There are no magic "technofixes" to solve the problem
of too many people and too little resources, but science and common sense
together teach us what works or what ought to. Ecologically appropriate
landscaping--plant materials and selection strategy--must vary according
to the climate of the region (e.g., humid vs. xeric) and human use.
Although "drought tolerance" is a popular misnomer for plant materials
in the landscape, there are excellent varieties such as turfgrasses which
serve us, while surviving mostly on natural rainfall. Turfgrasses are
being developed with reduced need for irrigation; this might not "save"
water but it is ecologically appropriate and imperative.
If we suspend thinking
of the new grasses as products, and see them as a process, then we can
do a better job of integrating the parts of our future landscape.
Once species and cultivar have been selected, establishment technique
either conspires to destroy it, or promotes the plant's--and peoples'--success
in this environment. Maintenance of established landscapes can create
a vicious cycle of pest problems, thatch accumulation, excess mowing requirement,
and too frequent irrigation need. Understanding the physics and
biology of water conservation can save other resources, and is fun.
the gross inadequacy of most landscape sprinkler systems, plant variety
selection will have relatively little impact on water conservation compared
with the obvious: design and manage the irrigation system
so that it provides even coverage.
Statement of Problem
selection is a tool for water conservation, but it has its share of mythology.
This article briefly covers the theory of drought resistance in plants,
variety selection, and its application to landscape water conservation.
Everyday Florida examples help us to understand the facts behind the theory.
Two assumptions are obvious
and must be accepted: (1) landscape plants consume water and survive within
the limits of moisture from nature (rain) and from management (irrigation)
and (2) water applied in excess of this plant use results in runoff to
canals and ponds, or percolation to the surficial aquifer.
Research has also shown
that: (1) landscape plants vary in the water that they use, and/or their
survival in drought, according to species and cultivar, and according
to environmental conditions and (2) landscape soil moisture storage is
limited by soil characteristics and on the depth and efficiency of roots.
There are apparently
four methods by which managers could affect the use of water in the landscape:
(1) change their watering practices; (2) select and use different species
or varieties of plants, e.g., (a) varieties which use less water or (b)
varieties with deeper root systems and larger available soil moisture
supply; (3) modify the soil or topography to increase the available soil
moisture; and (4) change the microenvironment to encourage lower evapotranspiration.
This article deals principally with method #2, although it emphasizes
the importance of and the interaction among all four methods.
Drought resistance and water use
Drought resistance is
"the generic term used to cover a range of mechanisms whereby plants withstand
periods of dry weather" (Paleg and Aspinall, 1981). In this broad, simple
definition, drought resistant plants are the ones that survive irrigation
curtailment, even during dry weather.
Drought tolerance is
the ability to withstand a drying stress which penetrates plant tissue,
while drought avoidance is the ability to avoid the drying stress (i.e.,
desiccation). Most higher plants, including most landscape plants, are
intolerant of desiccation. Examples of drought tolerance occur in lower
plants, e.g., fungi, some pteridophytes (e.g., resurrection fern), and
the dried seed. Otherwise, higher plants "normally remain turgid . . .
therefore possess drought avoidance." (Levitt, 1980). Except for a few
rare adaptations, it is generally incorrect to refer to drought tolerance
of landscape plants, because for all purposes it is nonexistent.
There are, however,
many examples of drought avoidance as a mechanism of drought resistance.
Some plants are water saving drought avoiders. Cacti conserve water, and
therefore resist drought, by exchanging gases at night. Bahiagrass conserves
water during dry spells, and thereby resists drought, by reducing leaf
area. Bahiagrass can go into permanent wilt, lose all its leaves, and
still come back. It has protected stems which, although not tolerating
desiccation, are well protected, and will initiate new leaves when rain
returns. St. Augustinegrass can avoid drought stress and conserve water,
by wilting temporarily. However, St. Augustinegrass does not come back
from permanent wilt. Unlike bahiagrass, St. Augustinegrass which is allowed
to defoliate will die.
Deep rooting, is a drought
avoidance mechanism within the general category of resistance (Levitt,
1980). The deeply rooted plant may be considered a water spender. Although
". . . it may appear contradictory to propose water spending as an adaptation
to drought . . ." (Levitt, 1980), this mechanism is obviously successful.
