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.