(CYNODON DACTYLON) Jodie S. Holt* Bermudagrass
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PHENOLOGY, BIOLOGY, AND COMPETITION OF BERMUDAGRASS (CYNODON DACTYLON) Jodie S. Holt* Introduction Bermudagrass, Cynodon dactylon, is one of the most serious weeds of the Poaceae, or grass family ( 4) . Among the many species of Cynodon that are called bermudagrass, ~ dactylon is recognized as the most ubiquitous, cosmopolitan, and weedy (3). Although the earliest introduction of bermudagrass into the United States was for pasture and forage use in the southeast, since that time farmers have also considered it a troublesome and invasive weed. The dual nature of bermudagrass as a weed and a crop is well recognized in California, where it is planted in lawns, pastures, and playing fields yet is also widely found as a weed in agrono~ic and vegetable crops, orchards, vineyards, roadsides, and landscapes (2). Around the world, bermudagrass is recognized as a weed in at least 80 countries in 40 different crops ( 4) . Interestingly, research and breeding programs to improve bermudagrass as a pasture and turfgrass may have also contributed to the weediness of this species where it escapes cultivation (4). Bermudagrass Growth, Development, and Reproduction The invasive and weedy nature of bermudagrass may be attributed largely to its perennial life cycle with prolific vegetative reproduction, the primary means of spread (13). Seed production also occurs in some varieties and cultivars and is an additional means of spread (3, 4). As a seedling, bermudagrass and other perennial weeds are functionally annuals in that no underground perennating or storage structures are initially present (13). Thus, the seedling stage of perennial weeds is the easiest to control. As the bermudagrass plant matures, however, production of stolons, or creeping horizontal surface stems, and rhizomes, or underground stems, becomes prolific (6, 7, 13). These vegetative structures are able to root at the nodes to form a dense sod of bermudagrass. Such rhizomatous perennial grasses are extremely difficult to control (2, 3). The seasonal pattern of growth and development (phenology) of perennial weeds such as bermudagrass will determine the effectiveness of any management strategy. In the spring, when new shoots are being produced, translocation of carbohydrates occurs from underground storage structures to aboveground actively growing parts of the plant (5). During flowering, which requires large amounts of energy, stored underground carbohydrate reserves are generally at their lowest point. Later in the *Assistant Professor, Department of Botany and Plant Sciences, University of California, Riverside. 192 summer, when leaves are producing carbohydrates, translocation occurs to underground storage structures. During winter dormant periods, underground stored food reserves are highest. This seasonal cycling of phloem translocation in perennial plants regulates the movement of many systemic herbicides throughout the plant, and thus, determines the appropriate time of application (5). The accompanying seasonal cycling of stored food reserves in perennial plants regulates the capacity of shoots to regenerate following damage to roots (5, 13). This phenomenon accounts for the effectiveness of repeated cultivation in depleting carbohydrate reserves and, thus, inhibiting further resprouting of perennial weeds. studies by Horowitz of bermudagrass early development demonstrated pronounced seasonality and temperature regulation of the life cycle of this species (6). Rhizome fragments planted in May through September sprouted, grew, and produced new rhizomes by 2 months of age, while those planted in January took 5 months to produce new rhizomes (6). Newly-formed rhizomes showed a high germination capacity, and one-node fragments germinated as well as those with several nodes. In an established bermudagrass sod, rhizomes formed more than 90% of the total underground dry weight of the plant, with most rhizomes occurring above 45 cm in the soil (6). Underground parts from the 1-15, 15-30, and 30-45 cm soil horizons constituted 62, 26, and 12% of the total rhizome weight. In further studies of the spatial growth of bermudagrass sprouts, Horowitz demonstrated that initial expansion of stolons and rhizomes was linear, followed by stolon branching and production of new shoots (7). Established sod developed in a circular pattern, with the fastest radial expansion occurring in the first year after sprouting (7). Over a 2 1/2 year period, a single sprout had spread to an average area of 25 m2 and some growth was found as far as 3.9 m from the original sprout (7). However, over 90% of the rhizomes on a mature plant were found within 2 m of the original sprout. These data illustrate the need to manage for belowground weed