Michigan State University Extension
Ornamental Plants plus Version 3.0 - 00001802
11/12/99
WATERING OF ORNAMENTAL PLANTS
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, but the small spaces in clay make it poorly
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
time during thunderstorms, but the water may fall on the
soil faster than the soil will absorb it. Excess 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 greatly 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 his 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 large
a overhang, the overhang prevents rainfall from reaching
them and they suffer from too little water.
Homeowners with swimming pools will sometimes water
their landscape plant 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 overwatered.
This information is for educational purposes only. References
to commercial products or trade names does not imply
endorsement by MSU Extension or bias against those not
mentioned. This information becomes public property upon
publication and may be printed verbatim with credit to MSU
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a commercial product or company.
This file was generated from data base ZZ on 01/19/00.
Data base ZZ was last revised on 11/12/99.
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