Examples include mesquite, which grows in the desert southwestern United
States, and a wide diversity of Florida natives and exotics. Under proper
establishment, many landscape plants can survive permanently with no irrigation,
because their root systems are sufficiently extensive to maintain turgidity.
Levitt (1980) provides several examples of grasses which are drought avoidant
due to greater water absorption. Bahiagrass also fits into this water-spending,
drought-resistant category. It has the deepest roots of any warm-season
turfgrass used in south Florida.
A deep-rooting, water-spending
drought resistance mechanism provides a practical tool for water conservation.
In a few words, you don't have to irrigate. The portion of irrigation
water that is generally lost to direct evaporation, can be conserved by
the use of non-irrigated landscape plants. Some additional savings might
come from a reduction in the luxuriant, water-demanding canopy that has
been shown (Kneebone and Pepper, 1984) to result from excessive irrigation.
Using drought avoidant, deeply rooted plants can allow for survival of
the landscape, while allowing for self regulation. Does this mean that
water has been saved?
No! In the immediate
sense, a dead landscape saves water, while a surviving landscape must
transpire. Are drought avoidant plants appropriate for the Florida environment?
Yes, they have been grown here for thousands of years.
Water in the Florida landscape
Florida is not a xeric
region, and yet water availability is critical. Although rainfall is sufficient
annually to grow almost anything, its seasonally variability puts a stress
on plants with shallow root systems, e.g., closely mowed turfgrass. In
Fort Lauderdale the 61 inches of annual rainfall more than satisfies the
44 inches of evapotranspiration (ET) for well-watered St. Augustinegrass.
Rainfall is seasonally variable, however, and about 32 inches of irrigation
annually are needed to make up for the seasonal deficit between evapotranspiration
and rainfall. Drought is typically most severe in April and May, but non-irrigated
St. Augustinegrass can be damaged at any time of the year, if irrigation
is not available.
Because of more variable
rainfall, seasonal variability of water stress is much greater in southern
Florida than in northern Florida. Soil moisture storage is least in southern
Florida, due to the sandy soils. For example, at turf plots in Fort Lauderdale,
there is only 3% soil moisture, by volume, in the top foot of soil. In
the absence of rain or irrigation, and in full sun, most cultivars of
St. Augustinegrass wilt 3 to 6 days after the last saturating rainfall.
Although there is no absolute limit to the effective root zone, the estimated
cumulative root-available moisture reserve for St. Augustinegrass (based
on days to wilt) is only about 15 mm (1/2 inch). The frequent need for
irrigation does not mean that turf areas need or use more water than deeply
rooted trees or other vegetation types, but it shows that turf is vulnerable
due to its more limited moisture reserve. In the deeper sands, where plant
available soil moisture is less than 2% by volume, deeper root systems
would not be an overwhelming advantage. Doubly the root system to 2 feet,
from 1 foot, would only add about 6 mm moisture, or about 1 day of transpiration.
So where are deeply rooted, drought avoidant plants likely to get water?
Dig almost anywhere
in Florida and you find moisture. Low lying entisols along the southeast
coast have an artificially drained water table which is often 4 to 6 feet
below the ground level. The widespread use of centrifugal pumps for lawn
irrigation is proof that water is close to the surface. This is verified
by the fact that almost every canal is filled with water to within about
5 feet of the ground level. The groundwater is often associated with a
limestone substrate. In southern Dade County the limestone is frequently
exposed, but is interrupted by "solution holes" filled with fine particles.
In central Broward County the same pattern occurs, but the rock is several
feet below the ground level. At Fort Lauderdale Research and Education
Center, the rock undulates from about 1 foot below the surface to sand-filled
solution holes which extend 6 feet deep or deeper. The surficial aquifer
at this location is at 4.5 feet. The rock and water table together are
important, because roots of turfgrasses and other plants are found attached
to the rock, extending in a yellowish marl layer down to the water table.
The marl layer is always wet, even after prolonged drought, and almost
certainly provides upward capillary movement of water. The proximity of
the water table and rock associated with fine particles means that many
deeply rooted landscape plants rarely need to be irrigated. Their roots
are the only pumps that they need.
The same generalization
is true for another major soil region of Florida, but for a different
reason. The spodosols of central Florida have a zone of slow percolation,
a "hardpan", which perches the water table nearer the plant roots. In
this region many sod farms efficiently produce turfgrass with no overhead
irrigation (except possibly right after plug planting). Home lawns in
this region can be cared for also, in some cases, with minimal irrigation.
