Article 9 of 12 from Trees: Dispelling Myths & Misinformation, prepared by the arborists of the Southwest Division of Bartlett Tree Experts as a community education initiative.
Picture it: Your century-old, feature live oak in the back yard, with its classic, sweeping limbs, provides the gift of shade for the enchanting garden you’ve installed under the canopy. To avoid disturbing the support roots, you’ve built a charming retaining wall that encircles the trunk about four feet out, filled it with rich soil, and planted willowy aspidistra inside. Not to worry, your irrigation people have installed bubblers inside the planter and spray heads under the rest of the canopy to nourish the shade-loving tropicals that live in harmony with the esteemed, protective oak.
There’s a lot wrong with this picture. If you’ve been following along, you’ve already “unearthed” at least one problem: soil around the lower trunk and root collar. Even if the root collar is properly exposed, regular irrigation on and around the trunk is not a good idea. Surround it with soil and plantings and you have the perfect environment for limiting gas exchange, constricting the flow of water and nutrients, and attracting harmful fungi and other pests.
About those tropicals under the canopy: Are their watering needs the same as the feature oak that’s protecting them? And how were those spray heads installed to irrigate them? Let’s start with the second question. Automatic irrigation systems can be good for trees if their installation and watering schedule are planned with care. Property owners should not assume an irrigation contractor understands where tree roots are located; the crew might just cut a trench through the critical root zone (generally from the trunk to the drip line). Alternative methods exist to install irrigation lines that do not harm tree roots, and it may be prudent to have the irrigation contractor consult with your arborist.
The other danger from an irrigation system is overwatering. Often systems are designed for turfgrass without regard for nearby trees. Excessive moisture can limit oxygen, cause root rot, encourage shallow root growth, and over time threaten the tree’s stability. Adjusting the schedule to provide a deep, slow watering less frequently can accommodate the moisture needs of trees and turf. About those tropicals luxuriating in “harmony” with the tree, see if your arborist advises canopy cleaning and thinning to let in some light. Then replace those plants with organic mulch. If you must, add a few carefully placed, drought tolerant perennials.
Now that we’re revisiting the root system, a discussion of soil structure and content is essential in understanding root health. Watch for our next article in this series.