How Much Should You Water Your Tree?

Healthy trees can grow anywhere, including cities, provided they receive enough water, soil, and sunlight. Research shows that vigorous urban trees keep people healthier, cool cities in summer, warm homes in winter, help kids learn better, decrease car accident rates, raise real estate values, and decrease crime dramatically. Even with these myriad benefits, recent heat and droughts in many parts of North America (and elsewhere) raise an uncomfortable but necessary question: how do you maintain a healthy tree when there is a shortage of water?

San Antonio, TX is an interesting place to look for an answer to this question, because the San Antonio Water System (SAWS) has higher current water needs than what’s available in its system. As a result, they have had to start purchasing water outside of their regional watershed, well-shed, and river-shed – and this has driven then to develop an efficient watering system that enables them to give trees what they need while minimizing the economic and environmental burden of purchasing water.

Mark Peterson, formerly of the Texas Forest Service and now with the San Antonio Water Department, was tasked with creating watering guidelines that would provide enough water for young trees to survive and grow, but not use any more water than absolutely necessary. Mark’s approach is what I’ll be sharing here.

Simple, But Not Easy

No matter how drought tolerant, native, or local a tree species is, almost all young to trees (typically 1 to 3 years old, or up to 5 years in Type I, Type II and especially arid regions) in man-made landscapes must be watered by people during the summer to survive and become established.  The complete extent of young tree roots in the first few years after planting is limited to the soil volume that the tree was last grown in (for example, a pot or container). Mature, established trees generally require less consistent care, but during droughts every tree must be monitored and watered adjusted accordingly.

If you are caring for young, recently planted trees, here are some good rules of thumb to follow (your mileage may vary depending on climate and tree species). Here is Mark’s watering regimen for newly planted trees.

Watering as a Science

Year Amount Frequency
YEAR 1
First month of planting Trunks smaller than 2” (5 cm):  1 gallon per inch of trunk diameter.

Trunks larger than 2” (5cm): 2 gallons per inch of trunk diameter.

Water three (3) times a week over the root ball.

 

Second month of planting Trunks smaller than 2” (5 cm):  1 gallon per inch of trunk diameter.

Trunks larger than 2” (5cm): 2 gallons per inch of trunk diameter.

Water two (2) times a week over the root ball.
Third month of planting Trunks smaller than 2” (5 cm):  1 gallon per inch of trunk diameter.

Trunks larger than 2” (5cm): 2 gallons per inch of trunk diameter.

Water once (1) per week over the root ball.

 

Fourth to ninth month of planting Trunks smaller than 2” (5 cm):  1 gallon per inch of trunk diameter.

Trunks larger than 2” (5cm): 2 gallons per inch of trunk diameter.

Water twice per month over the root ball.

 

YEAR 2
Hottest months Trunks smaller than 2” (5 cm):  1 gallon per inch of trunk diameter.

Trunks larger than 2” (5cm): 2 gallons per inch of trunk diameter.

Water twice per month over the root ball only. During a drought, water once weekly.
Cooler months Monitor and respond
YEAR 3
Hottest months Trunks smaller than 2” (5 cm):  1 gallon per inch of trunk diameter.

Trunks larger than 2” (5cm): 2 gallons per inch of trunk diameter.

Water twice per month, twice the width of the root ball. During a drought, water once weekly.
Cooler months Monitor and respond
YEAR 4
Hottest months Trunks smaller than 2” (5 cm):  1 gallon per inch of trunk diameter.

Trunks larger than 2” (5cm): 2 gallons per inch of trunk diameter.

Water twice per month, twice the width of the root ball. During a drought, water once weekly.
Cooler months Monitor and respond
YEAR 5
Hottest months Trunks smaller than 2” (5 cm):  1 gallon per inch of trunk diameter.

Trunks larger than 2” (5cm): 2 gallons per inch of trunk diameter.

Water twice per month, twice the width of the root ball. During a drought, water once weekly.
Cooler months Monitor and respond

 

For young trees, water the roots around the trunk (not the trunk itself, and not the area outside the root ball). I also recommend creating and maintaining a 3-foot wide, 1” to 3” (2.5 cm to 7.5 cm) deep organic (wood chip) mulch ring around the trunk for its entire life, to help maintain soil moisture.

