By Matt Russell and Gary Wyatt et.al
Buckthorn is one of Minnesota’s most damaging invasive plants. Landowners should be concerned if buckthorn is present in their woodlands because it is an aggressive invasive plant that outcompetes native vegetation and degrades wildlife habitat. Soybean growers should be concerned if buckhorn is present in nearby wooded areas because it serves as the overwintering host plant for soybean aphid eggs and the crown rust fungus.
Understanding the basics of buckthorn biology will help you to control its spread so that you have a healthy woodland and crop field. Despite the invasive nature of buckthorn, many landowners have had success in controlling it, but only after choosing the appropriate management techniques along with consistent follow-up treatments.
What is buckthorn?
Buckthorn may refer to one of two different species found in Minnesota: Common (or European) buckthorn (Rhamnus cathartica) or glossy buckthorn (Frangula alnus). The species arrived in Minnesota from Europe in the 1850’s and was planted as an ornamental plant typically used in hedges. Today, both species are listed as restricted noxious weeds in Minnesota. The sale, transport, or movement of buckthorn is prohibited statewide.
Buckthorn is a shrub or small tree that can reach 25 feet tall and takes an oval form. They are noticeable in woodlands because they can form dense thickets where few other plant species will exist in the understory.
The key identifiable feature of buckthorn is the “buck hoof print” that can be seen at the end of the twig. This hoof print is formed by two terminal buds and a thorn going down the middle. Buckthorn leaves are some of the first leaves to appear in the spring and the last to drop their leaves in the fall, a common trait among many woody invasive plants.
Where is buckthorn in Minnesota?
Common buckthorn is found on dry and moist sites, while glossy buckthorn is typically found on wet sites. Both species can grow either in full sun or deep shade.
Common buckthorn is widely distributed across central and southeastern Minnesota, with northern populations extending along the North Shore of Lake Superior and into northwestern Minnesota (Figure 1, left).
Glossy buckthorn has a narrower distribution compared to common buckthorn. It is abundant in the central and eastern portions of Minnesota (Figure 2, right):
Who is managing buckthorn?
Buckthorn is currently being managed by woodland owners, crop growers, and natural resource professionals. In a 2018 survey on invasive plants across Minnesota, common buckthorn was the most frequently reported invasive plant for both private landowners and public land professionals working across forest and agricultural settings.
In a recent online workshop on invasive plants offered by UMN Extension and the MN Department of Agriculture, agricultural and natural resource professionals were presented with a hypothetical budget of one million dollars to spend on invasive plant management. Professionals indicated they would spend over $400,000, or 40% of their entire budget, to control the two buckthorn species alone. This is a reflection of how professionals are eager to minimize the damage and control the spread of buckthorn.
Many organizations may be able to assist in buckthorn management. Your local Soil and Water Conservation District (SWCD) office may be aware of cost share opportunities through programs in the Natural Resources Conservation Service. Landowners with woodlands may be eligible for state-level cost-share assistance through the MN Department of Natural Resources.
How do you control buckthorn?
A number of chemical and non-chemical control methods are available for buckthorn. When controlling buckthorn, female plants should be targeted first. This is because female buckthorn plants produce the seeds from berries that are widely dispersed by birds and other wildlife.
In areas where buckthorn is not yet dense, small seedlings and trees can be pulled by hand or with tools such as a weed wrench.
In areas where buckthorn has formed a dense understory, mowing may be one option to reduce vegetation and evaluate re-growth. After mowing, resprouts and new growth will occur, so monitor the area and conduct follow up treatments as needed. Combining mowing with chemical treatments can also be effective.
Controlled burning may be an option on grassland or savanna sites. Landowners have reported moderate success with using burning to control buckthorn. Burning will need to occur every two to three years to be an effective management technique.
The use of goats and other livestock has become more popular in recent years for controlling buckthorn. Goats will graze buckthorn and other vegetation, which can help control the invasive plant. Research on using goats to control buckthorn is still emerging that investigates the long-term effects of grazing on buckthorn populations and what vegetation is replacing it.
Large diameter buckthorn stems can be cut at the stump with a chainsaw, brush cutter or other hand tool. After cutting, cover the stump with a tin can or black plastic to prevent future sprouting.
Following a cut stump treatment with brush herbicide can be effective for larger-diameter buckthorn stems. Herbicides containing glyphosate or triclopyr are recommended for buckthorn control. Apply the herbicide on the stump with a paintbrush, dauber, or low volume sprayer by covering an inch in from the edge of the outer bark. The center of the stump does not need to be treated.
Glyphosate is a non-selective herbicide, while triclopyr has a water- or oil-based formulation. The oil-based formulation works well for cut stump treatments.
Herbicide can also be applied directly to the bark using a basal bark treatment. This treatment works well for trees up to 5 inches in diameter. From the ground level up to 18 inches above the ground, wet the area with a low-volume sprayer.
Foliar applications are effective for smaller buckthorn plants. Spray buckthorn leaves until wet. The water-based formulation of triclopyr works well for foliar applications.
Follow label directions when using herbicides, wear recommended protective clothing, and avoid contact with non-target plants. Consider re-establishing native plants in areas previously dominated by buckthorn.
Cost and effectiveness of buckthorn treatments
In a 2018 survey on invasive plant management in Minnesota, public land professionals reported average costs per acre for both mechanical removal (e.g., by mowing) and herbicide treatments of buckthorn to be around $200 per acre. Manual removal (e.g., by hand pulling) of buckthorn was reported to cost over $600 per acre.
In that same survey, both public land professionals and private landowners reported on the effectiveness of different treatments for controlling common buckthorn:
- 72% of respondents said that herbicide treatments were extremely or very effective for controlling buckthorn.
- 50% of respondents said that manual removal treatments were extremely or very effective for controlling buckthorn.
- 23% of respondents said that mechanical removal treatments were extremely or very effective for controlling buckthorn.
When is the best time of year to control buckthorn?
Buckthorn control treatments can occur at any point in the year, but a few specific times tend to be optimal for the best results. Late summer and throughout the fall is the best time to cut and chemically treat buckthorn stumps. Foliar applications of herbicide on buckthorn are well suited in October after native foliage has gone dormant.
If using chemical treatments in the fall or winter, follow herbicide label instructions for the appropriate temperatures in which to apply chemicals. Oil-based herbicides are usually best for fall and winter applications.
Buckthorn is one of Minnesota’s most ecologically and economically damaging invasive plants. As a noxious weed that outcompetes native vegetation in woodlands and serves as an overwinter host for soybean aphid, controlling buckthorn can provide a healthier woodland and improved soybean yields. A variety of chemical and non-chemical treatments are available to control buckthorn, each with varying effectiveness and costs.
The right treatments will depend on the severity of the buckthorn infestation and time and resources available. Buckthorn seeds can persist in the soil for up to five years, so monitor the affected area annually after initial treatment. Conduct follow-up treatments if needed.Source : umn.edu