There are several factors to consider for this basic and essential practice.
Irrigating, watering, soaking, moistening, flooding, hosing, wetting, spritzing, spraying…all place water in a tree’s root zone. Of all the maintenance practices in the green industry, irrigation has to be the most basic. Even so, it may not be the simplest. Many variables may be out of one’s control, and considerations may not be feasible in all situations. Success is achieved by bearing in mind all aspects of putting water where it is needed — how much and how often.
Goal for both
Very few trees grow well in saturated or very dry soils. Whether the tree in question is a newly established specimen or one that has graced the landscape for many years, several factors are appropriate for both. The first is siting, the art of placing tree species where they will grow best or at least tolerate the conditions of the specific part of the landscape. Proper siting is one of the tried and true parts of the right plant, right place model.
The second is placing water near roots such that it is available as needed, but avoiding extremes. Moist, not soggy or dry, works for most; although some species tolerate wet sites (baldcypress, river birch, sycamore, swamp white oak, black gum, persimmon) and others (hornbeam, yellowwood, ginkgo, golden raintree, bur oak, Japanese pagoda tree, hackberry) accept dry sites quite well. The realization of these preferences (or at least tolerances) is helpful in overall maintenance of a property.
Growers commonly misunderstand that even species that tolerate dry soils still need adequate moisture in the establishment phase for good root development. Otherwise, the roots simply do not establish and the tree fails … not from a lack of water — from a lack of roots. Once a tree is established, the dry or wet preferences take on greater importance. Determining what promotes healthy tree growth in the first year or two compared to maturity is imperative.
Devices for new
To ensure that newly expanding tree roots receive sufficient water for growth, a multitude of devices have been created. The ones that work the best are those that deliver consistent, predictable flow with minimal problems with clogging and uneven distribution. Regardless of whether it’s a hand-held hose sprinkler or a sophisticated drip-line connected to an in-ground water sensor, newly expanding roots and root hairs must stay moist.
Certainly, some devices are more efficient than others. As with the culture of vegetables, where wetting areas without crop roots is undesirable (it’s wasteful and promotes weeds that compete for sun, water and nutrients), a desirable goal is to moisten the soil containing new tree roots as they expand laterally, stopping a couple of inches beyond the furthest outward root. Soaker hoses, drip emitters and water bags can be efficient if used properly and adjusted as needed. Hose-end sprinklers and most lawn-watering equipment also can place water where needed, but, unless turfgrass also is present, they tend to be wasteful and inefficient.
Devices for old
Once established, a great majority of trees can get by on natural rainfall, especially if well-sited and adapted for the region where they have been planted. But, in dry years, supplemental devices can be utilized to provide what Mother Nature hasn’t. A common device for this purpose is a quick coupler for hoses and above-ground sprinklers that can be adapted to meet temporary needs. When considering well-established specimens, the most important consideration is the presence of turfgrass surrounding the tree. If a tree is in the middle of a well-irrigated lawn or turf area, it’s likely that adequate soil moisture will be present. Probing the soil in various points under the canopy, drip line and beyond will help to get a handle on how moist the soil is at any given time. This should be done periodically during the growing season, as turf maintenance schedules can vary with the owner.
Moisture delivery to turf/tree landscapes should be monitored routinely through the use of catch-can devices that measure the output of the irrigation system at the ground level. Natural rainfall also should be taken into account when considering if supplemental water should be applied for established trees.
Considerations for both
Mulch: Mulch is that ever-present, no-brainer material to apply after installing new trees and around established trees for the benefits of suppressing weeds, keeping the roots cooler and retaining moisture. The caveat with mulch is that these benefits should be preceded with “in general, mulch will provide…,” not “always will provide.” It’s important to periodically check the condition and functioning of the mulch from time to time, as the depth and composition will add benefit or cause problems. Usually, a little (2 to 3 inches) of a quality wood chip mulch holds in moisture and slowly decays, returning nutrients to the soil solution. However, too much (4 to 8 inches) can either congeal and act like a thatch roof and repel water or hold in too much and promote decay of the root flare. Too much mulch on the base of the tree also can create a good habitat for mice and other rodents.
Location: Location, location, location was the cry of the 1970’s real estate market, and even rings true today. In relation to the tree care industry, most trees need 900 to 1,000 square feet of pervious surface over the root zone for healthy root growth, supported by adequate water infiltration and air penetration. Knowing the size and location of the root zone is crucial to understanding and treating the area of active water uptake.
Monitoring: The best equipment in the world is wasted if it is not used correctly. Frequent checking is necessary to see if the emitters are clogged, broken, running too long/not long enough or delivering water to off-target sites such as driveways and parking lots. Regular adjustment of tree irrigation systems is a valuable and prudent service to offer clients, one that helps to meet basic needs of the plant and puts money in the company’s checkbook. Adjustments can be simple or complex, involving replacement of worn parts and changes in design and distribution pattern.
Competition with turf: In many landscapes, trees are co-located with turf. As such, they often share the space for rooting as well as an irrigation system. Since turf gets the first crack at it, trees have to get by on what moves down below the turf roots, which is just fine, as most trees only require about a third as much as turf does.
The key to understanding the influence of the turf competition is a keen awareness of the proximity and interconnectedness of the turf and tree roots and that in order to grow well, each needs its share. In addition to monitoring for efficiency and good working order of the delivery system, it’s wise to probe the soil routinely to check for the soil moisture level. Long screwdrivers, stiff rebar rods and cut-off golf clubs are good for this purpose.