Habitat
The common name for all the East African rhizomatous species of Cynodon is Bermuda grass (Harlan 1970, Burton and Hanna 1985). Most of these species originated and have remained in southeast Africa. Cynodon dactylon however, has become a "ubiquitous, cosmopolitan weed" (Harlan and de Wet 1969). The large intra- specific variability in Cynodon dactylon is represented by four varieties which have remained endemic to their original locale, and by two varieties, most notably Cynodon dactylon var. dactylon (from here on referred to by the species name), which have spread to other countries (Harlan 1970).
Cynodon dactylon grows throughout the warmer regions of both hemispheres. In the United States it occurs at elevations under 6000 feet, primarily in waste places, agricultural fields, and roadsides (Crampton 1974, Humphrey 1977, Burton and Hanna 1985). Although widespread, this species "thrives only under extreme disturbance and does not invade natural grasslands or forest vegetation" (Harlan and de Wet 1969). In areas of low rainfall it commonly grows along irrigation ditches and streambeds (Crampton 1974, Humphrey 1977, Burton and Hanna 1985). Bermuda grass, especially the cultivar Coastal, is extremely drought tolerant, however moisture significantly increases its growth rate (Humphrey 1977, Burton et al. 1987).
Ecology
Warm temperatures are necessary for the plants to thrive, and long periods of freezing weather or short durations of extremely low temperatures are detrimental to the plants (Gould 1951). Average daily temperatures above 24 C are necessary for substantial growth and temperatures of 38 C result in maximum growth rates (Holm et al. 1977, Burton and Hanna 1985).
In addition to high temperatures, Bermuda grass requires high light intensities. Cynodon dactylon needs direct sunlight in order to grow and dies out with increased levels of shade. This characteristic can be utilized in the control of Bermuda grass (Holm et al. 1977).
Cynodon dactylon tolerates a wide range of soil types and conditions. Growth is greater on heavy clay soils than on light sandy soils in dry regions; this may be due to the greater water holding capacity of the clay (Burton and Hanna 1985). Bermuda grass can survive long periods of flooding, but little to no growth occurs without adequate soil aeration (Burton and Hanna 1985). It grows on soils with a wide range of pH values, however alkaline soils are tolerated more than acidic ones. Growth is promoted by the addition of lime to soils with a pH of 5.5 (Burton and Hanna 1985). A large amount of available nitrogen is required for maximal above-ground growth; this element is often the limiting factor for Cynodon dactylon. Nitrogen fertilizers are routinely used in order to increase the forage and turf value of Bermuda grass (Humphrey 1977).
The drought and alkali tolerance, and high temperature and sunlight requirements of Bermuda grass explains its success in the Southwest; it is the most common and best performing grass in Arizona (Gould 1951). In southern Arizona Cynodon dactylon grows abundantly along sandy washes and near alkaline seeps (Gould 1951). A rapidly growing variety, which can grow over hedges 2 m tall, was introduced to Hawaii and Arizona in the early 1900s. A substantial amount of the world's salable seeds of this "giant" Bermuda grass is grown near Yuma, Arizona (Kearney and Peebles 1951, Harlan and de Wet 1969). Large amounts of Bermuda grass, including the giant-type, grow along the edge of Roosevelt dam in Arizona, where it survives submergence under water for part of the year and provides food for cattle during the dry periods (Munz and Keck 1959).
Shading drastically affects both above- and below-ground growth (Burton et al. 1959, McBee and Holt 1966, Schmidt and Blaser 1969, Hart et al. 1970, Burton et al. 1988). In Georgia, forage yield is dramatically reduced after the middle of September, with an average June yield of 2907 kg/ha and an average October yield of 295 kg/ha (Burton et al. 1988). Daylength and solar radiation, but not rainfall and minimum temperature, were significantly correlated with forage yield; 64%, 43% and 29% of the normal light intensity resulted in a reduced annual dry matter yield of 68%, 42% and 30%, respectively, of unshaded plants (Burton et al. 1988). Half the amount of rhizome and root growth occurred in the 64% shade treated-plants than in the control plants (Burton et al. 1959). Tall dense trees greatly reduce Bermuda grass growth, and complete canopy cover eventually kills the grass (Burton et al. 1959, Schmidt and Blaser 1969). The decrease in growth due to shading is intensified by high temperatures; this may be explained by an increase in respiration rate relative to photosynthetic rate (McBee and Holt 1966). Increasing the level of nitrogen while maintaining a low light setting results in a further reduction in growth; nitrogen fertilizer increases the retarding effect of low light on shoot, root and rhizome yield, and decreases the amount of reserve carbohydrates while increasing the amount of crude proteins (Burton et al. 1959).
