   Landscape Tree Irrigation - August 2, 2000 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County I thought I had this topic covered in my May 31, 2000 column, but I am still seeing many trees and suffering from lack of water. You can blame it on an early spring and a warm dry winter or blame it on La Nina, but you can't resurrect the dead. The victims have mostly been pines, but Arizona cypress, mimosa, cottonwoods, and other various woody plants have also succumbed. It may be too late to save your trees this year, but for those that had near death experiences, read on. These weather conditions will occur again. Water stress symptoms are slow loss of leaves during the growing season from the bottom upward or from the inside outward. Decreased growth may also be evident. Pest and disease problems are also common on drought stressed trees. Trees may require as much as three times as much water in the summer as in winter. Deciduous trees (those that lose their leaves) require less water in winter. Evergreens do best when irrigated in winter as well as summer (those green leaves require water for photosynthesis). Large, fast growing species require large quantities of irrigation and should only be used in appropriate ways (near water or in sub-irrigated areas) or in parks where the public can benefit from the shade produced. Newly planted trees need more frequent irrigation initially to become established. Natives and drought-adapted species can be slowly acclimated to the planting site. These plants can survive drought but look best in a landscape when given some supplemental water. However, drought can stress and lead to death of native trees in the wild. Mulching under trees conserves water and cools the soil. Mulch also decreases water evaporation from the soil surface, yet allows it to infiltrate into the soil. It also prevents some weeds from becoming established. As large an area as possible should be mulched (the roots can extend up to three times the diameter of the canopy). Chipped woody material, leaves, gravel, landscape fabrics, or even cardboard can be used as mulch. The soil surface should not have plastic under the mulch material. This not only prevents water from soaking in, but also prevents oxygen from getting to the living roots. Basins are a common way of irrigating trees. Three to four inches of water can soak in and provide long lasting irrigation. This works fine in most cases, but water should be applied to soil above the root system and should not be concentrated at the base of the trunk. This can lead to butt rot. If this is occurring, then build a small berm just away from the trunk and a larger berm at the tree's drip line. Bubblers can be used to fill the basin, but the berm must be level to prevent overflow. Drip systems are also commonly used to irrigate trees. They are great for establishing trees, but must be expanded if the tree needs irrigation throughout its lifetime. They can also clog, so check them often. There are expandable manifold-type drip heads that can be expanded over time. Soaker hoses can accomplish the same task, they simpliy have to be moved from tree to tree. Sandy soils are better drained and require more frequent irrigation with smaller quantities of water. Clay soils are the opposite: requiring less frequent irrigation with larger quantities. Soil compaction will decrease the ability of water to penetrate the soil. Minimize traffic and do not park cars above the tree's root system. If construction projects are scheduled, fence the area above the existing tree roots. Overall, I recommend planting drought-adapted tree species, using mulch, and letting the trees acclimate to their site. Weeds should also be controlled to minimize competition for water. Apply the water in early morning or evening. Do not spray water on the plant leaves at night as it could promote fungal diseases. How's that for a cookbook approach! Finally, I am writing about irrigation to encourage rain. At the time I wrote this (one week prior), it was still dry as a bone in the Verde Valley. I hope it works. If not, plan a picnic or wash your car. It's bound to rain then. The University of Arizona Cooperative Extension has publications and information on drought-tolerant trees and landscape plants. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 or E-mail us at mgardener@kachina.net and be sure to include your address and phone number. The Yavapai County Cooperative Extension web site is http://ag.arizona.edu/yavapai/. |