In coastal ridges throughout Florida, the water situation is not so favorable.
But in the major part of most urban areas (Jacksonville, Tampa Bay, Orlando,
and South Florida) the net irrigation requirement should be more than
adequate, even if our estimates were based on full-sun exposure (not universal
in urban areas).
If persons frequently
irrigate their lawns and other landscape areas, it does not necessarily
mean that the plants need that much water, especially in Florida. In all
of Florida there is a rich tapestry of native vegetation which obviously
has survived through wet-dry cycles with no irrigation. Exotic vegetation
has been introduced, some with even greater drought avoidance than the
native plants. Such exotic and native vegetation, turfgrasses and trees,
can already be grown in many cases with no supplemental irrigation. While
the development of more drought avoidant vegetation is important, it appears
to be equally essential: (1) to clearly understand the ecological and
water relations of already available landscape plants; and (2) to apply
common-sense in their maintenance.
Limited turf areas?
It is no accident that
grasslands predominate in areas receiving less than adequate rainfall
(Fig. 1). While survival characteristics (e.g., intercalary meristems)
are important, total water use differences are associated, as well. In
general, forested regions have about 45% higher transpiration than grasslands
(based on Larcher, 1980). Sawgrass, the native vegetation covering the
Everglades, is more demanding of water than turfgrass. Clayton (1949)
reported 68 inches ET from sawgrass, compared with 54 inches for bahiagrass.
has been meager work on water use of woody plants, compared with the wealth
of information on grasses. There is, however, no physiological basis on
which to speculate that limiting turf areas, and replacing them with trees
or shrubs, would conserve water. Because of simple thermodynamics, trees
would be expected to actually use more water, thus confirming studies
of real ecosystems (Fig. 1). Transpiration is driven principally by total
radiant energy flux. The heat of vaporization is 580 calories per gram
of water. This same value is true for transpiration from a tree, a turf,
or a wet sponge. Because of the constant heat of vaporization, and the
unchangeable radiant energy from the sun, a green growing canopy will
consume about the same amount of water per unit area, regardless of whether
it is covered by trees or turf. The openness of a tree canopy (causing
lack of resistance to diffusion), and its roughness (causing turbulent
air flow) would be the only obvious difference in the two vegetations'
evapotranspiration. For these reasons, we would expect a tree canopy to
transpire slightly more than a turf canopy. This is true, based on ecology,
even though it refutes the popular mythology, that "turfgrass wastes water".
People waste water.
There have been other
alternatives proposed for reducing turfgrass areas, and thereby "save"
water. One recommendation has been to replace turfgrass with gravel in
order to reduce transpiration (South Florida Water Management District,
1978). This would be effective in a shortsighted way, but would be environmentally
unsound, because it would involve other problems, e.g., weed control.
Wood chips are also effective means of reducing evapotranspiration, but
are temporary because they decompose and become overrun with weeds. Any
surface which does not transpire or evaporate will contribute to heat
buildup. It has been shown that wood chips underneath trees increase tree
evapotranspiration, compared with an understory of turfgrass (James Beard,
personal communication). The reason for this is obvious; the increased
heat accumulated by the woodchips radiates back up to the undersides of
tree leaves. As stated previously, the heat of vaporization of water is
the same for turf and trees. The environmental air conditioning of trees
and turf is proportional to their evapotranspiration. The old adage says,
"there ain't no such thing as a free lunch."
When arguments to reduce
turfgrass areas as a means of water conservation have run up against irrefutable
laws of physics, other facts have had to be changed to agree with the
One example involves
the supposed maintenance inefficiency of turfgrass compared with groundcover.
In this argument (currently being developed by the South Florida Water
Management District), it costs more to maintain turfgrass areas than it
does to maintain groundcover areas (Teets, Xeriscape Florida 90, Orlando,
20 September 1990). The major assumption in this argument is that a typical
5,000-square-foot lawn requires mowing 36 times per year, at 6 hours per
mowing, at $22.50 per hour. By multiplication, it can be shown that it
costs $4860 per year just to mow a typical lawn. For anyone who has a
lawn, it is obvious that these values are inflated by many times. As an
example, the estimated cost of just one cutting of a typical lawn would
be $135, or about as much as an inexpensive lawnmower! Published data
(Van Zomeren, 1983) shows that the minimum rate of professional lawn cutting
on a college campus is actually 22,000 square feet per hour. The figures
of the South Florida Water Management District overestimate the time factor
by 2,540 %.
