European (Common) Buckthorn
Rhamnus cathartica
Description:
Shrub or small tree reaching height of 25' (7.6m); trunk diameter up to 10" (25cm); crown spreading and irregular; bark gray to brown, rough textured when mature and often confused with Prunus sp. Inner bark yellow. Twigs often tipped with a spine. Small black fruits up to 1/4" (0.6cm) diameter containing 3-4 seeds. Leaves broadly elliptic, rounded to pointed at the tip, and toothed. Upper and lower leaf surfaces smooth. Upper leaf surface dark glossy green. Leaves stay green late into fall, after all other leaves have fallen.
Concern:
R. cathartica was introduced to North America as an ornamental shrub. It is native to Eurasia. The fruit causes a severe laxative effect, quickly distributing the seeds through birds. Common buckthorn invades mainly woodlands and savannas, although it may also be found in prairies and open fields. Once established buckthorn crowds or shades out native shrubs and herbs, often completely obliterating them. European buckthorn control is also of interest to small grain producers; the shrub is an alternate host of the crown rust of oats. MN Interagency Exotic Species Task Force 1991 future threat ranking of SEVERE, current threat SEVERE.
The City of Lake St Croix Beach has a Buckthorn Weed Wrench for resident use. A deposit of $100 is required for a one week usage period.
Controls:
Late March to Early May: Prescribed fire is one method proposed for controlling buckthorn in natural areas. In the upper Midwest conduct burns as soon as leaf litter is dry; resprouts will be less vigorous due to low carbohydrate levels. Burning every year or every other year in established stands may be required for 5-6 years or more. Unfortunately buckthorn seedings often grow in low litter areas, unsuitable for prescribed fire more often than once every 3-4 years. In dense stands seedlings and saplings may be cut and dropped on site, creating fuel for future fires. Buckthorn seedlings appear to be very vulnerable to fire, perhaps due to their poorly established root structure. Fire will top kill a mature plant, but resprouting does occur. Uprooting of smaller seedlings with a weed wrench is another non-chemical control.
May to October: Excellent results are reported using 1part Garlon 3A (44.4% triclopyr) in 1 part water with dye on cut stumps. MN DNR Region V State Parks found a 1:4 dilution of the above also effective. Product label suggests avoiding the spring sap flow. Garlon 3A undiluted applied to cut stumps between first budding in May, through summer, to hard freeze in fall was 95% effective in preventing resprouting (Boudreau and Willson). Hennepin Co. MN parks report 1:1 Garlon 3A:water with antifreeze solution controls buckthorn during winter application.
Mid-August to October: MN DNR Region V State Parks Resource Management has been using a fall cut with immediate stump application of a 1 part Roundup (41% glyphosate) with 5 parts water solution using a hand sprayer. Initial checks indicate a >85% control overall. Kline, 1983 in Wisconsin, used a 1:1 water:Roundup solution on cut stumps in August/September with 100% control.
Winter: 1 part Garlon 4 (61.6% triclopyr) with 4 parts of an oil, such as Penevator, and dye on cut stumps was reported as very effective by Region V State Parks staff. Frill application is also effective. Experiments at the University of Wisconsin Arboretum report good results using a 12.5% active ingredient solution on cut stumps, or a 6% active ingredient solution basal bark treatment to stems <3 inches dbh. Pathfinder, a ready to use formula with 13.6% triclopyr also works.