Restoration Potential: Although Cynodon dactylon is considered the world's weediest grass, eradication appears to be feasible. Bermuda grass poses no problem in undisturbed, cold and shady areas. Thus if land adjacent to invaded areas remains covered by natural vegetation, Bermuda grass will be unable to spread into it.
Management Requirements: Cynodon dactylon, in disturbed sites, is a competitive and invasive weed. The best management strategy is to remove all plant parts at first sighting. Invasion will be limited by tall plants. Spot herbicide and manual tilling may be adequate in controlling native fields with patchy weed distribution. A more drastic control plan is necessary in sites heavily infested with Bermuda grass. The appropriate manipulation is dependent on the location, humidity, temperature, soil type and precipitation at the specific site. Burning, herbicide application, clipping and shading have all been effective in controlling Bermuda grass under various conditions. In order to prevent the sprouting and establishment of the remaining Bermuda grass rhizomes, native plants and shade material should be installed immediately after the eradication stage.
The best management practice is to avoid the initial invasion of Cynodon dactylon by limiting soil disturbances and maintaining a vegetation cover. Areas where the soil and native plants are kept intact should have little problem from Bermuda grass since it mainly invades disturbed lands (Crosswhite pers. comm., Diamond pers. comm., McWhorter pers. comm., Weigel pers. comm.).
Management strategies depend on the extent of Bermuda grass and the height of the native vegetation.
1a. Sites with established Bermuda grass where restoration projects include re-vegetating with tall-stature plants: A single season of controlling the Bermuda grass following by the transplanting or seeding of native plants will most likely be a sufficient control measure. Periodic spot control or shade mats placed around the young plants may be necessary during the establishment of the native plants. As already stated, the control technique employed is dependent on the site parameters. Herbicide application in conjunction with tilling and desiccation may be the most effective control technique.
1b. Sites with established Bermuda grass where restoration projects include re-vegetating with short-stature plants: A more severe eradication procedure is necessary when the native vegetation will not shade the Bermuda grass. Several repeated tilling and herbicide applications may be required to remove the maximum amount of underground rhizomes and stolons prior to the re-vegetation phase. Many years of spot control may be required until all remaining Bermuda grass is removed or until the ground is covered by the native vegetation.
2a. Uninfested sites with complete canopy cover surrounded by areas containing Bermuda grass: Most likely this situation will not require any active management since Bermuda grass rarely invades undisturbed sites.
2b. Uninfested sites with bare or unshaded spots surrounded by areas containing Bermuda grass: If the site is undisturbed than most likely Bermuda grass will not invade it. However, careful monitoring may show this to not be the case. If appropriate, planting of tall plants between the invaded and non-invaded sites may prevent spreading into the exposed area. Removal of the weed in the adjacent land may be required if invasion occurs frequently.
Several techniques are helpful in controlling Bermuda grass. Depending on the extent of coverage by the weed and on the site parameters a combination of the following manipulations may aid in controlling Bermuda grass: mowing and clipping, tilling and plowing, burning, shading, and chemical control.
MOWING AND CLIPPING: Removal of the aerial portion of perennial plants may slow the growth by limiting the accumulation of carbohydrates (Lorenzi and Jeffery 1987). The temperature, moisture and clipping frequency influence the amount of subsequent growth. Cutting Bermuda grass on hot, dry days has a much greater inhibitory effect than cutting on cool, moist days (Prine and Burton 1956, Jameson 1963). Weekly clippings at soil level during the moist season reduced yield by 50% whereas clipping during the dry season reduced the yield by 65% (Prine and Burton 1956). Monthly clipping of Bermuda grass reduced the amount of regrowth in the following year, whereas bi-weekly clipping from spring through winter resulted in the complete inhibition of regrowth the following year (Horowitz 1972c). In addition to reducing the regrowth of shoots, the initial clipping inhibited the formation of flowering stalks (Jameson 1963, Horowitz 1972c). The removal of aerial portions of the plant as a control measure is only efficient on small scale problem sites due to the labor and time intensity of the necessary frequent repeated clippings.
