Bermudagrass, Cynodon dactylon (L.) Pers, is a warm season perennial
grass with a creeping growth habit. Bermudagrass spreads both by stolons
and rhizomes which readily root and tiller at the nodes. Common, Sahara,
Cheyenne and Sonesta are seeded varieties of bermudagrass. Other varieties,
including Tifgreen, Tifway, Tifdwarf, Texturf-10, Santa Ana and Midiron,
must be established from vegetative stock, sprigs or sod.
All varieties of bermudagrass turn straw-colored and become dormant during
winter months. During late fall soluble sugars are converted to starch
granules and stored in the stolons, roots and rhizomes during winter months.
Where winter temperatures frequently drop below 10°F, bermudagrass
is subject to significant winterkill. Midiron bermudagrass is significantly
more cold tolerant than others.
Green-up and recovery of bermudagrass begins when nighttime temperatures
remain above 60° for several days in the spring and soil temperature
reaches 65° at the 4-inch depth. The grass makes rapid recovery in
the spring by producing new shoots from the nodes of previously dormant
stolons and rhizomes. Research by Beard and DiPaola at Texas A&M University
suggests that early spring green-up of bermudagrass is accompanied by a
rapid dieback of old roots and production of new roots. This phenomenon
of new shoot production, root dieback and new root production describes
spring transition in bermudagrass and accounts for the vulnerability of
the grass to low temperatures, herbicides and competition from winter grasses
during the transition period.
The normal growth pattern for bermudagrass is shown in Figure 1. The date
of occurrence for each phase of growth-spring green-up, rapid growth, fall
hardening, and winter dormancy-may vary several weeks depending on the dates
of the first and last killing frosts. Thus, the length of the growing season
may range from 5 to 8 or 9 months, depending on the length of the frost-free
period. Obviously, in some areas of the county, bermudagrass may stay green
year-round.
In the Transition Zone of the U.S. between cool season and warm season grasses,
bermudagrass is subject to severe loss of stand (winterkill) during the
dormant period. Winterkill may be the result of direct low temperature
kill, desiccation, disease, traffic, competition with cool season grasses
or a combination of these factors. Where winterkill is a major concern,
management practices during the critical fall and spring periods can have
a significant impact on the amount of winterkill incurred.
In addition to the cyclic patterns of shoot and root growth in bermudagrass,
energy reserves show a definite cyclic pattern. Energy reserves accumulate
during the fall when photosynthetic activity is high and shoot growth is
greatly reduced. Energy reserves, principally starch, are gradually depleted
during winter months. In the spring, energy reserves are rapidly depleted
during the spring green-up and root dieback period. During summer months,
energy reserves accumulate very little since they are repeatedly used for
regrowth following mowing. Again, management practices can have a significant
influence on the reserve energy status of bermudagrass throughout the year.
New shoot production, root dieback with subsequent root production and depletion
of energy reserves put bermudagrass in a vulnerable condition during spring
green-up. A significant change in temperature, untimely applications of
fertilizers or herbicides or competition from cool season grasses could
result in a significant loss of bermudagrass turf during the spring transition
period.
Environmental Factors Affecting Spring Transition. Temperature,
shade, moisture, soil conditions, competition and traffic are the major
environmental factors affecting bermudagrass recovery in the spring. Temperature
has both direct and indirect effects on spring recovery. Low temperature
kill of bermudagrass occurs somewhere below 10°F depending on the variety
of bermudagrass. Low temperature kill likely occurs during winter months
and should not be considered a part of the spring transition phenomenon,
although it would certainly affect spring recovery.
Bermudagrass begins to green up in the spring when nighttime temperatures
exceed 60° for several days (soil temperature at 65° or above
at the 4-inch depth). When these conditions occur lateral buds (tillers)
found at the nodes of bermudagrass stolons and rhizomes break dormancy,
carbohydrate reserves are converted to soluble sugars and the first new
leaves appear. About this time some of the old roots of bermudagrass begin
to deteriorate and new roots are produced at the node.
