Watering Shrubs  

By Michigan State University - Extension

All plants need water.  When rainfall does not provide      

water in the necessary amounts, the gardener must.  When    

to apply water and how much are decisions anyone growing    

ornamental plants must make.                                

All soils have air spaces, and it is in the air spaces      

that soil water is stored.  When water falls on the soil,   

it moves into the spaces in the soil.  The rate at which    

the water moves into the soil is called the infiltration    

rate.  After rain or watering, gravity pulls water out of   

the largest air spaces deeper into the soil.  The more      

quickly this happens, the better the soil drainage.  Sandy  

soils have many large air spaces, so they are  well-drained.                                                      

Water is lost from the soil in two ways: either it          

evaporates from the soil surface, or it is absorbed by      

plant roots and moved to the leaves where it is transpired  

to cool the plant.  The combination of evaporation and      

transpiration is called evapotranspiration.  As water is    

lost from the soil, soil particles hold the remaining       

water more tenaciously and plants have a harder and harder  

time absorbing it.                                           

The progressive drying of the soil can be seen by           

watching the plants.  After rain or irrigation the plants   

look fine.  As soil moisture levels decline, plants may      

wilt briefly during the hottest part of the day but         

recover quickly as temperatures fall.  As soil moisture     

declines further the wilting becomes more pronounced, but   

the plants usually recover during the cool nights.          

Eventually the soil becomes so dry that the plants are      

wilted all the time.  This level of soil dryness is called  

the permanent wilt point.                                   

The landscape manager essentially manipulates the           

relative amounts of air and water in the landscape soil by  

irrigating.  Because roots need both air and water yet      

occupy the same space in the soil, the gardener must try    

to maintain a balance between the two.  This can be         

complicated by soil types, the weather, types of plants     

and the capability to provide the proper amounts of water.  

When is it Time to Water?                                    

Watering will need to be done more often on sandy soil      

than on clay.  You must also take into account the amount   

of rainfall since the last watering.  One way to approach   

the decision of when to water is to try to replace the      

water lost through evapotranspiration.  This  would         

involve applying about an inch of water in any week         

without rain.  This would have to be adjusted depending     

on soil type (drainage) and weather (sunny or cloudy).      

Any consideration of the amount of rainfall that occurred   

in the previous week must also include how the rain was     

distributed.  Large amounts of water can fall in a short period of   

time during thunderstorms, but the water may fall on the    

soil faster than the soil will absorb it.  The excess water merely   

runs off.  The amount of water captured by the soil can be  

far less than the amount that fell.  An inch of rain        

falling in a very short period of time on clay soil  with   

a low infiltration rate does not translate into an inch of  

usable water for plants.                                    

How Should The Water Be Applied?                            

Apply water in any way that provides the maximum amount of  

usable water in the shortest period of time.  Two factors   

must be considered: the rate of application and the manner  

in which the water is applied.                              

The rate of application should not exceed the       

infiltration rate of the soil.  If the application rate is  

too high, much of the water will run off the irrigated      

area into the street or other areas where it will do no     

good.                                                        

You need to determine how long it takes your watering       

system to apply an inch of water over the entire area       

being irrigated.  A simple way to do this is to run the     

sprinkler and catch water in containers at various points   

under the sprinkler pattern.  Keep track of the time it     

takes to collect about an inch of water in all of the       

containers.  Most sprinklers do not apply water evenly,     

so more water will accumulate in some containers than in    

others.  But this will give a very rough idea of how long    

it takes your watering system to apply an inch of water.    

The next consideration is whether the soil will accept an   

inch of water in one sprinkling.  It may be necessary to    

move the sprinkler or stagger waterings over more than one session. 

  The water should be applied in the shortest       

possible time, however, not spread out over the entire      

week.                                                       

The best way to water with overhead sprinklers is to start  

in the morning and stop early in the day so that the        

plants have time to dry off before the cool night           

temperatures occur.  Foliage that remains wet for long      

periods, especially in cool night air, is vulnerable to     

attack by disease organisms that require moisture to get    

established.                                                

Conserving Soil Moisture                                    

Conserving soil moisture is one way to reduce the effects   

of dry weather or to reduce the amount of watering          

necessary or the frequency of watering.  The most widely    

used method of conserving soil moisture is mulching.        

(More information on mulching can be found in that section  

of this data base.)                                         

Watering Problems                                            

A number of problems on landscape plants can be influenced  

by soil moisture, either how much water is applied or how   

it is applied.                                               

Overhead sprinkling is a common practice in most            

landscapes, but water that sits on the leaves during the    

night can lead to increased disease problems in              

susceptible plants.  Applying water overhead shortly after  

applying pesticides may wash off much of the spray.         

Shrubs in the foundation planting may suffer from serious   

water-related problems.  Plants placed directly in front    

of a downspout can develop root problems during rainy       

weather due to the large amount of water put in their root  

zone from the down spout.  The other extreme also occurs    

in foundation plantings.  If plants are placed under a large  

overhang, the overhang prevents rainfall from reaching    

them and they suffer from too little water.                 

Homeowners with swimming pools will sometimes water         

their landscape plants with the water used to backwash the   

filter.  The water is no problem, but the chemicals in the  

water can injure plants.                                    

Keeping the lawn growing can involve frequent watering,     

especially if underground sprinklers attached to a time    

clock are used.  It is impossible to water the lawn         

without also watering the other plants in the landscape.    

When the lawn is being watered frequently, check the root   

zones of trees and shrubs to make sure they are not         

getting too much water.  Plants growing in areas that may   

be slightly lower than the rest of the lawn are most        

likely to be over watered.