Beneficial turfgrass, healthy environment
From an environmental
viewpoint, rather than limiting turfgrass areas, it would be more appropriate
to expand turfgrass areas, in some cases. Examples include church and
swap-shop parking lots, swales which collect stormwater runoff, slopes
subject to erosion, and areas near buildings. It would be so much better
to use turfgrass to filter particulates and oily residues from asphalt,
than to run that water out to the nearest oceangoing canal. Turfgrass
areas can facilitate the movement of cool, mosquito-abating breezes. This
is important near houses, especially with the increased risk of St. Louis
encephalitis. Turfgrass areas are increasingly more important in personal
security, because of the risk of home invasion in most urban areas of
Florida. Turfgrass areas also provide safety and sanitation, for outdoor
play activities, for movement of people and vehicles, and for inspection
of sensitive infrastructures, e.g., bridge embankments. While other environmental
plants provide some of these benefits, a sod-forming turfgrass is most
dependable, for these situations. Turfgrass areas blend in with other
vegetation types, and provide a transition for the use by butterflies,
birds, and mammals, including people who come to observe.
Water quality and water quantity impact on the landscape
Water quantity is closely
related to water quality. Ultimately, this is the urgent reason behind
landscape water conservation. In low lying coastal areas, if the pressure
or "head" of freshwater is relaxed, the aquifer can be penetrated by salt
water. Because fresh water is lighter than salt water, there is a small
margin of safety. The porosity of the underlying rock, and other factors
(presence of canals and water control structures) can affect the rate
of saltwater intrusion. Hydrogeologically, Florida is like a sponge filled
with freshwater, floating in a buoyant ocean of salt. As long as the inflow
of freshwater considerably exceeds the use, water quality is adequate.
During dry periods there is a risk of reduced water quality.
In southern Florida,
the previously mentioned landscape factors work together with geographic
factors to make landscape water use a very major concern: greater seasonal
variability in plant water stress, sandy soils, closeness to the ocean,
the presence of large population centers, and the demand for luxuriant,
drought sensitive vegetation. These factors explain the great involvement
of public agencies (e.g., water management districts) in landscape water
conservation in southern Florida.
During extended dry
periods water users must restrict consumption. Landscape irrigation traditionally
receives relatively greater restriction than some other water uses, and
such restriction has the potential for harming landscape plants. At Fort
Lauderdale, commonly grown Floratam St. Augustinegrass goes into permanent
wilt (i.e., remains wilted in the morning) only 19 days after irrigation
curtailment. Thereafter, the turf loses about 3% coverage per day. St.
Augustinegrass, mainly Floratam, represents about 234 thousand acres of
turfgrass in the South Florida Water Management District alone. Water
curtailment to the millions of St. Augustinegrass lawns could result in
great damage. Landscape water use is important because it is large (an
estimated 200 billion gallons irrigation requirement in south Florida
St. Augustinegrass lawns) and because of the potential for great harm
when it is curtailed.
Breeding for drought resistance
Previous research on
drought resistance has emphasized physiological characteristics of plant
tissue. Examples of such research include the selection for varieties
with fewer stomates. It was found that such selection was ineffective
in reducing water use, because it was associated with an increase in stomate
size. Leaf resistance has been shown to be a minimal factor in St. Augustinegrass
turf water loss, and so there is little basis on which to expect that
selection for stomate characteristics would be effective.
Possibly, the fascination
for physiological mechanisms of drought resistance has been in the belief
that science can change the underlying processes by which plants grow.
In fact, the search for other magic bullets has been repeatedly unsuccessful.
Compilation of some few studies which have compared species (Table 1)
shows that the maximum species variation (6.2 mm day-1 for buffalograss
vs. 7.4 mm day-1 for St. Augustinegrass) would only result in 16% savings
even if you could grow buffalograss successfully in Florida. While species
differences are so inconsequential, variations among cultivars within
species have been even more elusive. The only major demonstrated differences
in water use among grasses have been studies which compared C3 species
e.g., tall fescue with C4 species, e.g., St. Augustinegrass (Biran et
al., 1981). The result has been that cool-season, C3 species have been
shown to use about 45% more water than warm-season, C4 species. While
this is important, it is not practically relevant, because we wouldn't
grow tall fescue in Florida.
