Water is often the most limiting growth requirement for trees and shrubs in the landscape. Trees and shrubs are mostly water, which is a major part of its cells, is used in chemical reactions and photosynthesis, and aids in the movement of materials throughout the plant. However, most water is lost from the plant through transpiration from pores on the leaf surfaces. As the water is lost through transpiration, it creates the pull necessary to move the water from the roots to the leaves. Water is also essential to cool the plant, as it is lost from the leaf surfaces.
In Iowa, during the establishment period of one to two years, supplemental watering during periods of drought and high temperatures will increase survival and improve tree health and growth. Drought conditions can lead to tree decline, pest problems and in some cases damages so severe that the plant will die. In addition, excessive water can cause very similar problems with tree growth and survival. Tree roots require oxygen to survive and excess water can cause their decline and death, resulting in tree mortality.
When first planted, trees often have insufficient root systems or at least their roots are not expansive enough to gather enough water for good growth and survival. Tree roots mostly occupy the upper 16-20 inches of soil; this is the region to add supplemental water. Ideally, watering should begin when this soil region becomes dry and can no longer supply the needed water to the plant. One can also used the “hand” method to determine moisture condition of the soil. Simply feel a handful of soil from a depth of 6-12 inches to determine its moisture level. In Iowa, an average time frame is to water new trees every 7 to 14 days if less than one inch of rainfall is received during the same time period. Sand soils dry out faster than clay soils. As the root systems expand, the trees will require less frequent watering. By the end of the first growing seasons, most trees and shrubs should have sufficient root systems to survive without supplemental watering. Supplemental watering may be required for very large planting stock and for newer plantings during periods of drought during the second year of establishment.
When watering is necessary for new plantings, use enough water to soak the entire root system to that root depth of 20 inches or more. The best watering technique is to use soaker or drip systems, ensuring that applied water enters the soils rather than running off. Surface soaking allows the tree more chances to absorb water and helps maintain nutrient cycling by moving nutrients down to the root system. The use of wands to deep soil water is not as efficient as watering from the top and often results in incomplete watering. It is important not to over water with respect to frequency. Allow the root system to dry out moderately to stimulate root expansion and growth. Use 3 to 4 inches of organic mulches to help conserve water.
Turf watering and tree watering are sometimes in conflict. Proper turf watering is too frequent and usually insufficient to penetrate down to the tree’s root system. This pattern may encourage the tree to develop a shallow, ineffective root system. Where possible, strive to isolate the tree roots away from turf watering system or when planning the irrigation system, have a separate zone for the trees.
In some cases, especially on clay soils, frequent turf irrigation will keep the upper root zone of the tree so wet, that roots suffer mortality and the trees decline and die. Trees perform best when their root system is soaked, either naturally or with irrigation and then allowed to dry significantly before receiving more water.