LAWN IRRIGATION WITH AUTOMATIC SYSTEMS
A.J. Powell, Jr.
Department of Agronomy
Automatically time-controlled underground
lawn irrigation systems have become readily available in Kentucky. When
the systems are properly designed and installed, they can be effectively
used to grow high-quality turf. In addition to applying necessary water
for plant growth, proper irrigation can
•provide water to move pesticides and
fertilizers to the root zone,
•help reduce thatch by maintaining
a moist environment for microbial degradation,
•cool the plant surface and air that
surrounds the home or commercial building, through the process of plant
When compared to hand watering with
hoses and sprinklers, these automatic systems give more uniform coverage
and can reduce water loss by not watering sidewalks, streets and buildings.
In addition, irrigation can be automatically scheduled to avoid
peak residential water demand, which is important when using municipal
Beware of Overwatering
Since Kentucky receives an average
of about one inch of precipitation per week during the summer, automatic
irrigation systems mainly supplement rainfall rather than providing all
the moisture that turf needs. Consequently, irrigation should almost never
be programmed daily, every other day, etc. It would be better to let the
lawn suffer or die naturally without water, than kill it with expensive
overwatering. Overwatering causes shallow roots, nutrient loss, disease-susceptible
turf and very severe problems with. weeds like nutsedge, nimblewill, bent
grass, annual bluegrass, oxalis and crabgrass. Turf roots must have a balance
of water and air - overwatering excludes air. Further, due to diseases
such as Phytophthora Root Rot and Pythium, overwatering stimulates damage
to susceptible ornamentals such as taxus, junipers, azaleas, rhododendrons,
Japanese hollies, hemlock, dogwood, geraniums, chrysanthemums, etc. Other
ornamentals such as pin oaks, willows, bald cypress, red maples and sweet
gum are much more tolerant of overwatering. Ornamentals planted in drainage
areas and planted too deeply will be most severely affected by overwatering.
How Much Water?
Most Kentucky soil can hold about 2/3
inches of plant extractable water in the surface 4 inches where most turfgrass
roots persist. Although this is less than the turf will likely use during
a week, it is equivalent to about 400 gal of water/1000 sq ft of lawn.
The difference between the Total Amount Needed for evaporation and transpiration
per week and the Amount of Plant Extractable Soil Water plus weekly Rainfall
is approximately equal to the Amount of Irrigation Needed to maintain maximum
Total needed - [soil water + rainfall]
= irrigation needed
It is commonly suggested, as a rule-of-thumb.
that turf needs about one inch of rainfall or irrigation water/week from
May through September. Long term weather data, relating water loss and
rainfall, indicate that most Kentucky lawns need less than 1/4 inch of
irrigation water/week as an average. However, such long term weather averages
mask the increased need for irrigation during periods of obvious drought.
Weather Variations and Water Cost
Kentucky's weather has been extremely
variable in recent years. A weekly water deficit occured 10 times during
the relatively wet 1985 and 18 times during an extremely dry 1983 (Figures
1 and 2). Also note that almost every deficit was more severe in 1983 than
in 1985. If you irrigated each time the 2/3 inches of available soil water
was depleted, and if you applied 2/3 inches each time, then you would have
irrigated 15 times in 1985 and 42 times in 1983. Even in the very dry year,
42 irrigations averaged less than 2 times/week during this 26-week period.
At 400 gal/1000 sq ft, 42 irrigations equals 16,800 gal/1000 sq ft without
considering any evaporative losses during irrigation. At a cost of $1.70/gal
and excluding sewage charges, the water cost per 1000 sq ft in 1983 and
1985 would have been $29 and $10 respectively.
Good Judgment Needed
Although an automatic timer may be
set to irrigate several times a day, everyday and at various time intervals-DON'T.
Instead, use good judgment to decide when irrigation is needed, then either
(1) manually set the clock to automatically rotate through the different
stations (valves/zones), (2) program the system for one irrigation to occur;
for example the next morning, or (3) set the automatic timer to provide
one or two irrigations per week with either automatic or manual cutoff
if significant rainfall or mild weather occur. However, it is best and
most economical to initiate watering only when drought becomes evident.
When is Turf Too Dry?
Dryness is evident when
•turf on high spots and/or south slopes
starts to show some chlorosis or turns bluish gray in color,
•footprints remain in the grass long
after being made,
•soil from the root zone is dry or
•no rain has occurred for about one
week and hot, dry, sunny, windy weather persists. The very best way to
determine if irrigation is needed is to probe the surface soil and determine
if the surface inch or two is visibly dry. Economical soil water tensiometers
may soon be available to electronically initiate irrigation when soil becomes
The following guidelines may help you
use your irrigation system effectively:
1.Not all sites in a lawn will need
the same frequency of irrigation. If, for example, you are watering a flower
bed under a roof overhang, you may need irrigation even during rainy spells,
especially on the east and north sides of buildings. Probe the soil and
2.When necessary to irrigate, apply
about 2/3 inch of water. Check the irrigation rate and uniformity by placing
straight edge cups or pie pans around several lawn areas. Also probe the
soil to see if it is wet approximately 3 to 4 inches deep after irrigation.
If surface run-off occurs before irrigation is complete, apply only 1/2
of the amount in future irrigations and let the system recycle after a
few hours. An efficient irrigation wets only the turfgrass root zone, does
not saturate the soil and does not cause run-off
3.More frequent irrigation is usually
required on the hotter, south-facing slopes, where 1/2 inch or greater
of thatch is present, when growing Kentucky bluegrass rather than tall
fescue, when mowing at 1 1/2 inch height rather than 2 1/2 inch height,
and certainly during the hottest time of year.
4.During summer vacations and other
summer periods when close attention cannot be given to initiate each irrigation,
consider setting the clock to apply about 2/3 inches of water two times/week.
During fall and spring, since temperatures are cooler, the lawn can usually
go 2 to 3 weeks without significant rainfall or irrigation.
5.Water can be applied anytime during
the day without damaging turf. However, evaporative water loss during irrigation
is much higher during the heat of day. Early morning watering is often
advantageous since it removes dew and guttation water which often encourage
disease problems. If a turf disease is evident, avoid late evening watering
that would prolong leaf wetness. Otherwise watering during late evening
or night causes no problems.
Kentucky's continuously changing environment
provides a challenge to planning an efficient turfgrass irrigation program.
After a few weeks of experience, using the guidelines mentioned above,
you may tend to develop a sixth sense concerning irrigation. At that point,
irrigation becomes an efficient and effective tool in developing a high