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Control of the Invasive Exotic Rhamnus cathartica in Temperate North American Prairies.. Page 5 of 11 and early May as low carbohydrate levels should reduce re- sprouting vigor (Dziuk 1998). Considering the time, energy, and safety considerations involved, it is usually only larger restorations relatively far away from human habitation that employ fire to control R. cathartica (Moriarty 2000). One impediment to conducting a burn is that R. cathartica usually shades out understory species, reducing the amount of leaf litter found in the area and consequently limiting the ability to employ fire as an effective means of control (Dziuk 1998). A sufficient accumulation of dead plant material is necessary to move the fire from one place to another. Chemical There is much information available about herbicides that are often used to impair the growth and development of R. cathartica. Restoration practitioners and interested members of the public should carefully read all manufacturers' directions prior to using a herbicide and use proper safety equipment during application. The best time of year to use herbicides on target plants is just after the native grasses and wildflowers have gone dormant (Boudreau and Wilson 1992). Herbicide applications may be done early in the season just after the trees have leafed out and before the trees have begun to bud, but those conducted in the fall or early winter appear to be the most effective (Aho 2000, Bohnen 2000, Solecki 1997). Applications conducted at temperatures below zero degrees Celsius (32 degrees Fahrenheit) will likely experience reduced effectiveness (Solecki 1997). Many of these chemicals such as Garcon and Roundup are unrestricted in their usage and can be purchased without a special license (Apfelbaum 1984). Table 1 is a list of the most thoroughly tested and commonly used herbicides available along with their potential impact on the surrounding environment. Contol Method Efficacy There is no clear consensus on the most effective means of control, but the application of herbicides to newly cut stumps seems to be an emerging preference. Conversations with several local restoration practitioners add further insight into the usage of herbicides. Mike Aho, a naturalist at Sibley State Park (located 150 km northwest of Minneapolis, MN near the town of New London), has found basal bark and cut stump application of Garlon 4 to be a reliable means of controlling R. cathartica (Aho 2000). Brush application in the fall, as opposed to spraying, resulted in no non - target plant mortality. In the initial phase of eliminating the mature trees capable of reproduction, chemical application to stumps yielded 100 percent mortality while basal bark treatment yielded slightly less efficacy. Some of the trees with a basal diameter greater than seven cm did not fully succumb until the second year after application. Garlon 3A, Tordon RTU, and Roundup have also been used by Aho in the past. Despite the high efficacy achieved, Garlon 3A has greater persistence because of its amine salt component in the environment and the higher user risk. In addition, painful blistering immediately ensues following contact with even a drop of the solution. Aho found application of Tordon RTU to stumps in the winter produced good results as well. So although year round usage is an advantage, Tordon's potential as a major groundwater contaminant renders it less desirable (Dziuk 1998). His use of Roundup yielded the least reliable results. Some stumps that received an application of the manufacturer recommended medium strength (30 -50 percent) solution managed to re- sprout. Field results obtained by Julia Bohnen, a University of Minnesota Landscape Arboretum restoration manager, contradict those found above (the arboretum is located in Chanhassen, MN, about 30 km west of Minneapolis). Bohnen (2000) achieved excellent results with late summer to early winter applications of 50 percent strength Roundup to cut stumps. Late summer to early winter treatment with a basic utility-grade spray bottle, available at any hardware store, succeeded in confining the application to R. cathartica and limiting non - target plant mortality. Achieving desired results with a herbicide that is relatively harmless to the environment is indeed attractive. She has also used the foliar treatment herbicide, Krenite S, to control seedlings and young saplings and, similar to results obtained by other practitioners, achieved a very low efficacy. Likewise John Moriarty, currently a restoration manager for Ramsey County Parks in St. Paul, MN, has also observed low efficacy of Krenite S. Its past use in a secondary herbicide application at Highland Park in Bloomington, MN, a southern suburb of Minneapolis, proved totally ineffective (Moriarty 2000). Regarding Roundup, Moriarty has also found it to be a less reliable means of control, especially at lower strengths. Instead he highly recommends the use of Garlon 3A, or more specifically its weaker commercial version, http: / /www.hort. agri. umn.edu /h5015 /00papers /gale.htm 2/15/01