Summer Transition and Tree Establishment
Anyone who has endured the often abrupt transition from spring to summer in the desert southwest knows it can be brutal and unforgiving for trees and shrubs. To attempt to address these challenges once they have appeared risks severe damage to plants that are not easily reversed. Preparing trees for the high heat and low humidity of desert summers begins in the landscape design phase and tree installation. Water applied to soil will follow one of three fates. Gravity and other physical forces in the soil eventually pull some water to depths out of the reach of plant roots. Another portion evaporates from the surface and just slightly below the surface of the soil. The third and most important fraction is for water to be absorbed by plant roots. Proper design of the water delivery system, followed by careful, site specific irrigation scheduling are a critical part of tree establishment and long term, vigorous tree growth and development.
The depth and distribution of irrigation water dictate the depth and distribution of plant roots. Roots only grow into moist soils. If the distribution area of irrigation water is limited, even in the most ideal soils, the root system will be similarly limited. A single, point source of water, like that delivered by a drip emitter, produces a cone-shaped profile of wet soil with the point of the cone being directly beneath the emitter. The distribution of emitters then will dictate the distribution of water and directly influence rooting. The design and installation of traditional irrigation systems has favored economics over horticultural considerations. All too often irrigation systems are either designed or installed as if the tree is already established and at its mature size when transplanted, with emitters placed near the trunk or towards the outer edge of the original rootball. In these situations what appears to be adequate above ground growth can be achieved by simply running such a system for long periods of time. The long-term consequence will be trees that have large canopies, stout trunks and very small root systems. These trees are highly prone to wind throw or vulnerable to root rotting diseases.
Ideally the irrigation system design should allow expansion outward as the canopy and root system grows and matures. Tree roots can extend from 1 1/2 to four times the diameter of the tree canopy. It’s obvious that as the tree grows the irrigation system should be expanded to deliver water over the ever-expanding root zone. Extending the irrigation system beyond the original rootball should begin at installation and be monitored through establishment and maturity. Shortly after transplanting, emitters closest to the trunk should be closed or capped and those near the drip line should remain open. The capping of these emitters allows the soil nearest the trunk to dry out and firm up, helping to avoid wind throw. The emitters watering the soil profile near and beyond the drip line will be irrigating the newly developing feeder and anchoring roots. This management approach encourages greater root distribution, a larger total root mass, and a healthier root system that is physically proportional to the trees above ground trunk, branch and foliage mass.
Irrigation schedules must be responsive to the water needs of the plant as influenced by the plant's stage of development, air and soil temperature, relative humidity, other available sources of water (rainfall, site drainage, subsurface water) and the nature and structure of the soil. Another consideration is surrounding hardscape elements such as walls, planter boxes, parking lots and sidewalks that can shade or store and reflect the suns intense rays. With these factors in mind it’s clear that irrigation scheduling is a site specific and, in many instances, a plant species specific tack.
It is unlikely that even the most sophisticated irrigation system can precisely meet the exact need of all the plants in a typical desert landscape. A reasonable goal when scheduling should be to replenish lost or absorbed water at a frequency that allows the root profile to proceed through a cycle from saturated (immediately following the application of water) to slightly dry soil (when most soil moisture has either drained through or been taken up by the plant). The length of this cycle, or the interval between irrigations, will be determined by the plant species present, their stage of maturity and the water retention characteristics of the soil.
While tree staking is not typically recommended, in instances where tree transplanting/establishment occurs during the summer monsoon and Haboob season, securing trees with stakes may prove prudent. In this, as in all other landscape applications, tree staking should be used only in very isolated circumstances and even then only for short periods of time. Planting detail should specify the removal of tree stakes at the conclusion of the summer monsoon.
