You too can root out this invasive species

By Mary Maguire Lerman 

Photographs of the Mississippi River gorge in the Twin Cities prior to 1968 show slopes heavily covered by native shrubs. Common buckthorn was popular even then as a hedge, but it tended to mind its own business. Then in the late 1960s carbon dioxide levels rose sharply, and buckthorn, which is able to sequester carbon dioxide, suddenly became a very aggressive plant. By the mid 1970s, it had invaded woods throughout the metropolitan area.

Buckthorn is known as an invasive species, and it harms the natural environment in several ways. First of all, it secretes exudates from its roots, leaves, fruit and bark that suppress the growth of other plants. Second, songbirds build nests in its low branches that are easily reached by predators, leading to a decline in the songbird population.

Another thing about buckthorn is that it is shallow-rooted and will not stabilize slopes. When the slope below University of Minnesota Hospital collapsed several years ago, it may have been partly due to buckthorn. The slope in that area was covered by buckthorn with no understory plants, so when heavy rains came—along with runoff from the hospital—the entire slope collapsed onto West River Parkway.

Common buckthorn produces star-shaped clusters of chartreuse flowers in late May. The berries that develop from the flowers are green initially, but ripen to black in late summer. Each berry contains four seeds. When birds consume the berries, they have a severe purgative reaction and drop the purplish black excrement, leading to the spread of the invasive species.

Research has shown that birds that consume buckthorn berries can only fly about a third of a mile before emptying their intestinal tract of the buckthorn seeds. Those who live within a third of a mile of the river gorge or another natural area and have buckthorn in their yard are likely contributing to the spread of buckthorn.

Buckthorn also promotes the spread of soybean aphids, which damage soybean crops. In the summer of 2001, farmers in southern Minnesota reported damage to their soybean crops. The soybean aphid had come west from Wisconsin.

 

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buckthorn removal
A closeup of a common buckthorn hedge with ripened black berries.

Buckthorn is known as an invasive species, and it harms the natural environment in several ways.

One day an entomologist biking along the former East Calhoun Parkway in Minneapolis rode through a swarm of these insects. He stopped, pulled out a magnifying glass and identified them as soybean aphids. He wondered what soybean aphids were doing in the heart of the city. He did some checking and discovered that soybean aphids spend the winter as eggs on the buds of common buckthorn.

The aphids had come to the metro area to lay their eggs, and in their wake came the predatory Asian ladybugs. Those who have buckthorn in their yards are likely to see Asian ladybugs trying to get into their house in the fall. In the spring the soybean aphids hatch on the buckthorn, and the winged insects then fly to the soybean fields where they reproduce asexually.

What can be done?

How can you help stop this environmental damage? Check your yard twice each year for buckthorn seedlings that have sprouted. Look below where birds perch—under trees and below gutters or fences. Year-old buckthorn seedlings are easy to pull out by hand, provided the soil is moist. Pull gently on the stem twice and then pull hard and you can remove the entire seedling down to the root.

If you have buckthorn hedges or small trees of buckthorn (from 6-20 feet tall), they can be easily removed with the help of a tree service equipped with a large claw. Soak the soil beneath the buckthorn for two to three days beforehand to help ensure that it is removed down to the root. Then, add compost to the soil and replant with another hedge species if desired.

Shorter buckthorn can be cut down above the ground or dug out by the roots with the help of a weed wrench. If you cut it down, you have two options to prevent resprouting: Apply 25 percent glyphosate to the perimeter of the stump with a foam paintbrush (there is no need to paint the entire stump), or cut the buckthorn 4 inches or so above the ground, cover the stumps with a large metal can down to the ground, and with long nails secure the cans to the stumps and leave them in place for several years. If buckthorn suckers develop below the cans, remove them weekly.

For more information on buckthorn management, visit the Minnesota Department of Natural Resources website at tinyurl.com/y67p7u9s.

Writer Mary Maguire Lerman is a retired horticulturist for the city of Minneapolis Parks and Recreation Board and a past chair of the Minnesota State Horticultural Society.

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