For mature trees (>25 years), or those with a trunk more than 12″ (30 cm) in diameter, water deep and occasionally. About 10 gallons per 1 inch (2.5 cm) of trunk diameter per week (ex., a tree with 12″ DBH would receive 120 gallons) during drought. If there is unlimited water, there are records of trees absorbing 150 gallons of water in a single day.

Watering as an Art

In addition to the (human-driven) watering recommendations described above, there are environmental and design decisions that can set trees in the built environment on a more secure course for getting their irrigation needs met.

Select tree species that, over the long term in typical summer weather (not droughts), won’t require supplemental watering.

The urban landscape is full of small humps, bumps, and pimples that don’t serve to gather and contain water runoff. By thoughtfully altering these forms via slopes, pipes, and berms, we can turn the entire pervious landscape into a tool for draining water to tree planting areas.  This would be a paradigm change for watering trees and managing stormwater worth billions of dollars, and billions of gallons of water, nationwide.

All trees need water during droughts. DeepRoot published some thoughts about this last summer that are worth re-reading. Trees that have access to larger volumes of loamy soil will be able to withstand dry periods better because of the water reserves the soil can contain (remember that sandy soils will drain quickly and require more frequent irrigation).  Evergreens need heavy watering going into the winter, and need watering during winter droughts.

Sometimes annuals or bulbs can look nice planted under a tree. But the tree is paying a price in root damage (caused by planting and removing flowers) and water competition for that temporary beauty. After tree establishment, I do not plant anything under trees within 10 feet of the trunk.

Watering Tools

There are a great number of available tools for watering trees depending on your needs, budget, and other site considerations.

Passive

  • Slow release watering bags (e.g. Gator Bags).
  • Rain leaders, or scuppers, can be directed towards tree trunks or below ground into the tree soil mass.
  • Flexible downspout extender can be directed towards tree trunks.
  • Clean 5 gallon bucket. Fill with hose and time speed of fill – this will tell you how many gallons per minute are being applied. A typical municipal fill = 5 gallons un 2-5 minutes
  • Rain barrels with flexible hoses attached.

Active

  • Automatic irrigation can be great for watering hard-to-get-to trees and can be set to run occasionally for long periods of time using drip, bubbler or soaker hose.
  • Harvest cisterns – sump pump.

It’s important, particularly with mature, established trees, to water the entirety of the soil volume, even the part under paving. If there is no automatic tree watering system (bubblers, drip), I suggest using a soil watering needle with a watering hose connected.

Timing

Effective tree watering always takes place relatively slowly. (For this reason, pop-up rotary sprinkler head systems for lawns, that only turn on for a few minutes a few to several  times a week, are not the best type of watering for trees). If you use automatic irrigation to water your trees, set them to run for much longer periods of time using drip, bubbler, or soaker hose.

Still not sure?

The above are just guidelines; you should use your own experience, common sense, and (if appropriate) input from a professional when applying these to your site. Some simple questions can help you assess how much and how frequently to water your trees. Think about the following as a place to get started:

  • Are the trees young and newly planted, or mature and established?
  • How much precipitation does the area receive? How intense and frequent are the storms?
  • How warm is the average daily|high temperature in the hot season?
  • How much soil are the trees planted in?
  • What type of soil are the trees planted in?
  • Are the trees growing in a street, median, parking lot, lawn?
  • What moisture conditions does the tree prefer?
  • How does water get into the tree opening?

If you’re wondering what trees do with all that water, on hot or windy days in the summer, a whopping 95 percent of the water that the tree consumes, when available, is turned into mist by the leaves (a process called evapotranspiration). The remaining 5 percent is used to photosynthesize to manufacture sugars for food.

 

Thanks to Mark Peterson at San Antonio Water System, Dr.  Edward (Ed) Gilman at the University of Florida, Dr. Gary Watson with the Morton Arboretum, Jim Urban with Urban Trees + Soils, and Colorado State University Extension.