Carbohydrate levels in Bermuda grass do not generally follow a consistent pattern (Weinmann 1961). The cultivar and the environmental conditions greatly influence the reserve carbohydrate quantity and quality. In general, total available carbohydrates in the rhizomes increases in the fall, peaking between November and December, decreases in late winter and begins increasing in late spring, reaching a second, but lower, peak in May, and then decreasing in the summer (Horowitz 1972b). In Mauritius, the carbohydrate reserves do not decrease in the late winter, instead they increase steadily from fall to spring, and then the pattern fluctuates the rest of the summer depending on the variety (Rochecouste 1962a). Seasonal rhizome bud germination does not appear to be correlated with the carbohydrate level (Horowitz 1972b).
Temperature affects the level of carbohydrates by altering the ratio between the respiration and photosynthetic rate, thus influencing the growth rate (McKell et al. 1969). The greatest amount of growth occurred at 30 C/24 C (day/night temperatures) whereas the greatest amount of starch in the stem bases and rhizomes of Coastal Bermuda grass occurred in the 13 C/7 C treated-plants (McKell et al. 1969).
Increasing the level of nitrogen results in a decreased amount of reserve carbohydrate (White 1973, Burton et al. 1959). Nitrogen fertilizers increase the glucose in leaves by decreasing the amount of sucrose and fructosan in stems, stolons, rhizomes and roots (Adegbola and McKell 1966). Nitrogen fertilizers are used to increase the above ground growth of Bermuda grass. When nitrogen is limiting, and the growth conditions unfavorable, fructosans accumulate in the rhizomes (Adegbola and McKell 1966). Storage carbohydrate utilization in nitrogen metabolism is thus connected with increased shoot growth (Schmidt and Blaser 1969).
An increase in nitrogen fertilizer from 0 to 900 pounds per acre results in an increase in height (2.5 inches to 6.5 inches), percent protein, yield (1.6 tons to 11.0 tons of hay), stem length (6.0 to 17.0 inches), internode length and node number, and a decrease in leaf percentage and seed head frequency (5% to 2%) (Prine and Burton 1956). An increase in nitrogen from 0 to 80 kg/ha results in a 5 times greater above-ground biomass (Skousen et al. 1989).
Bermuda grass is susceptible to desiccation. Long rhizome fragments and dormant stolons require long periods of drying in order to destroy the activity of the buds (Webb 1959, Horowitz 1972d). Air drying of one-node rhizome fragments for seven days resulted in the inhibition of sprouting and a 53% weight loss, however three-node rhizome fragments continued to sprout after seven days of desiccation (Horowitz 1972d). Actively growing stolons are more susceptible to desiccation than post-dormant stolons. Greater than 48 hours of drying over an ammonium chloride solution kills actively growing stolons, whereas greater than 96 hours is required to destroy post-dormant fragments (Webb 1959). The critical moisture level for stolons is 39% and for the harder to control rhizomes 15% (Webb 1959, Horowitz 1972d). Bermuda grass rhizomes cannot be drowned. Submergence of fragments for eight days in running water or four weeks in stagnant water had no effect on sprouting ability (Horowitz 1972d). Thus water is likely to be an efficient means of spreading rhizomes.
