Healthy trees can grow in even the most challenging urban environments, provided they receive enough water, soil, sunlight, and appropriate care. Vigorous urban trees help keep people healthier, cool cities during the summer, reduce energy use, support wildlife, raise property values, and make communities more welcoming.

Yet heat and drought in many parts of North America and elsewhere raise an uncomfortable but necessary question: How do you maintain a healthy tree when water is limited?

San Antonio, Texas, offers an interesting example. The region’s hot climate and recurring drought make efficient landscape watering a priority. The San Antonio Water System has therefore developed practical guidance intended to give young trees enough water to survive and become established without using more water than necessary.

Mark Peterson, formerly of the Texas Forest Service, helped develop the watering approach shared here. Although it was created for South Texas conditions, its gradual, year-to-year strategy offers a useful framework for understanding how a tree’s watering needs change as it becomes established.

Simple, But Not Easy

No matter how drought-tolerant, native, or well-adapted a tree species may be, most young trees in constructed landscapes require supplemental watering during dry weather. This is particularly important during the first one to three years after planting — and potentially longer for large nursery stock, trees in arid climates, or trees growing in difficult urban conditions.

Immediately after planting, most of a tree’s water-absorbing roots remain within its original root ball. As new roots grow into the backfill and surrounding soil, the area that must be monitored and watered gradually expands.

Mature, established trees generally require less consistent care. During prolonged drought or extreme heat, however, even established trees may need supplemental water. The appropriate amount and frequency depend on the species, climate, soil, available soil volume, drainage, and current weather conditions.

Watering as a Science

The following schedule is based on a water-conscious approach developed for San Antonio’s warm, drought-prone climate. It offers a useful year-to-year framework, but it should not be treated as an automatic prescription for every tree and every site.

Rainfall, temperature, wind, soil texture, drainage, tree species, planting season, and the size of the tree at planting all affect how quickly the root zone dries. Before watering, check the soil several inches below the surface. If it remains moist or saturated, wait and check again.

Establishment period	Where to water	Starting frequency
Year 1: First month	Directly over the root ball and immediate backfill	Three times per week
Year 1: Second month	Root ball and immediate planting area	Twice per week
Year 1: Third month	Root ball and surrounding planting soil	Once per week
Year 1: Months 4–9	Gradually expanding root zone	Two to four times per month, depending on weather
Year 1: Cooler months	Root zone	Monitor soil moisture and respond
Year 2: Hottest months	Root ball and surrounding soil	Two to four times per month; as often as weekly during drought
Year 2: Cooler months	Expanding root zone	Monitor soil moisture and respond
Year 3: Hottest months	Approximately twice the original root-ball width	Two to four times per month; as often as weekly during drought
Year 3: Cooler months	Expanding root zone	Monitor soil moisture and respond
Year 4	Broad developing root zone	Water during hot, dry periods if the tree is not fully established
Year 5 and beyond	Broad root zone, especially near and beyond the dripline	Monitor established trees during drought and extreme heat

Recommendations synthesized from tree-establishment and irrigation guidance published by the City of San Antonio, San Antonio Water System, University of Minnesota Extension, University of Maryland Extension, and University of Florida IFAS. Actual watering needs vary by climate, soil, tree size, rainfall, and site conditions. Note on amount per watering: For the San Antonio-based approach, approximately 1 gallon per inch of trunk diameter per watering event is a reasonable starting point. Other climates and planting conditions may require more. Apply water slowly and adjust the amount based on soil moisture, drainage, rainfall, and whether the entire root ball is being moistened. Never irrigate soil that remains saturated.

 

For young trees, concentrate water over the root ball and immediate planting area during the first weeks after planting. Apply the water to the soil around the tree, not directly against the trunk. As roots expand beyond the original root ball, gradually increase the area being watered.

A layer of organic mulch can help retain soil moisture, moderate soil temperature, reduce competition from turf and weeds, and protect the soil from compaction. Apply approximately 2 to 3 inches (5 to 7.5 cm) of wood-chip mulch over as broad an area as practical, while keeping the mulch several inches away from the trunk and root flare.

For mature trees, there is no single gallons-per-inch formula that works everywhere. Water slowly across a broad portion of the root zone, particularly around and beyond the canopy dripline. The objective is to moisten the upper portion of the soil profile where most water-absorbing roots are located without leaving the soil saturated.

Watering as an Art

In addition to direct watering, several environmental and design decisions can place urban trees on a more secure course.

Select tree species that are well suited to the climate, soil, available rooting space, and typical rainfall of the site. A tree that is appropriately matched to its environment should require less supplemental irrigation after establishment, although even drought-tolerant trees may need water during prolonged dry periods.

Provide as much healthy, loamy soil as practical. Trees with access to larger soil volumes are generally better equipped to withstand dry periods because the soil can capture and store more water. Soil texture matters as well: sandy soils drain quickly and may require more frequent watering, while clay soils retain water longer and are more vulnerable to overwatering and poor aeration.

