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Watering established trees and shrubs

Kathy Zuzek

The importance of water to trees and shrubs

large shade tree can release 40,000 gallons of water

K. Zuzek, UMN Extension

A large shade tree can release 40,000 gallons of water into the atmosphere in a year's time

As you stand in your garden or landscape, you are surrounded by water on the move within plants. This is due to a solar-powered process called transpiration. Water is absorbed from soil into the roots of plants and then transpiration pulls streams of water molecules up into the plant through inter-connected tissues within roots, stems and finally leaves. Water then moves from the leaves into the atmosphere as an air-cooling vapor through tiny openings called stomata that are found primarily on the bottom surface of leaves. Transpiration is a powerful process that lifts enormous quantities of water against gravity to the tops of even our tallest trees. It is also the main path of water loss in plants. At least 90% of the water in plants is lost from leaves through transpiration. The remaining small portion of water is lost through other plant parts or is used during the processes of plant growth and development.

Water is vital to plant growth and development because it:

Because of the importance of water to woody plants and the large amount of water that is constantly lost due to transpiration, it is important to know how to water trees and shrubs to maintain active growth, health, and vigor. During periods of consistent rainfall, a well-sited and well-established tree or shrub will need little supplemental irrigation. During long periods without rainfall, established trees and shrubs can decline or die without timely irrigation. As Minnesota’s climate changes, drought is expected to become more common. More frequent irrigation of trees and shrubs will be needed to ensure the health and survival of these long-lived plants.

Watering established trees and shrubs

root zone of an established tree is as wide as or wider than the tree canopy

The root zone of an established tree is as wide as or wider than the tree canopy

Where to water trees

The growth pattern of a tree’s root system resembles spokes of a bicycle tire with roots radiating through the soil from a central point below a tree trunk. Root spread will continue in actively growing trees until roots hit some kind of impediment to growth (paved roads, sidewalks, highly compacted soil, low oxygen levels, etc.) In order to water trees effectively, it is necessary to know how far roots have spread.

Roots grow and spread quickly in young and middle-aged trees with trunk diameters of 10 inches or less. What little information is available on root spread in older trees with trunk diameters of 10 inches or greater indicates that root spread slows in larger older trees. Trunk diameter or trunk circumference are the most accurate tools for estimating root spread. After measuring trunk diameter or circumference, the root spread diameter listed in Table 1 can be used to estimate root zone width for irrigation purposes. To measure the trunk's diameter, use a ruler or yardstick. To measure the trunk's circumference, use a measuring tape. Be sure to measure the diameter and circumference 4.5 feet above the ground.

Table 1. Trunk diameter or circumference are useful tools for estimating root spread of trees

dbh = trunk diameter 4.5' above ground (inches) Circumference of tree 4.5' above ground (inches) Root spread diameter (feet)
2 6-7 12-14'
4 12-13 20-22'
6 18-19 26-28'
8 25-26 28-30'
10 31-32 30-32'
12 37-38 32-34'
14 44-45 32-34'
>16 >50-51 34-36'
Measure trunk diameter with a ruler or yardstick and trunk circumference with a measuring tape.

K. Zuzek, UMN Extension

Measure trunk diameter with a ruler or yardstick and trunk circumference with a measuring tape.

A root zone is the top 18

A root zone is the top 18" of soil within the root spread area

Although a tree’s root system can extend down several feet below the soil line, most of the small feeder roots responsible for water uptake are located in the top 12-18 inches of soil. This 12-18 inches of soil within the root spread area is called the root zone and serves as a reservoir of water for trees between rainfall or irrigation events. When rainfall is absent for extended periods, soils will dry first at the ground surface. If lack of rain continues, drying will progress into deeper soils and irrigation will be needed to resupply the root zone with water.

The root zone of a shrub is 1.5 to 3.5 times as wide as the shrub canopy and 12-18

The root zone of a shrub is 1.5 to 3.5 times as wide as the shrub canopy and 12-18" deep

Where to water shrubs

Very little is known about the size of shrub root systems. As with trees, roots radiate out and down from below the base of the shrub until they hit impediments to growth. The larger the shrub canopy, the larger root spread will be. Root systems of established shrubs are approximately 1.5 - 3.5 times as wide as the plant. The majority of small feeder roots responsible for water uptake are near the soil surface, probably in the top 12-18 inches of soil.

When to water trees and shrubs

The best time to water a tree or shrub is when the top 6-9 inches of soil in the root zone are dry. Using a spade or hand trowel, dig a small hole under the plant canopy and feel the soil. If the soil is cool and moist 6-9 inches below the surface, no water is needed. If the soil is dry, it is time to water.