The reasons that the
water use approach to resistance breeding is inappropriate for the Florida
setting are several: (1) differences in water use efficiency are rarely
observed; (2) in notable instances these differences contradict obvious
drought survival differences; and (3) most of this research has been done
in western states where total water supply is limited.
A practical method for
selecting drought avoidance in Florida grasses is to grow them under conditions
of limited, or no, irrigation. This has been shown (Busey, 1989) to be
useful in selecting drought resistant polyploid St. Augustinegrasses,
and has resulted in the recognition of FX-10, which survives for extended
periods with no irrigation. In the first two years of total curtailment
of irrigation, FX-10 had very little loss of stand, compared with Floratam,
which was almost completely wiped out. The grass has a patent pending,
and the first plant patent applied for by the University of Florida. FX-10
is not a grass which never has to be irrigated. After 2.5 years with no
irrigation at Fort Lauderdale, several FX-10 plots have lost substantial
coverage. If anything, it would be important to have an efficient irrigation
system for the few times a year that one would need it. The number of
required irrigations per year would vary greatly with microenvironment.
The potential advantages
of FX-10 require continued evaluation before a blanket recommendation
can be made. However, FX-10 has additional advantages besides drought
resistance. FX-10 is moderately resistant to the Floratam-killing chinch
bug (Busey, 1990) and is more shade tolerant than either Bitterblue or
Floratam. This shade tolerance could be quite important for urban areas,
in view of the increasing tree canopy, and the desire of persons to enjoy
turf, but protect themselves from sun exposure. FX-10 is lower growing
than Floratam. (It is not known how low growth habit might translate into
mowing energy requirement.) FX-10 retains a distinctively more bluish
color than Floratam, which would be an esthetic alternative. While FX-10
is no "wonder grass", it appears to be a dependable alternative to Floratam.
The technology was transferred to the Florida Sod Growers Cooperative,
which has the exclusive rights to produce FX-10. Currently there are 22
growers statewide who are producing FX-10. The grass was distributed for
experimental purposes to extension faculty in 34 Florida counties, as
well as private cooperators.
Educational opportunities - a common-sense approach
Even before more drought resistant grasses become available, there are
many common-sense things that can be done to conserve water in the landscape.
Here are some examples:
1. Treat the landscape
(including turf) as a process, not a series of products. Encourage the
sensitive awareness of landscapes as ecological communities. This should
include an awareness of the benefits of turfgrass when it is used appropriately,
as well as other vegetations when they are used appropriately. This should
also include the treatment of other organisms as components of an ecosystem,
not as pests. Avoid reliance on "technofixes". Landscape biology can help
in conservation of water and other scarce resources (e.g., petroleum)
and is fun.
2. Turn it off. Homeowners
could reduce irrigation use by relying on "single- event" timers, rather
than more technologically complex systems. While moisture sensors are
very compatible with commercial turf irrigation, it is easier for most
homeowners to use the bioassay, "let your lawn tell you when to water."
3. Reduce nitrogen and
phosphorus fertilization to the minimal amount needed to prevent weed
encroachment. Excessive reliance on color cues results in excessively
high application rates. This is associated with premature wilt, greater
risk of chinch bug and sod webworm infestation, and excessive cutting
4. Avoid hard curbs
and hill mounds. Structures such as these direct rain water to storm drains,
canals, and, very quickly, the ocean.
5. Properly establish
turfgrass and other landscape areas. This involves proper irrigation design
and installation, careful grading, and timely weed control. The best,
most well adapted plant material, will die or perform very poorly if it
is root bound, poorly pruned, contaminated with nematodes or torpedograss,
or planted over buried debris.
6. If it dies, grow
something else. Many plants, including exotics and natives, are poorly
adapted to the disturbed soils of urban areas. Consider this a good selection
7. Suspend the adoption
of buzzwords. Acronyms and other catch phrases are an effective marketing
tool. Until Florida horticulturists can agree on the message that is trying
to be conveyed, it would be wise to withhold the acronyms. The healthy
landscape is a process, not a product.
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of 11 turfgrasses as affected by mowing height, irrigation frequency,
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Stenotaphrum germplasm: Resistance to the polyploid damaging population
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