COMPETITIVE RELATIONSHIPS: Plant residues and actively growing plant parts of Cynodon dactylon may pose a direct threat to the growth of neighboring plants (Friedman and Horowitz 1970, Horowitz and Friedman 1971). Light textured soils mixed for four months with extracts from decaying Bermuda grass plants caused an inhibition of radicle elongation in barley and mustard seedlings (Friedman and Horowitz 1970). Incubation of test plants for two months with Bermuda grass results in a high degree of inhibition (Horowitz and Friedman 1971). In addition to the importance of the duration of exposure, is the concentration. The inhibition is proportional to the concentration of plant material. In general, root growth and germination are both affected by decaying residues and actively growing Cynodon dactylon plants (Horowitz and Friedman 1971). The importance of the allelopathic substances produced by Bermuda grass in the field is unclear. Threats due to completely decayed residues should not be overlooked.
In addition to the allelopathic effects of Cynodon dactylon is the direct competition for space and nutrients by this rapidly growing perennial grass. Bermuda grass's notoriety as a tremendous colonizer comes from the spreading ability of both the rhizomes and stolons (Horowitz 1973). The open growth pattern of Bermuda grass's stolons provides for greater land coverage than seen with species which lack stolons, such as Sorghum halepense; the average monthly area increase in the warm season for Cynodon dactylon and Sorghum halepense is 1.6 m2 and 1.3 m2, respectively (Horowitz 1973). Aerial growth from shoots, tillers and previous season's rhizomes produce an abundance of stolons, which in turn produce more shoots, rhizomes and roots (Horowitz 1972a). This growth pattern explains the tremendous spreading capacity of Bermuda grass; the highest monthly area increase was 6 m2 during July and August (Horowitz 1972a). However, the average area increase for Cynodon dactylon is only 0.9 m2 per month. This growth rate is far less than other perennial grasses; Cyperus rotundus has a mean area increase of 2.8 m2 per month (Horowitz 1972a).
Rhizomes grow in the same configuration as the above-ground growth and are not found growing outside of the sod perimeter. The subterranean dry weight averages 0.6 kg per m3 of soil within a 1 m radius from the center of the plant. Rhizome depth is comparable under the center of the plant and at the edge of the sod. The depth of penetration is restricted by compaction and aeration (Horowitz 1972a). With roots extending from stolons and rhizomes, a vast area can be utilized for uptake of water and nutrients.
The competitive ability of Cynodon dactylon depends on the competing plant species and the nutritional level of the soil. Bermuda grass yields were reduced by 40%, 27% and 13% when grown with Johnson grass, Torpedo grass and Cogon grass, respectively for one year (Wilcut et al. 1988). However Bermuda grass had a greater inhibitory effect on the competing plants, with a reduction in the yield of Johnson grass, Torpedo grass and Cogon grass by 55%, 38% and 43%. After two years of competition Johnson grass reduced the yield of Bermuda grass by 99% (Wilcut et al. 1988). Native vegetation recovery, due to the competitive ability of knotgrass (Paspalum sp.), began within one year after cattle were removed from a riparian ecosystem in which Bermuda grass was abundant (Richter pers. comm.).
Cynodon dactylon and Acacia smallii were grown in mixed and mono-culture plots, with and without added fertilizer in order to study the competitive ability and mechanism of the two species (Cohn et al. 1989). C. dactylon grew 1.5 to 2.4 times larger in mixed cultures than in mono-cultures, with a yield increase of 30% to 50% when grown with Acacia. Cynodon dactylon's competitiveness is thought to stem from its ability to reduce the level of nutrients to below the necessary amount needed by Acacia smallii; this assumption is based on the increase in Cynodon dactylon's growth in the mixed over the mono-culture treatments, its drastic increase in the fertilized, mixed culture plots, and the growth reduction by 70% to 90% of Acacia smallii in the fertilized mixed plots (Cohn et al. 1989).
Studies on competition in mixed plots of wheat and Cynodon dactylon showed similar intra- and inter-specific competition for nutrients when plants were planted at the same time (Ramakrishnan and Kumar 1971). Stunted growth of Cynodon dactylon occurred in high wheat density plots. A reduction in dry weight, leaf area and seed output was most likely due to the large size of the wheat plants which caused shading of the Bermuda grass. Low density mono-culture plots of Cynodon dactylon promoted early vegetative spreading growth with delayed reproductive development, whereas in high density plots the period of vegetative growth was shortened and floral development was hastened. In addition, seed production decreased with increased densities of Bermuda grass (Ramakrishnan and Kumar 1971).