So long as the temperature remains favorable during this transition period,
recovery progresses until complete green-up of the turf occurs. Depending
on the temperature, complete green-up may require 2 to 6 weeks (the higher
the temperature, the faster the process is completed). However, if periods
of favorable temperatures are interrupted by occasional freezing temperatures,
green leaves may be killed and new leaves must be produced. These intermittent
freezes delay spring recovery and place a greater demand on the reserve
carbohydrates in the stolons and rhizomes of bermudagrass. If, for some
reason, these reserves are low at the beginning of spring green-up, energy
reserves may be transported from one tiller to support leaf production on
another. When this occurs to a significant extent some tillers die and
a thin turf develops. Then, surviving tillers must grow and spread to fill
in where other tillers died. This is a common occurrence in many areas
where bermudagrass is grown and 20 to 80 percent of tillers may be killed.
Rate of recovery is, of course, directly related to the percent loss of
tillers.
Indirectly, temperature affects spring transition in bermudagrass through
its influence on nutrient availability and on disease activity (and nematodes).
The effects of these organisms on plant growth are well known and will not
be discussed at this point. Fungicides and nematocides can be used in the
spring to reduce their effect on spring recovery of bermudagrass.
Shade also affects spring transition in bermudagrass by weakening the plant
(reducing energy reserves) and increasing its susceptibility to winterkill.
Bermudagrass growing in partial shade does not accumulate as high a level
of energy reserves as that growing in full sunlight. Thus, the grass is
slower to recover in the spring and more susceptible to environmental stress
during this period.
Shade also has the effect of prolonging exposure of the grass to low temperatures,
snow cover and other extreme environmental conditions. Prolonged exposure
to these environmental stresses increases winterkill in bermudagrass and
delays spring recovery.
Where shade is a factor, management should be adjusted to enhance bermudagrass
recovery in the spring. Fertilizer rates should be lower in bermudagrass
growing in shade to reduce depletion of energy reserves. Also, mowing heights
should be higher in shade to increase total energy reserves and to increase
insulation or protection of the stolons during winter months. Herbicides
should not be applied to bermudagrass growing in partial shade.
Moisture, either too little or too much, also has an effect on spring transition
in bermudagrass. During dry periods in late winter and spring, significant
losses of lateral buds or tillers, can occur through desiccation. Thus,
as in the case of low temperature kill, complete recovery of bermudagrass
is delayed until the surviving tillers fill in the dead areas.
Dry spring conditions also slow the mobilization and transport of soluble
carbohydrates to lateral buds. Thus, leaf production is delayed until moisture
becomes available. Timely irrigations will overcome the delay in the spring
recovery associated with dry conditions.
Perhaps more serious than dry conditions which can be overcome by irrigation,
wet soil conditions also delay spring recovery in bermudagrass. Under wet
soil conditions in spring the conversion of energy reserves in the plant
to soluble sugars is very inefficient. Where bermudagrass is exposed to
prolonged wet conditions in the spring, energy reserves are depleted with
little new growth produced to support the plant. Thus, some tillers are
killed and a weak, thin turf develops. The solution to wet soil conditions
is to provide for effective surface drainage, and, in some situations, install
underground tile drains.
Compacted soil conditions delay spring recovery by restricting oxygen availability
to the root system. This has the same effect on grass as wet soil conditions.
Soil compaction can be alleviated in early spring by core aeration. Core
aeration should be delayed until dangers of hard freezes are past.
Competition from winter weeds, particularly annual bluegrass, can also retard
recovery of bermudagrass in the spring. A dense stand of winter weeds shades
bermudagrass and competes with bermudagrass for moisture and nutrients.
Furthermore, when annual weeds die in spring the turf is weak and susceptible
to invasion by crabgrass. Thus, weeds should be controlled to reduce competition
with bermudagrass and speed its recovery in spring.
Traffic not only increases soil compaction but increases wear on dormant
bermudagrass turf. Heavy traffic removes much of the dormant bermuda cover
and exposes the stolons and crowns to greater temperature and moisture stress,
increasing winterkill due to low temperature and desiccation. The effect
traffic has on compaction further compounds its effect on spring transition.