Eliminating aerial growth reduces the carbohydrate availability in the rhizomes. However understanding the annual carbohydrate cycle is not helpful since rhizome sprouting is not correlated with carbohydrate levels, thus clipping should proceed throughout the entire growing period (Horowitz 1972b). The depletion of carbohydrates is related to nitrogen concentrations. Nitrogen fertilizers decrease the amount of carbohydrate reserves; clipping increases the translocation of nitrogen from the soil to the roots and carbohydrate reserves are depleted during the above-ground utilization of nitrogen (White 1973, Skousen et al. 1989). Care must be taken to remove all aerial growth repeatedly throughout the growing season when clipping and mowing are used as a control measure.
TILLING AND PLOWING: Hand hoeing is practical only where the concentration of Bermuda grass is low. Shallow cultivation using sharp hoes, shovels, knives or hand pulling will remove the plants and rhizomes from the upper portion of the soil without dividing or pulling up deep rhizomes (Heathman et al. 1986, Lorenzi and Jeffery 1987). This technique is impractical in large scale infestations. Repeated plowing throughout the summer growing period will fragment the rhizomes and bring them to the surface; this will aid in the desiccation of the rhizomes and stolons (Burton and Hanna 1985). Small actively growing rhizome and stolon fragments are susceptible to drying within one week (Webb 1959, Horowitz 1972b). Hot, dry weather facilitates desiccation. An alternative to desiccation is freezing. Tilling in the winter will expose the rhizomes to freezing temperatures (Lorenzi and Jeffery 1987).
BURNING: Inconsistent results have been obtained on the effects of burning as a control for Bermuda grass. In general, if conducted at the correct time, burning will slow down the growth of perennial grasses (Lorenzi and Jeffery 1987). A late winter fire in Georgia drastically reduced the amount of Bermuda grass (Odum et al. 1973). However most burning experiments conducted in swampy areas result in the increase in abundance of Bermuda grass (Rensburg 1970). In wet areas, the rhizomes are protected from the heat of the fire. Due to the variable outcome of burning, this method is not recommended for controlling Cynodon dactylon.
SHADING: This plant requires high light intensity to thrive. With high levels of shade the plant can no longer grow; thus shading can be used as a control method (Kearney and Peebles 1951). Increased amounts of shade results in a decrease in the following: underground carbohydrate level, root weight, rhizome weight and herbage yield (Jameson 1963). Plants grown under 65% shade resulted in a 68% reduced yield (Burton et al. 1988). Results on the survival of Bermuda grass growing under trees indicate that the grass will die when completely shaded by closed canopies (Burton et al. 1988). Successful weed control resulted from the use of mats (bought at garden supply shops) which cover the ground and shade the Bermuda grass growing around the base of irrigated trees (Tiller pers. comm.). A thick layer of organic or inorganic mulch may provide adequate shading.
CHEMICAL CONTROL: Herbicides are helpful in controlling Bermuda grass. However, pre-emergence herbicides are not recommended. These reduce the competition of annual grasses allowing the rhizomes and stolons of Bermuda grass to thrive (Holm et al. 1977). An increase in the growth of Bermuda grass is seen in fields where pre-emergence herbicides are used to control annual weeds (Horowitz 1972a).
Glyphosate (commercial name -- Roundup, produced by Monsanto) is mildly toxic and decays rapidly in the soil. This foliar spray, which should not be used in galvanized steel sprayers, is absorbed in the leaves and translocated to growing regions throughout the plant (Kelly 1983, Ross 1986). Glyphosate, sprayed from helicopters, trailer sprayers or backpack sprayers, at a concentration of 2% will result in an 85% to 95% control after the first year (Hamilton pers. comm., Heathman pers. comm., Silberman pers. comm., Wildman pers. comm., Kelly 1983).
The best time to spray is when the carbohydrates are being translocated down to the rhizomes at the time of maximum rhizome growth. A downward movement of the herbicides most likely coincides with the spring and fall rhizome growth period (Rochecouste 1962b). Silberman (pers. comm.) and Brookbank (pers. comm.) recommend the fall spraying of herbicides for maximum effectiveness.
