By Susan Krzywicki.
Root zone irrigation for trees has been a staple of conventional gardening, and this technique is being used to mimic nature in order to create optimal conditions for oaks. Two proponents of this method are Don Rideout, active member in the San Diego Chapter of the California Native Plant Society and former member of the state-wide committee for California Native Plant Week, and Kay Stewart, former board chair of the San Diego Chapter of the California Native Plant Society and local landscape architect. They're both well-versed in the whys, whens, and hows of using water tubes (sometimes called water pipes) for deep root zone irrigation of oak trees.
Theory Versus Practice
Kay describes how and when oaks naturally take in water to set up an understanding of how the water pipes emulate nature. She says, “California’s evergreen oaks grow where surface water from rain arrives in cool weather, and lots of water is available year round deep in the soil of canyons or alluvial fans below canyons. Oaks draw that deep water up to the surface during the hot seasons so the surface roots can do their magic tricks that require oxygen." As such, oaks do well when they can regulate the amount of water available to surface roots, and they are otherwise susceptible to fungi in gardens that are irrigated multiple times per week in the summer. Kay notes that a proper oak watering regimen includes surface irrigation two or three times per month, allowing time for the soil to dry out in between, and should use water tubes for consistent deep water delivery.
Regarding the purchase of water tubes, gardeners can take the DIY route and fashion water tubes out of PVC pipe with holes poked into it or purchase pre-fabricated stakes with holes and mesh filters to keep dirt out of the shaft.
As for setup, the pipes need to be within reach of a hose that can run into them for each irrigation session. Kay adds, "They need to be placed initially no more than 12 inches from rootball of the tree, far enough away to allow the trunk to expand." The pipes should be incrementally moved away from the trunk. A sure sign that a tree has found groundwater is if, after slowly growing for several years, it begins to grow much faster. At this point, the watering tubes may no longer be necessary.
Don’s Do It Yourself
Before installing water tubes to irrigate his oaks, Don observed that the trees dropped twenty to thirty percent more of their leaves in late summer. What appeared at the time to be drought stress was visibly thinning out the canopy. When Don first installed the tubes, he filled them as often as twice per day, but now that the trees are healthier, the tubes are only filled every other day.
About this new approach to watering his oaks, Don says, "This system is like injecting medicine right into the blood stream, versus an oral medication that must be digested. The key is that it mimics the situation where oaks normally thrive. Oaks seem be happiest when they can touch water about 24 inches deep, as if they were situated on a river bank. Notice that oaks tend to locate twenty to fifty feet from a water source and you can see what we are re-creating.”
Considering that a small amount of water is being delivered per tree using water tubes (around four gallons every other day), it makes a big impact. It is a great technique, as the water is delivered deeper than the warm top layer of soil where pathogens are more frequently found. Don finds he has a healthier oak and the canopy stays denser over the late summer period. Stressed trees are vulnerable trees, so this may help gardeners to maintain specimen oaks even when the next drought rolls around.
Susan Krzywicki is a native plant landscape designer in San Diego. She has been the first Horticulture Program Director for the California Native Plant Society, as well as chair of the San Diego Surfrider Foundation .