Obviously, when all environmental factors that effect spring transition
(temperature, moisture, shade, compaction, competition and traffic) are
combined the effect can be striking in terms of bermudagrass recovery.
The winters of 1983-84, 1989-90 and 1993-94 produced severe losses of bermudagrass
in the transition zone. All of the above mentioned environmental factors
were involved in those losses. Losses of bermudagrass were most pronounced
where northern exposures, shade and traffic were involved.
Cultural Effects on Spring Transition. In addition to environmental
factors that affect spring transition, cultural practices also have a significant
influence on spring transition in bermudagrass. Practices such as fertilization,
irrigation, and mowing that promote leaf growth enhance recovery of bermudagrass
in the spring. Fertilization practices have been shown to have striking
effects on spring recovery.
High rates of soluble nitrogen can be detrimental to spring recovery of
bermudagrass. Timing of the spring application of nitrogen is important.
The application should be after the danger of a severe freeze is past,
but before complete recovery of bermudagrass. Generally, nitrogen should
be applied after 20 to 30 percent green-up of bermudagrass. Nitrogen applied
at that time will promote leaf growth which, in turf, stimulates stolon
and root development. Very early applications of soluble nitrogen increase
the risk of injury from a late spring freeze.
In addition to the effect of nitrogen on spring recovery, potassium has
been shown to reduce winterkill and promote early spring recovery. High
potassium levels in bermudagrass increase carbohydrate storage in stolons
and rhizomes and reduce its susceptibility to low temperature kill and to
desiccation. Potassium also promotes root development in the fall and early
spring. In addition to the level of potassium in the grass tissue, the
ratio of potassium to phosphorus in the plant can have an effect on spring
recovery. A high ratio of potassium to phosphorus promotes early spring
recovery; whereas, a high phosphorus to potassium ratio has been shown to
increase winterkill and delay spring recovery in bermudagrass. Potassium
should be applied with nitrogen in late fall and early spring in a one-to-one
ratio.
Iron deficiency can also cause a delay in spring recovery in bermudagrass.
Iron deficiencies are most pronounced in the early spring when soil temperatures
are low. Soils with a high pH, high phosphorus level, or high moisture
level are particularly prone to show iron deficiencies in the spring. Soil
compaction also contributes to iron deficiencies. Where these conditions
occur foliar applications of iron at the time grass begins to green up in
the spring can promote recovery of bermudagrass. All of these soil conditions
are common to bermudagrass turf situations on golf courses, sports fields
and lawns.
Other nutrient deficiencies such as sulfur or magnesium that may occur in
turfgrass should be corrected through fertilization. Since bermudagrass
has been shown to have a restricted root system in early spring, soil nutrient
deficiencies at that time have a pronounced effect on spring recovery of
bermudagrass.
Irrigation practices through the winter and spring affect spring recovery
of bermudagrass. Even though top growth is dormant during the winter the
roots continue to take up water to keep the crown and stolons alive. Desiccation
of bermuda during winter months is a major cause of slow recovery in spring.
During extended dry periods, dormant bermudagrass should be occasionally
irrigated to maintain moisture in the rootzone. Soils should not be kept
wet, however.
In the spring when bermudagrass begins to break dormancy, even short dry
periods can have a detrimental effect on recovery. The soil should not
be allowed to become dry during the spring transition period if rapid green-up
and recovery is desired. Although moisture stress has been shown to promote
deep rooting during summer months, moisture stress during the critical transition
period would cause a significant delay in spring recovery.
Mowing practices that promote leaf growth also reduce the time required
for complete recovery of bermudagrass in the spring. Where bermudagrass
is mowed close such as on golf courses, raising the mowing height in the
fall as little as 1/16-inch could make a significant improvement in spring
transition. However, where bermudagrass is overseeded with ryegrass, close
mowing in the spring reduces competition by the ryegrass and increases
sunlight exposure to bermudagrass. Very close mowing of overseeded golf
greens during the spring will improve the recovery of bermudagrass.
Vertical mowing to reduce thatch should be done before bermudagrass greens
up in the spring, but after the danger of a prolonged cold period is past.
Otherwise, vertical mowing should be delayed until bermudagrass has completely
recovered.
