this can cause a number of problems. Direct spray against trunks over the long term can physically damage bark and underlying tissues, impairing the movement of internal water, nutrients, and carbohydrates. In addition, death of the underlying tissue will eventually lead to sloughing or peeling of bark, opening another entry point for decay fungi.
Irrigation needs of trees also change over time. Irrigation timing and location of emitters that might be appropriate for a newly planted tree can lead to overwatering once the tree has established a root system outside the original planting hole. Years later, as the tree grows and water needs increase, this same irrigation may become insufficient. In addition, emitters should be moved further and further from the trunk as the tree grows in order to apply water where the densest root growth occurs. A final problem with outdated or overgrown irrigation systems occurs when trunk diameter growth causes a physical conflict between the trunk and the irrigation system, leading to leaks in the system or girdling of the stem.
Water quality, particularly when using reclaimed or recycled water, is another common problem for trees. Trees are often far more sensitive to salts and other dissolved solids than turf species. Use of reclaimed water is desirable from many other perspectives, and should not be discouraged. The potential negative impacts of reclaimed water on tree health are dramatically reduced when irrigation is applied at a low frequency and high volume that is appropriate for trees. This allows for leaching of harmful water contents rather than concentration of these salts in the upper soil.
All of the issues mentioned so far can cause stress and lead to slow decline of trees, but they also can attract pests that will lead toward much more rapid tree mortality. Many insect pests of trees, including bark beetles, wood borers, and ambrosia beetles, are more attracted to trees under stress. Stressed trees release chemicals, commonly gaseous alcohols, which attract these insects. In fact, recent research has shown that trees growing in saturated soil conditions for just a few hours are far more likely to be attacked by boring insects compared to those growing in non-saturated conditions.
The best solution for many of these problems is to create separate irrigation zones for trees and turf. Irrigation zones for trees should run very infrequently, often only once or twice a month depending on location, for durations that allow moisture to reach 30-45-centimetre soil depth. The exact amount of time and water will depend on soil texture, weather, tree species, and many other factors. In addition to leading to healthier and longer-lived trees, removing trees from the typical turf irrigation pattern will save significant amounts of water.
Another great way to improve overall tree condition in landscapes is to create mulch rings around trees. Organic mulch, particularly fresh arborist wood-chip mulch, is one of the simplest and best soil treatments for trees for many reasons, several of which are related to irrigation. In addition to removing competition from turf in the critical root zone of trees, mulch helps to reduce evaporation, and moderates soil temperatures. All of these factors will reduce the overall need for supplemental irrigation, thus reducing the potential for excess irrigation. Mulch also has the side benefits of acting as a slow-release nitrogen source, protecting root zones from compaction, and protecting tree trunks and surface roots from physical damage often caused by turf maintenance equipment. As mulch slowly decomposes, it adds organic matter deeper into the soil profile, which improves soils’ structure and drainage.
Overall, it is possible to create landscapes where both trees and turf can survive and thrive. Just remember that trees and grasses are very different plants with different needs. When irrigation design only accounts for one plant type, the other will likely suffer.
Dr. Drew Zwart is a plant pathologist and physiologist with the Bartlett Tree Research Laboratories, and provides research and technical support for the Bartlett Tree Expert Company in Canada.