In a perfect world, the tree watering fairy would make a stop at each customer’s property once a week, insert probes into the soil to determine the level of soil moisture, lay out some soaker hoses and flip the switch to deliver the right amount of water if it were too dry and pass on by if the moisture content was “just right.”
In the real world, Mother Nature often delivers either too much water or too little, causing trees to suffer. As arborists, we can try to smooth things out by supplementing or reducing, but it’s often easier said than done.
Basic tenet, basic problem
Speaking of Mother Nature, let’s take a step back and take a look at how things work in the natural world.
In landscapes designed by nature, there are trees in groups (such as woodlands or forests) and grasses in fields (or grasslands). In most natural landscapes, trees and turf are not together. Taking the cue from Mother Nature, we would be wise to separate trees and turf.
Save for a few exceptions, most broadleaf and evergreen trees require less water than turf that is maintained under medium or moderate to high levels of input. As a result, if trees and turf are growing together, it’s common for the trees to receive too much water. The same potential exists for lawns that are grown under low-maintenance conditions, although, in these situations, the need for water for trees and turf is usually more closely aligned.
Zealous property owners
If you spend $3,000 on a turf irrigation system, you want to use it. Not only do you want to use it, you want to see it run. So, in addition to the early morning, two to three times per week, half-inch to one inch per week in the absence of natural rainfall watering that university extension specialists and educators recommend, many property owners enjoy coming home from work, flipping the switch on the irrigation controller, pulling the tab on a cool one and delighting in sitting on their deck to experience the relaxing whoosh, whoosh, whoosh of water pulsing from turf irrigation heads.
In addition to the negative enthusiasm generated by watching water splash on the turf and trees, a very high majority of turf irrigation systems fail to deliver water evenly across the turf/groundcover/perennial flower surface. Why? Just like any other mechanical system, they are full of moving parts, parts that break or fail to function as designed.
The most common ones are heads that don’t rise completely above the surface, but there are many, many others – such as heads that spray the street instead of the lawn, tree roots that grow around the supply lines, bent risers, pressure valves that deliver too much or too little, cracks and leaks in pipe joints and mismatched heads within the same zone. All of these and more add up to inefficiency. The average residential irrigation system is about 60 to 65 percent efficient. Golf and sports turf fare much better, in the 75 to 85 percent range. In addition to systems running too long or too short, they over or under apply, adding insult to injury.
Over-irrigation has consequences. The central concern is a slow and steady decline in trees’ health, visible in yellow foliage, limp foliage, thin foliage, softened root and shoot tissue and slow growth. These symptoms are often observed on trees with other causal agents such as compacted soils, ripped/torn roots and fill soil placed over the established grade; however, improper irrigation often exacerbates the effects of these influencers.
With these phenomena in mind, it pays to ask the question: “What does an over- or under-watered tree look like?” Generally, it just looks sick: low in vigor, lackluster, pale and unthrifty. As ISA instructor Skip Kincaid says, it is “lacking vitality.” As such, it becomes a sort of nebulous construct, and often is made more tangible by taking a closer look at evident and demonstrative causes, both biotic and abiotic.
How to fix the problem
Fortunately, there are several strategies that can greatly reduce the negative influences of turf irrigation effects on trees.
First, the tried and true concept of right plant, right place (RPRP). Most arborists are used to hearing “right tree, right place,” but RPRP broadens it a bit to include all plants in the landscape. In terms of irrigation, the central goal is to identify the specific water needs of each plant grouping and install plants with similar needs together.
Not only is RPRP and separation of turf and ornamentals good for plant health, it’s good landscape design. In a design sense, separation of turf and ornamentals takes on an aesthetic construct, with the term “mass/void” commonly used to describe where masses of plants (often lawns) are placed adjacent to showy ornamentals such as trees, shrubs and perennial flowers.
In many cases, if not most, arborists find themselves in finished, thoroughly established landscapes, where initial landscape designs are long gone and forgotten. In these scenarios, encourage property owners to retrofit bad designs into ones that support tree health. Specifically, to remove sickly trees in the midst of a solid mass of turf, to remove thin turf under trees and replace it with either mulch or shade-tolerant groundcovers, to overhaul turf irrigation systems to meet the individual water needs of all plants in each zone and monitor and make adjustments as landscape plants grow over time.
One last way to minimize the effects of poor irrigation is to teach. In particular, to teach property owners the two basic rules of turf irrigation, which, if combined with the separation of turf and ornamentals, will go a long way toward achieving the goal of growing healthy trees.
First, water to the bottom of the roots — water should be applied in a manner such that the entire root zone is filled, where the air spaces between soil particles are temporarily filled with water. When this occurs, all plant roots are moistened, not just the ones on top. If only a portion of the root zone is watered, the part that is not will slough off due to desiccation. If water is applied to a depth greater than the deepest roots, water will be wasted; lesser amounts are not adequate. Both will result in root dieback.
The second guideline is very simple, to keep the roots moist. Much like the old bedtime story of the Three Bears, the goal is not too dry, not too wet, just right — moist. Probing the soil with a screwdriver or chisel will help determine the depth of the penetration of the water after application, as well as the relative moisture content of the soil.