Overseeding Effects on Spring Recovery. Overseeding bermudagrass
with cool season grasses is a common practice on golf courses, sports fields
and lawns. The cool season grasses provide color and increase wear tolerance
of bermudagrass turf during winter months. However, cool season grasses
also compete with bermudagrass during spring transition and significantly
delay recovery of bermudagrass.
During early spring when bermuda is beginning to green-up, cool season grasses
are at their peak growth period. Cool season grass tillers that may have
been initiated in the fall and winter develop very rapidly in the spring
and produce a dense turf cover. In contrast, bermudagrass is just beginning
to break dormancy and produce new shoots from stolons and rhizomes near
the soil surface. Consequently, cool season grasses absorb much of the
sunlight, and very little reaches new bermudagrass leaves. Likewise, the
root system of cool season grasses is very active compared to bermudagrass
in early spring. Thus, bermudagrass recovery is significantly delayed
by competition from cool season grasses.
Until environmental factors, particularly temperature, change to favor bermudagrass,
cool season grasses remain dominant. When this change occurs in late spring,
turfgrass managers should use cultural practices (vertical mowing, aeration,
fertilization, topdressing, close mowing, etc.) to suppress cool season
grasses and promote bermudagrass. Otherwise, the change from cool season
grasses to bermudagrass occurs too abruptly and results in a poor transition.
Where suppression of cool season grass is desirable, turf should be aerated
with a coring-type aerator when soil temperatures at the 4-inch depth reaches
65°F . The turf should also be vertically mowed throughout the transition
period to reduce the leaf surface of cool season grasses. Initially (March),
turf should be vertical mowed weekly to thin the cool season grasses and
increase the exposure of bermudagrass to sunlight. During the following
several months (April and May) twice monthly vertical mowing should be adequate
to thin the cool season grasses and reduce competition with bermudagrass.
Fertilization practices during the transition period are also critical to
bermuda recovery. So long as temperatures favor growth of cool season grasses,
nitrogen rates should be kept at a minimum to maintain acceptable color.
When temperatures increase to favor bermudagrass (60° nights and 75°
or higher daytime temperatures), nitrogen rates should be increased to promote
bermudagrass recovery. Applications of nitrogen every two weeks at a rate
of 1 pound per 1,000 sq. ft. should be made in May and June, or until bermudagrass
has completely recovered.
If cool season grasses continue to dominate bermudagrass through April,
aerate and lightly topdress overseeded sites in early May. Light, frequent
vertical mowing should also continue until bermudagrass becomes the dominant
grass. Close mowing during the transition period will also weaken the more
competitive cool season grasses, such as perennial ryegrass.
Watering practices during the transition period are critical on overseeded
sites. Daily applications of water that wet only the top 2 inch of soil
should be avoided after temperatures favor bermudagrass recovery. Light,
frequent applications of water favor cool season grasses and extend their
period of dominance over bermudagrass. On the other hand, thoroughly wetting
the soil 6 to 8 inches deep and allowing the cool season grasses to show
signs of moisture stress before applying additional water will give bermudagrass
a competitive edge over cool season grasses.
Disease also Prevents Bermudagrass Recovery. Turfgrass diseases
such as Pythium, brownpatch, dollar spot, leaf spots, and nematodes can
cause significant losses of bermudagrass in the spring. Bermudagrass turf
should be observed closely during spring transition for symptoms of these
diseases. Also, preventative applications of fungicides should be made
to critical turf areas such as golf greens and sports fields.
In areas where spring dead spot has been a problem, turf management practices
that reduce the severity of the disease should be followed. Dethatching
bermudagrass turf, keeping nitrogen application rates to a minimum and applying
fungicides in the fall have been shown to reduce spring dead spot severity.
Nematodes interact with turf diseases to produce significant losses of bermudagrass
during winter and early spring. Nematodes weaken the grass and increase
its susceptibility to disease infestations. The combination of nematodes
attacking roots and disease organisms attacking leaves, stems and crowns
can produce losses of turf in irregular patterns. On golf greens and other
critical turf sites, treatments should be made in fall and spring to reduce
nematode population.