L. Peter MacDonagh is the director of science + design at the Kestrel Design Group.

Henk Sijgers / CC BY-NC 2.0

5 comments

  1. As someone who deals with street trees and new trees in my own yard, I found this information very helpful–Thank you. I’m surprised though by what I consider to be the very low quantity of water–for a tree in its 4th year and less than 2″ diameter, “1 gallon per inch of trunk diameter twice per month, twice the width of the rootball”. For a 2″ dia. caliper tree that would equate to 2 gallons spread over 5.5 square feet!?

  2. We watered over 500 mature trees in City parks in the worst part of the 2011 drought and learned from experience. The volume and frequency of water recommended in this article is insufficient for young and mature trees, even for survival mode when water is scarce. The goal when watering is to provide a moist environment where roots can grow, the equivalent to 1 inch of rain. The volume of water to apply can be figured out by watering until the soil half the way between the trunk and the edge of the canopy (1/2 Critical Root Zone) for a mature tree, or in the rootball of a young tree, is moist 3 inches deep for survival mode, or deeper (typically 5 inches), up to 12 inches deep, depending on how much water is available. The frequency or time to water again is figured out in the same manner. Gently insert a hand trowel or screwdriver in the soil to feel the moisture. In Austin, TX, in average a mature tree needs to be watered slowly for 2-3 hrs. every 3 weeks when outdoor temperatures are 90C or hotter, and every 3-4 weeks when it’s cooler, provided that it hasn’t rained. A young tree needs to be watered twice weekly in Summer and once weekly when its cooler. In average, this amounts to 5 gallons per caliper inch up to 10 inch diameter. A mature tree needs 15 gallons per caliper inch. Note that a tree 15 inches in diameter or larger can’t be watered with a soaker hose because the soaker hose needs to be laid on a spiral pattern on the entire outer half of the root zone to water this area evenly. Only 200-300 ft. of soaker hose can be connected due to the pressure drop from the holes. The best way to water a mature tree is with drip tube laid on a spiral pattern on the outer half of the root one, or with an oscillating sprinkler set at medium. Inserting tubes in the root zone is dangerous (could poke a hole in a root) and water only travels 12 or 18 inches horizontally depending on soil type, so many tubes have to be inserted or the same tube has to be inserted repeatedly. Directing water from roof spouts and the landscape to the tree is OK as long as the water is not allowed to pond at the base of the tree.

    • “The simple method for homeowners and professionals alike that I described is based on Ed Gilman and Gary Watson’s research and was modified by me for South Central conditions like San Antonio and Austin. It is intended for Tree Establishment only, when the only roots available to absorb moisture are in the root ball, and later, within the planting site. A minor note, I failed to emphasize to Peter that the total amount applied does not change over time but varies with events. For example, a 1.5” tree will be watered initially with a total of 4.5 gallons per week (3 events) and the same at the end (1 event); thus, mimicking the concept of “frequent but light, changing to infrequent and deep”.

      Mr. Fossum is correct that mature tree watering is done by square feet of root area. However, with limited water and logistics, water must be applied at the most effective root area, that is, the tertiary roots. This area is at the canopy drip line and beyond. For mature, well-established, native species, my recommendation is 1 ¼ inches per month when necessary. This accepts historic rainfall (approximately 2 inches during July and August) and assumes soil depth of 6” of clay loam soil , the predominate soil in the area. If necessary, additional events may be added to reach historic norms.

      Finally, I must respectfully disagree with Mr. Fossum’s description of the lateral movement of water within the soil. Lateral movement may be only 18 – 24 diameter inches in a sandy loam but will be 36-48 diameter inches in a clay loam. See Irrigation Association and the ISA Certification Study Guide. Knowing the lateral movement allows for more effective watering and less logistics.”

      Mark A. Peterson
      Conservation Project Coordinator
      San Antonio Water System

  3. Had professionally planted one 12′ spruce and one 10′ ConFir the same day. Heavy root balls on both. Watered both per instructions.
    ConFir began to die in 3 weeks. Spruce doing fine. It’s been 5 weeks and ConFir has bit the dust. Spruce doing fine. “Tree Farm” says I probably over watered. Don’t think so. So why does one tree die and the other do just fine? They are planted 20 feet apart in same soil bed.
    Source not willing to make it right. Any advice?

Leave a Reply

Your email address will not be published. Required fields are marked *