RESPONSE TO MANIPULATIONS AND ABIOTIC FACTORS: The effects of fire on Cynodon dactylon are variable and dependent on the season and prevailing environmental conditions at the time of burning. Odum et al. (1973) burned a four year old fallow field during the late winter in Georgia. The burn resulted in a drastic reduction in Bermuda grass from 14.7 g/m2 to 0.2 g/m2 as compared to the increase in Johnson grass from 0.2 g/m2 to 27.4 g/m2. However, with the exception of extremely dry conditions or long periods of fire suppression, both of which result in hot fires that may damage the rhizomes, most rhizomatous grasses, such as Cynodon dactylon tend to benefit from fire (Rensburg 1970). Winter burning of Bermuda grass is performed in several southern states in the U.S. in order to hasten spring growth, resulting in increased yield and quality of forage (Hardison 1974).
The increase in the amount of Cynodon dactylon due to cattle grazing is well documented (Weinmann 1961, White 1973, Belsky 1986a&b). Unlike many other plants, intensive grazing on Bermuda grass results in an increase in carbohydrate accumulation in the below-ground structures (Weinmann 1961). This explains the rapid regrowth and establishment that was seen in overgrazed plots in the Serengeti National Park (Belsky 1986a&b). Grazing does not significantly affect growth of rhizomatous and stoloniferous plants that have a prostrate growth form (White 1973).
Clipping may have a greater affect than grazing on Cynodon dactylon due to the potential for removal of all tillers and shoots (White 1973). The mowing of Bermuda grass three times a week throughout the growing season had no significant effect on the carbohydrate content or weight of the rhizomes and roots; however, systematic cutting of each individual aerial structure with a scissor resulted in a significant reduction in the reserve carbohydrate level and weight of the below ground structure (Weinmann 1961). Removal of greater than 40% of the shoots reduced root growth and many roots failed to resume growth when severely clipped (Risser et al. 1981). Plants with prostrate growth and high, fluctuating levels of reserve carbohydrates, such as Cynodon dactylon, are difficult to control by clipping (Weinmann 1961).
Reproduction
VEGETATIVE GROWTH: The principle means of propagation of Cynodon dactylon is through the rhizomes and stolons (Holm et al. 1977). These structures are often severed from the plant by burrowing animals and animal hooves; the fragments are then transported by contaminated animals, hay, and machinery, as well as by running water (Holm et al. 1977).
The following is a description of the general life-cycle of Cynodon dactylon. In the spring when the temperature begins to increase new stolons elongate and aerial shoots sprout (Horowitz 1972a). The characteristic prostrate growth of Bermuda grass lasts for one to several months, early in the season, before flowering culms develop. Most of the lateral growth, produced in concentric circles from the original rhizome, occurs throughout the summer (Horowitz 1972a&b, Holm et al. 1977).
Single-bud rhizomes were planted and monitored throughout several growing seasons (Rochecouste 1962a). In the first month a primary shoot and four roots develop from the rhizome. Elongation of the internodes on the shoot is followed by the development of up to twenty buds per node. As many as 12 tillers sprout and three dormant rhizome buds develop from these shoot buds. Horizontal growth commences when the primary shoot and tillers reach 10 cm to 15 cm long, resulting in the formation of stolons. New stolons and roots are continually formed at the nodes of the spreading stolons. The dormant rhizome buds at the basal node of the primary shoot begin to grow at the commencement of the wet warm season. These rhizomes growing deep in the soil provide an over-wintering structure as well as, when they surface, additional above-ground growth (Rochecouste 1962a).
