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Monday, August 31, 2015

Converting to Desert Landscapes Can Damage Existing Trees

Q. We removed half of our lawn with a 15 year old Chilean Mesquite in the middle which has done very well up to this point. Besides water from the lawn it had its own water supply located near the trunk. During grass removal, roots were chopped and six small plants with drip emitters in the rock mulch surrounding the tree. Will the tree be okay now that the front half sits in rock with only the plant emitters providing water.

A. The short answer is it will not. You need to supply more water to this tree or it will begin to drop its leaves and the branches will begin to die back.
Mesquite roots growing deep for water.
            Now the long answer. Chilean Mesquite is among a group of plants, called phreatophytes, which have the potential to develop a very deep root system when growing in the wild along arroyos. In the case of mesquite, 200 feet or more. This is if the tree is in the right location together with deep, infrequent rains that help establish roots to that depth. Arroyos, or desert gullies, concentrate rainwater in one location pushing water to great depths with the roots of these plants not far behind.
            Phreatophytes like mesquite when grown with water that is applied frequently grow rapidly, vigorously with a very dense canopy. In many home situations, trees do not develop deep roots because the water supplied to them, such as your lawn, is applied only to the surface few inches.
            During 15 years of growth, the vast majority of roots will grow in a “mesh” 12 to 18 inches just below the lawn. The six irrigated plants planted under the canopy will help somewhat but not enough. Removal of tree roots also reduces generally speaking, most trees can lose as much as 50% of their roots and still recover provided they get adequate amounts of water.
            My hunch is your mesquite will start dropping leaves at the onset of hot weather and you will see limb death in the canopy. The roots will try to reestablish themselves wherever they can find water but the canopy will die back because of root loss and inadequate amounts of water.
Tree dieback after converting from lawn to desert or rock landscape.
            What should you do? During this hot weather you should put a hose out there and irrigate the rock beneath the tree about once a week during hot weather. This is a stopgap measure.
            You might consider installing a “bubbler and basin” around the tree in the future to provide more water. Use an irrigation valve previously for the lawn for the water source to bubblers. An irrigation bubbler is installed 2 feet from the trunk. If this basin is quite large, two bubblers located in this basin might be needed to fill it. Each time you irrigate, fill the basin.
Mesquite blown over because shallow rooted due to lawn and flower bed
            It is important that the basin constructed is level and wide enough to lie on top of about half of the area under the canopy of the tree. A level basin, or berm, is built around the trunk approximately 3 to 4 inches high of the tree with the trunk at its center.
            The bubbler is a type of emitter that pushes out usually 1 to 2 gallons a minute. Drip emitters emit gallons in hours, not minutes, so this is a large amount of water applied in one spot in a very short. Of time. This is why the basin or berm is needed.
            If the tree is on a slope, then install the basin around the trees so that it is level. The water from the bubbler must flood the basin and be contained by the basin for it to work well. This may take 10 to 15 minutes with bubblers and anywhere from 20 to 30 gallons every time the tree is watered.
            In midsummer when it's hot this, watering might be once a week to every 10 days or so for desert trees like mesquite. Adjusting how often you water and how much is applied each time will determine how fast the tree grows and how dense the canopy is.
            If you begin to irrigate less often, but apply more water each time, you will slowly encourage the roots of desert trees like mesquite to grow deeper.


  1. Very interesting. We are planning on removing our lawn and changing over to desert landscaping but now I am wondering how the old, large Modesto Ash trees will fare from this change? Do you think it would be an issue? I tried to find out about their root system, it seems like it is shallow, but won't that be an issue as the roots will be damaged and then covered with hot rocks?

    1. I am not telling you not to convert to desert landscaping but be cognizant that established trees can get hurt in the process and many landscapers do not know how to convert from lawns to desert landscaping with existing large trees.

      If you have large trees in an established landscape you have some options.
      1. Leave the lawn surrounding the big trees and remove lawn where there are no big trees.
      2. Remove lawn and spiral in-line drip tubing around the existing trees out to a distance of their drip line (spread). However, if you do this you should put this drip tubing on a separate valve and run it longer and less often than drip going to other plants.
      3. Put LOTS of plants beneath these large trees and drip irrigate them to assist the existing trees with enough water. This is above and beyond having emitters for the large trees, too.
      4. Use a lawn irrigation valve to feed bubblers to existing trees and form a basin around the trees to capture water from the bubblers. This is called basin/bubbler irrigation and is a form of flood or border irrigation. The basins must be level and flat and be three to four inches thick.