Evergreen trees and shrubs should enter winter with adequate soil moisture when autumn has been dry. Supplemental winter watering may occasionally be appropriate, but only when the soil is dry, unfrozen, and able to accept water.

Repeated digging and seasonal planting beneath trees can damage roots and create additional competition for water. If other vegetation is planted within the root zone, choose compatible plants and minimize disturbance to the soil.

Silva Cells and Stormwater Irrigation

One of the smartest ways to support urban trees is to make use of water already falling on the site. When runoff from streets, sidewalks, roofs, and other hard surfaces is directed into the planting soil within Silva Cells, that water can be slowed, filtered, and made available to tree roots. This passive irrigation can reduce demand for potable water while allowing the same soil volume to support healthy tree growth and perform at-source stormwater management.

But stormwater should generally be treated as supplemental irrigation unless the system has been specifically designed and verified to meet the tree’s water needs during dry periods. Rainfall does not arrive on a tree’s schedule, and the periods when young trees need water most may be the same periods when little runoff is available.

Newly planted trees still need a reliable establishment-watering plan, regular soil-moisture monitoring, and access to supplemental water when rainfall is insufficient. Rainwater can do meaningful work, but it should not be expected to meet every irrigation need by chance.

Watering Tools

There are many ways to deliver water to trees. The best choice depends on the tree’s age, the site, available water, maintenance responsibilities, and the extent of the root zone.

Direct Irrigation

• Slow-release watering bags can deliver a measured amount of water directly over the root ball of a newly planted tree.
• A clean 5-gallon bucket with small holes near the bottom can provide a similarly slow application. Time how long it takes to fill the bucket so you know how much water is being delivered.
• A hose set to a slow trickle can be moved around the planting area to distribute water evenly.
• Drip irrigation, bubblers, and soaker hoses can apply water slowly while reducing runoff and evaporation.

Captured Rainwater and Passive Irrigation

• Rain leaders, roof drains, and scuppers can be directed into the tree’s planting soil or connected soil volume.
• Flexible downspout extensions can direct roof runoff toward the root zone while keeping water away from the trunk and building foundations.
• Rain barrels and cisterns can collect runoff for later irrigation.
• Grading, curb cuts, permeable paving, and other site-design strategies can help runoff reach tree planting areas rather than immediately entering the sewer system.

Automatic Irrigation

Automatic irrigation can be useful for trees in difficult-to-reach locations or in landscapes with formal maintenance programs. Drip lines, bubblers, and soaker hoses should be placed to serve the developing root zone and adjusted as the tree grows.

Automatic systems should not simply run on the same brief schedule used for turf. They should be inspected regularly and adjusted for rainfall, season, soil moisture, and changing root-zone size.

For trees growing in extensive connected soil systems, water should be distributed throughout the accessible soil volume rather than only at the tree opening. If a soil-watering needle or root feeder is used, insert it only shallowly and reposition it throughout the root zone to avoid delivering water too deeply or in only one location.

Timing and Application

Effective tree watering generally takes place slowly. Water must be applied at a rate the soil can absorb without running across the surface or draining below the active root zone.

Early morning is usually the most efficient time to irrigate because temperatures are cooler and less water is lost to evaporation. Late-day watering may also be appropriate when water is applied directly to the soil without leaving foliage wet overnight.

Brief lawn-irrigation cycles are rarely sufficient for trees. Whether using a hose, bubbler, drip line, or soaker hose, apply water long enough to moisten the active root zone. Then allow oxygen to return to the soil before watering again.

After watering, use a trowel, soil probe, or screwdriver to check how deeply the water penetrated. The soil should feel cool and moist — not muddy or saturated — several inches below the surface.

Still Not Sure?

These recommendations are guidelines. Your own observations — and, when appropriate, input from a qualified arborist or local extension professional — should determine how they are applied to a particular site.

Consider the following questions:

• Is the tree newly planted or mature and established?
• How much rain has the site actually received?
• How hot, dry, or windy has the weather been?
• How much soil is available to the tree?
• What type of soil is present, and how quickly does it drain?
• Is the tree growing in a street, median, parking lot, lawn, or open landscape?
• What moisture conditions does the tree species prefer?
• How does water enter and move through the planting area?
• Is runoff being directed into the soil?
• Does the soil several inches below the surface feel dry, moist, or saturated?

Most of the water absorbed by a tree eventually returns to the atmosphere as water vapor through transpiration. The smaller portion retained by the tree supports photosynthesis, cell structure, nutrient transport, growth, and other essential biological processes. This movement of water is also one reason healthy urban trees are so valuable: it helps cool the surrounding environment while supporting the canopy growth that allows trees to provide their many other benefits.

Thanks to Mark Peterson, Dr. Edward Gilman at the University of Florida, Dr. Gary Watson with the Morton Arboretum, Jim Urban with Urban Trees + Soils, and Colorado State University Extension for their contributions to the guidance that informed the original article.