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Control of the Invasive Exotic Rhamnus cathartica in Temperate North American Prairies.. Page 4 of 11 tree species and crowding out the native herbaceous species found in the understory (e.g., Thalictrum dioicum, T dasycarpum, Sanguineria canadensis, Trillium grandiflorum, Maiaenthemum canadense, Trientalis borealis, Uvularia grand j7ora, Polygonatum commutatum), R. cathartica has altered in a short period of time the vegetation composition of those forest communities it has invaded. Besides the primary concern for the degradation of natural areas and urban landscapes, R. cathartica poses an additional threat when proximate to agricultural fields. It is the alternate host of the crown rust fungus, Puccinia coronata, which, under wet conditions, can severely reduce oat field yields (Munkvold 1996). Control Methods Due to its prolific nature, R. cathartica is a difficult exotic species to eradicate. Several types of control are currently being used and tested in areas affected. They include mechanical, physical (use of fire), and chemical methods of control. There is no clear consensus on the most effective means of control, but herbicide application to a newly cut stump seems to be an emerging preference. These methods must often be employed repetitively because of R. cathartica's persistent re- sprouting ability. The capacity for extended dormancy in R. cathartica seeds, an average of 6 years, also necessitates the use of repetitive treatments (Archibold et al. 1997). In most restoration efforts, a combination of at least two of these methods is utilized. Mechanical The most environmental friendly and time - consuming method of R cathartica removal is via mechanical means (MNDNR 2000). If resources are limited, initial efforts should be concentrated on removing the female, or fruit producing, trees. This will at least minimize addition to the existing seed bank, thus reducing the amount of time spent in the future on seedling removal. Removal of trees with a base diameter of up to seven cm can be accomplished with commercially available jaw -type pullers known as "weed wrenches" or "broom pulls" (Ness 1989). Those individuals with a base diameter larger than seven cm will likely have to be cut with a chain saw or hand saw and the remaining stump dug out with a shovel. If the stump is left in the ground then subsequent visits to remove any re- sprouting will be necessary. Seedlings that are less than 1 meter (m) tall can usually be pulled by hand. Pulling trees up from the roots will prevent them from re- sprouting and avoid the need for further treatment. Hand pulling is most effective immediately after significant rainfall when the ground is well saturated. Wet soil allows for most of the root to be removed with minimum effort (Rebuffoni 1992). Safety measures should always be taken to avoid injury when removing the trees. Experts advise using safety goggles and thick work gloves to protect against the thorns (Rebuffoni 1997). In areas where there are large numbers of seedlings, mowing can reduce plant number and vigor (USGS 2000). Mowing is done in early June and late August for three consecutive years and usually results in a reduction in stem height and numbers. Unfortunately, mowing can also impair the development of native forb species as well. In addition, mowing will not remove R. cathartica from the area; it will only keep populations under control and prevent the trees from producing fruit. Any time fruit bearing trees are cut after the fruit has ripened, the seeds should be removed from the branches and destroyed. Fruit that is allowed to remain on the tree has the potential to be eaten by animals or drop and sprout. Burning or simply putting the seeds in a sealed trash bag can remove potential buckthorn from the system. Specific instances of composting being used as a means of fruit disposal were not found for this paper, but it would probably be a good idea to try this method and place some of the resulting compost in gardening trays or pots to see if any R. cathartica seedlings are produced. Physical Prescribed or controlled burning is used to eradicate populations of R.cathartica in areas adapted to fire (Kline 1981). Areas that are suited to fire management are those that contain fire tolerant natives (e.g., older Quercus rubra and Q. macrocarpa with thick, fire retardant bark) have well- drained soils, and have enough litter under the trees to fuel the fire. A successful burn consumes the brushy understory, leaving the ground mostly bare with the larger fire resistant trees left intact. Controlled burns usually have the biggest impact on R. cathartica seedlings and the current year's seeds. The best time to conduct burns is between late March http: / /www.hort. agri. umn.edu /h5015 /00papers /gale.htm 2/15/01