Written by L Peter Macdonagh and published on https://www.deeproot.com/.

When you plant new trees in your yard, it very important to give the young trees excellent cultural care. Watering a newly transplanted tree is one of the most important tasks. But gardeners have questions about how best to do this: When should I water new trees?

How much to water a new 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 to be 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 previous 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 is 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.

Original post here https://www.deeproot.com/blog/blog-entries/how-much-should-you-water-your-tree.