growth at all times. Interactions of Bermudagrass with Other Plants Competition is the tendency of neighboring plants to make simultaneous demands for the same resources, such that the immediate supply of resources is below their combined needs (13). As in many perennials, vegetative reproduction enables bermudagrass to begin the growing season with abundant reserves of stored food, resulting in rapid early growth and resource capture at the expense of neighboring plants (13). Competition from bermudagrass, primarily for water and nutrients, has been documented in most crops in California (2). In cotton, for example, dollar losses due to bermudagrass in Kern County alone ranged from $98 to $165 per acre from 1981 to 1983 (11). In studies of bermudagrass competition in cotton in the southeastern United States, bermudagrass planted at densities of 1 to 16 plugs per 7.5 m of row had little effect on cotton during 193 the season of establishment, but in the second year resulted in 25 to 80 % cotton yield reduction, respectively ( 1). In these experiments, reducing cotton row spacing from 1. 5 m to o. 5 m increased cotton yield by 20%, largely due to the reduced impact of bermudagrass (1). In other competition studies, bermudagrass planted 3 wk before cotton resulted in 90% reduction in cotton fresh weight after 70 days, while bermudagrass planted 3 wk after cotton reduced cotton weight 24% in the same time interval (8). These and other data confirm that although bermudagrass is very competitive against most crops, it is intolerant of shade and therefore, a competitive crop or cultural system may be used to manage it (1, 2, 4, 8). Numerous studies have documented bermudagrass competition in orchards (9, 10, 14) . Experiments by Jordan showed increased soil moisture at depths up to 92 cm in valencia orange orchards when bermudagrass was 50 to 100% controlled (10). Other benefits realized from bermudagrass control included increased tree volume, trunk circumference, leaf area, number of new shoots, and new shoot length. Furthermore, fruit yield and quality were improved, and physiological stress was reduced, in citrus growing where bertnudagrass was controlled (10). In sour orange orchards, bermudagrass caused severe reductions in plant height, stem diameter, and leaf number of trees; added nitrogen reduced the weed effects only slightly. These results suggested that bermuQagrass competition in orchards was not for light, but for water and nutrientst ( 9) . Peaches were also affected by competition from bermudagrass, and the beneficial results from control treatments were similar to those seen in citrus ( 14) . Thus, the competitive effects of bermudagrass on crops are seen not only in reduced yield, but in many other parameters of growth, as well. Bermudagrass is considered to be one of the most severe competitors of all lawn grasses. In a bermudagrass sod, the shallow, lateral roots of neighboring trees and shrubs are forced to compete for water and nutrients, and reduced growth often results (15). Clearing an area of bermudagrass for a certain distance around newly planted trees and shrubs greatly increases their growth. Experiments with 4 woody species planted into bermudagrass sod showed that a cleared circle at least 75 cm in diameter greatly reduced bermudagrass competition and aided tree establishment (15). As seen for citrus, the addition of nitrogen fertilizer without simultaneous control of bermudagrass had little effect of tree yield or performance (9, 15). Many reports on the interactions of bermudagrass with other species suggest that allelopathy, or the production of phytotoxic substances, may be a mechanism whereby bermudagrass affects neighboring species ( 8, 9, 12, 14) . In most of these cases, however, allelopathy was proposed as an explanation when reduced growth of a particular species could not be explained entirely by competition for nutrients. In contrast, one study specifically designed to evaluate allelopathy showed a stimulation of soybean 194 germination by bermudagrass root exudates ( 12) • Clearly, no definite conclusion can be reached regarding allelopathic effects of bermudagrass without further experimentation using appropriate designs and methodology (13). Conclusions As this review has shown, bermudagrass possesses many characteristics that make it a successful and competitive weed and turfgrass. Many of these characteristics can be exploited when developing management strategies, however, in order to tip the competitive balance in favor of other desired species. In combination with chemical control measures, cultural management of bermudagrass as a turfgrass, forage, or weed is entirely possible. Literature Cited 1. Brown, S. M., T. Whitwell, and J.E. Street. 1985. Common bermudagrass (Cynodon dactylon) competition in cotton. Weed