A successful restoration project, restoring cottonwoods and willows to a 40 acre Bermuda grass pasture in California, resulted in maximum control with glyphosate alone (Tiller pers. comm., Silberman pers. comm.). Combinations of herbicides and tilling were less effective. In September while the plants where in full bloom, one week before spraying 2% glyphosate from a boom-sprayer, the field was irrigate to encourage growth. A 95% control was seen after the single application. Trees were planted later, placed on a drip irrigation system and fertilized. Hand weeding and spot herbicide treatment continued for the following year. The greatest regrowth occurred around the irrigated trees. Four feet by four feet shade mats (mulch) were placed around the trees to reduce the sprouting of the remaining rhizomes.
Management Programs: The following people are involved in either actively managing or planning the management of Cynodon dactylon:
Val Little, Preserve Manager, Hassayampa River Preserve, The Nature Conservancy, Box 1162, Wickenburg, AZ 85358; (602) 684- 2772.
Oren Pollack, Stewardship Ecologist, California Regional Office, The Nature Conservancy, 785 Market St., 3rd Floor, San Francisco, CA 94103; (415) 777-0487.
Ron Tiller, Preserve Manager, Kern Preserve, The Nature Conservancy, P.O. Box 1662, Weldon, CA 93283; (619) 378-2531.
Monitoring Programs: Presently no formal monitoring programs of Bermuda grass are known. The following people "eyeball" the distribution of the grass:
Val Little, Preserve Manager, Hassayampa River Preserve, The Nature Conservancy, Box 1162, Wickenburg, AZ 85358; (602) 684- 2772.
Ron Tiller, Preserve Manager, Kern Reserve, The Nature Conservancy, P.O. Box 1662, Weldon, CA 93282; (619) 378-2531.
Jeff Weigel, Director of Stewardship, The Nature Conservancy, P.O. Box 1440, San Antonio, TX 78295-1440; (512) 224-8774.
Management Research Programs: No specific research on Bermuda grass control in natural plant communities is being conducted presently. However, ongoing research on the eradication of perennial weedy grasses with the use of newly synthesized herbicides in the agricultural milieu takes place in most of the southern land-grant universities (Hamilton pers. comm.).
Management Research Needs: An extensive amount of information on Cynodon dactylon is available. The genetics, life-cycle, environmental requirements, phenology, beneficial and deleterious characteristics, and control of Bermuda grass are all well documented. However, the information pertaining to controlling this species pertains to agricultural crop fields and not to the natural environment. Most of these techniques are not economically or practically feasible in a non-agricultural setting.
Information on controlling Bermuda grass in a natural setting is needed. Two major stages, not necessarily temporally separated, are essential for restoring the native flora: eradication of the weed and encouragement of native plants, preferably large, if appropriate. The extensive rhizome system essentially prevents the complete removal of Bermuda grass, thus once controlled, periodic manipulation of the weed is necessary. The early establishment of native plants which can shade the Bermuda grass is important to eradication, maintenance and re-vegetation.
Information on both optimal manipulation and native competitor establishment must be specific for the problem site. The temperature, precipitation, humidity and elevation will determine the optimal control technique.
Experimental plots should be employed for long term studies of various manipulation techniques. Controlled burning at various times of the year and assorted repetition cycles from single burns to yearly repeated burns should be analyzed (Cox pers. comm.). Various schedules of mowing, grazing, tilling, desiccating and herbicide applications should be studied at different locations. The percent coverage, timing of shading and types of shading material, such as shade cloth, shade mats, trees and other plants, should be studied in order to maximize the shade sensitivity of the species. The effects on carbohydrate reserves of nitrogen amendments to shaded plants should be analyzed. The interaction of several manipulation techniques should also be examined.
Rapid recovery of native vegetation, once most of the Bermuda grass has been removed, is essential in order to prevent invasion by other weeds or re-sprouting and establishment of the remaining Bermuda grass rhizomes. Studies to determine the optimal native species to be used and re-vegetation schedule to be followed must be conducted (Newman 1989). If appropriate, the establishment of large native plants will provide both a shading device and re- vegetation material.