The first visible symptom in trees and shrubs that indicates a need to water dry soils is temporary wilting. During temporary wilting, leaves wilt and droop during the day but recover at night and appear normal again the following morning. Wilting occurs first in the top center portion of an established tree or shrub canopy and can be difficult to spot, especially in a mature, tall shade tree. Monitoring soil dryness and watering when the top 6-9 inches of soil are dry is more effective.

How much water to apply

Applying the proper amount of water is important to tree and shrub health. Under-watering causes a decline in the growth and health of trees and shrubs. Overwatering is wasteful and can be just as detrimental to tree and shrub health as under-watering, particularly in clay and compacted soils. Pore spaces between soil particles are shared by water and oxygen. Overwatering causes pore spaces normally occupied by oxygen to become filled with water and may cause root suffocation and a decline in tree or shrub health.

A watering regime that provides the correct amount of water to trees and shrubs is unique to every landscape. To determine the best watering regime for your landscape:

  1. Choose a time when the top 6-9 inches of soil is dry and there is no rain predicted for several days.
  2. Set several shallow containers (pie tins, cake pans, shallow cans, etc.) across the area to be watered. These will serve as a reservoir that can be used to measure one inch of water.
  3. Apply water with a sprinkler over the area until there is an average of one inch of water in each container.
  4. Record how long it took to apply this amount of water.
  5. Wait two hours and then slice into the soil with a sharp spade or shovel. Pull the spade/shovel towards you to create a big enough opening to reach in with your hand and feel the soil 6-9 inches below the ground. If the soil is cool and moist at this depth, the irrigation is appropriate.
  6. If you need to apply more water, add another half inch and check again to see if the top 6-9 inches of soil are moist.
  7. Continue this process until the top 6-9 inches of soil have received water.
  8. Record the total irrigation time for future reference. Water for this length of time during future irrigation events.
  9. Over the next several days, check the moisture level of your soil 6-9 inches below the surface of the ground every other day. If the moisture level is still adequate, the soil will feel cool and damp. Refill your hole and repeat this step every other day until the soil feels dry.
  10. Record the number of days it took for the top 6-9 inches of soil to dry. This provides you with the number of days between future irrigation events in the absence of rain.

Seasonal temperature shifts, soil characteristics, sloping terrain, and exposed sites with higher temperatures or winds, can all affect irrigation schedules. Soils will remain moist for longer periods of time in the cool days of spring and fall. Soils dry much more quickly in summer as temperatures and transpiration rates increase. You may want to test your soil’s water holding capacity in both spring and summer so that you can adjust the frequency of watering as the growing season progresses.

Sandy soils, with their reduced water holding capacity, may need smaller irrigation amounts applied over shorter intervals of time than loam or clay soils. For instance, if a clay soil requires one inch of water once/week, a sandy soil may require 1/2" of water applied every 3-4 days.

On sloped sites with clay or silt soils, water may run off of the soil surface before the desired about of water has infiltrated into the soil. This can be avoided by dividing irrigation into shorter events. The extra time between events will allow irrigation water to soak into the soil. Applying mulch that can serve as a sponge to absorb and hold moisture for slow-absorbing soils can also help to avoid runoff.

How to water

Watering of established trees and shrubs is most easily accomplished with overhead sprinklers that can be used to apply a deep watering over the entire root zone area after the top 6-9 inches of soil are dry. Avoid light watering as this practice promotes shallow root systems that are susceptible to summer heat and drought stress. Watering early in the morning minimizes water loss due to evaporation and wind drift.

Mulching trees and shrubs maximizes water uptake and tree health

Start mulch 12-18' from trunks or stems

K. Zuzek, UMN Extension

Start mulch 12-18' from trunks or stems

When trees and shrubs are grown in turf, competition for nutrients, water, and space occurs below ground between turf roots and woody plant roots. Turf wins because its dense fibrous root system prevents woody plants from producing water-absorbing roots in the top few inches of soil. As a result, woody plants grow more slowly in turf areas than in mulched or bare soil areas.

Woody plant growth and health improve when turf growing beneath tree and shrub canopies is removed and replaced with organic mulch:

  1. Eliminate turf and weeds from the base of the plant.
  2. Start mulch 12-18 inches from trunks or stems to prevent the formation of circling and stem-girdling roots.
  3. Beyond this area of bare soil, place a 3 inch layer of organic mulch such as wood chips, out to the edge of the tree or shrub canopy.
  4. Replace organic mulch every few years as it decomposes.

Mulching around trees and shrubs with organic materials (wood chips, pine needles, etc.) benefits trees and shrubs in several other ways. Mulch:

Deep mulch applications can be problematic because they may:


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