Rhizomes are the primary over-wintering structure (Holm et al. 1977). Cynodon dactylon's success as a weed is thought to be a result of the adaptive rhizome characteristics. The rhizome system is superficial as well as deep, which may account for the ability of this species to infest both arable and waste lands in a variety of conditions. Horowitz (1972a) found approximately 70% of the rhizome weight of two and a half year old plants in the upper 20 cm of the soil and no rhizomes below 40 cm (Horowitz 1972a). Other investigators report the existence of rhizomes 1 m deep (Holm et al. 1977). Concentric growth, outward from the original rhizome sprout, of the rhizomes corresponds to the circular above-ground growth pattern (Horowitz 1972a). A decrease in rhizome dry weight correlates with an increase in distance from the original rhizome. Greater than 60% of the subterranean weight (600 g/m3) occurs within 1 m of the plant center, 30% in the next meter, with the remaining 10% in the 2 to 3 m range (Horowitz 1972a). This growth pattern ensures both rapid spreading and strong establishment of the plants.
New rhizomes form only at temperatures greater than 15 C to 20 C; sprouting of rhizome buds is maximal at temperatures between 23 C and 35 C and is inhibited by temperatures below 10 C (Horowitz 1972b). No dormancy period is found in rhizomes; sprouting occurs once apical suppression is relieved by fragmentation (Horowitz 1972b). Growth of rhizome buds varies depending on depth in soil and age of rhizome. The year-long average sprouting rate for rhizomes in the top 15 cm of soil is 34% and for deeper rhizomes 24% (Horowitz 1972b). Young rhizomes sprout much more readily than do older ones. New rhizomes are capable of growing once several distinct internodes have developed. Subterranean growth begins earlier in the spring than aerial growth. The rhizome dry weight increases in April, followed by rhizome elongation (Horowitz 1972b). The total rhizome length of single node rhizome fragments planted in July increases from 36 m per plant in December to 95 m the following July, indicating greater rhizome growth in early summer than in late summer (Horowitz 1972b).
New stolons are formed at the basal node of shoots which had developed from over-wintering rhizomes (Horowitz 1972a). New stolons can grow 75 cm in the first six weeks after sprouting (Horowitz 1972a). Within three and a half months eight stolons from the main shoot and seven stolons from secondary shoots develop, resulting in 570 cm of stolon length (Horowitz 1972a). The initial stolons move away from the center shoot in straight lines. This is followed by the secondary stolons growing in all directions to form dense circular mats of sod (Horowitz 1972a). In two and a half years the average sod area of a single plant is 25 m2, with a maximum growth rate of 2 m2 per month in the summer months (Horowitz 1972a). Above ground growth is limited by temperatures below 18 C, and dies at -2 C (Horowitz 1972b). The high temperature requirements explain the five times greater increase in sod area for June through November than for December through May (Horowitz 1972a).
Roots develop from rhizomes and stolons. Roots produced at the distal end of the stolon are much longer and more abundant than those close to the original stem (Rochecouste 1962a). Depending on the cultivar, soil texture and nitrogen availability roots can reach 245 cm below the soil surface, however the majority of roots are found in the top 30 cm (Burton et al. 1954).
SEXUAL GROWTH: Asexual reproduction, not sexual reproduction, is responsible for the spread of most Bermuda grass. Most biotypes are infertile, and those that are fertile tend to produce sparse amounts of seeds (Rochecouste 1962a, Holm et al. 1977). However, southwestern United States varieties often have a good seed set. The tiny seeds remain viable after passing through livestock and after submergence in water for 50 days (Holm et al. 1977). There are approximately 4.4 million seeds in one kilogram (Holm et al. 1977). In California, 450 kg of hulled seeds are harvested per hectare in July (Burton and Hanna 1985). All biotypes, regardless of their fertility, produce inflorescences which range in height from 5 cm to 40 cm (Rochecouste 1962a). Inflorescences develop on the center shoot one and a half months after late spring planting (Horowitz 1972a). Inflorescences form during the summer and their production terminates in November; flowers occur throughout the sod at the end of the season, with a maximum of 99 inflorescences per plant. The sod area is proportional to the number of inflorescences with an average of 78 inflorescences per square meter of sod the first year. The second year of growth results in a drastic increase in the number of inflorescences with the maximum of 1125 per plant and an average of 87 inflorescences per square meter of sod (Horowitz 1972a).