Mystery of the Month - November

A dark green understory in late fall is a tell-tale sign of buckthorn.

By Carole Gernes, Ramsey Conservation District, Cooperative Weed Management Area Program Coordinator

Driving along scenic routes to check fall colors this year was incredible! Clear days and crisp nights with minimal rain kept the orange, yellow and red leaves visible for a beautiful extended fall. Did you notice areas where the understory would just not turn color? A dark green band of shrubs or small trees in the understory could be bad news – BUCKTHORN!




Woods in Battle Creek Park 1960.  Note the absence of buckthorn in the
understory area on the hillside.

Some of us remember what our forests looked like before the buckthorn invasion. Many of us growing up in the metro area can’t remember seeing woods without buckthorn, or have never experienced a woods without buckthorn. Not that long ago, fall offered unobstructed views of the leafy forest floor, around corners and outlines of hillsides and picturesque landscapes.

 
 Unfortunately, two invasive buckthorn species now call Minnesota “home.” Common or European buckthorn, Rhamnus cathartica, and glossy buckthorn, Frangula alnus, are not species we can eradicate from the state, but there is good news. More and more information has come to light about steps we can take to keep these pests under control.

 

Buckthorn Busting Basics

 

What is buckthorn and what does it look like? Common buckthorn is a chameleon of the shrub world. Here are some tips to help you identify it:

• The leaves can vary in size, but always have a few curving (not straight) veins and rounded teeth on the edge of the leaves 
• Leaves and buds can vary in their arrangement on branches. Some may be opposite (straight across from each other), sub opposite (almost straight across from each other) or alternate (staggered). Common buckthorn is unique in our area. Sometimes all three of these arrangements may be found on the same plant. 
• Buds are dark brown to black and hug the twig. When opposite from each other the two buds combined resemble the bottom of a deer hoof. 
• Often, there is a short sharp thorn at the tip of the twig. These may fall off or continue to grow over the years. 
• In the fall, common buckthorn leaves stay green and on the shrub, long after other trees and shrubs have turned color or lost their leaves. Waiting until fall colors are strong will help prevent accidental removal of beneficial shrubs.

Common buckthorn. Left: How common buckthorn got its name: when buds are opposite they resemble the bottom of a deer hoof. Middle: On the same branch we may also see alternate leaves or buds. Right: A small thorn is sometimes present on common buckthorn branch tips.

The Minnesota DNR has a great brochure to help you identify both species of invasive buckthorn:

http://files.dnr.state.mn.us/natural_resources/invasives/terrestrialplants/woodyplants/buckthorn_what_you_should_know.pdf

Are there beneficial plants that look like buckthorn? There are beneficial native shrubs and tree saplings that can be mistaken for common buckthorn, such as dogwoods and cherries.

• Dogwood leaves have curving veins, like common buckthorn. However, the leaves never have serrated edges. 
• Cherry and plum leaves do have serrated edges, but the “teeth” are sharp and not rounded. The leaf veins are more numerous and not curved like those on common buckthorn leaves. 
• Cherries and plums may have small, comma-shaped insect galls that grow up from the top of the leaf surface. If you see these, the plant is not a buckthorn. 
• Cherries may also have a black knot fungus growing on the branches. This fungus does not grow on buckthorn. 
• Cherries and plum leaves and buds are always in the alternate, staggered arrangement.

 

Glossy buckthorn is more difficult to identify. Its leaves will turn color and fall like our native trees. Identifying it may take a bit of practice or assistance. Be aware; glossy buckthorn saplings have white spots on smooth reddish bark, just like cherry saplings. Fortunately, the leaves do not have teeth like cherries, are widest toward the end and attach to the branch with pinkish petioles. Do not remove unless you are 100% sure it is glossy buckthorn.

Glossy buckthorn, Frangula alnus.

How can I protect native trees and shrubs? If you aren’t certain whether a shrub is buckthorn, do not remove it. Wait until fall when identification is easiest. When removing, flag cherries, plums, dogwoods and native tree saplings so you don’t accidentally remove them.

How can I get rid of buckthorn? Are some methods better than others?

• Pulling is no longer recommended. Although people have pulled buckthorn for many years, the Minnesota DNR Trails managers and others have noticed quick colonization by garlic mustard and other invasive plants/weeds after pulling. They no longer pull buckthorn. University of Minnesota researchers have confirmed this observation with a formal study, http://www.mipn.org/UMISC-2014/Tuesday/InvasivePlantEcology_Roth_Tues_210pm.pdf. Although plants fill in the void that pulling buckthorn leaves behind, areas where soil has been disturbed are more likely to be invaded with new weeds and buckthorn seedlings in following years. Using herbicides to remove buckthorn leads to quicker establishment of a rich native plant understory.  
• Cutting and treating stumps with specific herbicides is less likely to be followed by new invasions. Be sure to check the concentration of active ingredient in the lower right corner of the front herbicide label. Triclopyr should be 8 or 8.8% active ingredient. Glyphosate should be at least 25%. 
• Follow the label directions, with one exception: do not drill holes in the stump and pour in herbicide. Applying herbicide to the orange/yellow layer of fiber, just under the thin bark is sufficient to kill the plant. Buckthorn has very dense, hard wood. Herbicide poured into holes in the heartwood will not move to the root to kill the shrub. 
• Leaving the stumps and roots in place will also prevent soil erosion until new plantings become established.

The U of MN's buckthorn research and the DNR recommend not using buckthorn wrenches because the soil is more easily disturbed and is likely to be invaded with new weeds and buckthorn seedlings.

What should I plant after my buckthorn is gone? Areas that were infested with buckthorn may be bare after removal. There are many attractive, native shrubs and perennials which provide screening for privacy, nectar for pollinators and add interest to your landscape. 

• RWMWD Checklist of Common Invasive Plant Species and Replacements Brochure [link here]
• Information on planting to help support pollinators can be found on the Minnesota Board of Soil and Water Resources website: http://www.bwsr.state.mn.us/practices/pollinator/pollinator-tool4.pdf . Avoid planting shrubs or perennials that have been treated with systemic insecticides. 
• Most non-native, ornamental plants aren’t invasive, but it doesn’t hurt to be pro-active. If you are interested in planting a non-native plant, check online to make sure it hasn't become invasive in other areas of the U.S. A good source of information is http://www.invasive.org/weedcd/. 
• Cost share programs for shoreline or native plant restorations may be available to help. Contact your local watershed district or county conservation district for more information. If your site is in Ramsey-Washington Metro Watershed District, check out our BMP Incentive Program at this link on our website.