The Trouble With Backyard Buckthorn

Invasive buckthorn takes over natural areas and crowds out more desirable species. Here's how to battle back, beginning in your own back yard.

By John J. Moriarty

Looking out over the back yard of our new house, I saw a lush green woods. On closer inspection, I found that the green was almost all buckthorn--no small oaks, aspens, or other native trees. The wildflowers and ferns were small and widely scattered. I knew I had to do something to change the direction of my woods, or buckthorn would take over completely.

An all-buckthorn wood lot would be a boring place. The lack of diversity would limit the usefulness of the woods for wildlife, because of a lack of certain foods and cover. It would also be boring because you would see only green. There would be no blue, red, or yellow of wildflowers, nor gold, red, and yellow of fall leaves.

Buckthorn woods are also unfriendly because they are normally dense with many low branches. These branches have numerous thorns, making it unpleasant to try to walk through.

Lastly, I feel that native woodlands should have native species.

Buckthorns are nonnative shrubs or small trees that have become a problem in Minnesota's woodlands and wetlands, where they crowd out native plants. The problem is extensive in the metro area, the southeast, and many other parts of the state. Wood lots, trails, roadsides, and back yards can have heavy infestations.

Of the two species of exotic buckthorn that have become established in Minnesota, the European or common buckthorn (Rhamnus cathartica) is the most widespread and problematic. Originally imported as an ornamental, it has not been sold since the 1930s, when it was found to be a carrier of oat crown rust, an oat disease. Garden centers sell the glossy or columnar alder-buckthorn (R. frangula) in several varieties--including columnar and fernleaf--as a horticultural plant. Several Twin Cities nurseries have voluntarily stopped selling this tree because of its ability to escape cultivation. A problem in wetlands, glossy buckthorn will also spread into upland sites.

European buckthorn has small alternate simple leaves, black berries, and a flaky bark. The side branches have short terminal spurs, or thorns, which are literally a pain if you try to remove the plant.

Glossy buckthorn has leaves similar to those of European buckthorn, but it has smooth bark and no spurs. The fruits are less numerous than those of European buckthorn.

During the summer, buckthorns sometimes resemble a number of small trees, including wild plum and some cherry trees. Fall is the easiest time to identify buckthorn. The leaves of buckthorns do not change color until late in the fall, after most other trees have changed. The berries usually stay on the branches until March.

Many species of birds, especially robins and cedar waxwings, eat the plentiful blue-black berries and help spread them to other areas. Despite this effective means of spreading, more than 90 percent of the seeds land under the parent tree. The ground under mature buckthorn trees can have about 75 seeds per square foot. (In comparison, corn is planted at less than five seeds per square foot.)

High densities of seeds result in 15 to 20 seedlings per square foot or up to a half million per acre. Wood lots with this density of seedlings are a green carpet of buckthorn. At this level of infestation, not much else grows.

Early Intervention

Buckthorn in your back yard is most easily controlled before it spreads. If you have only one or two buckthorn trees, remove them right away.

A large area with many trees to control presents a bigger problem. If you have more than five acres of buckthorn, contact your local resource manager for advice. The Department of Agriculture, Department of Natural Resources, Minneapolis Park Board, and Hennepin Parks all have undertaken large-scale control programs on their properties.

Control of buckthorn in large heavily infested areas is not yet economically feasible. Several state and regional agencies are beginning to research control methods, including biological control, which uses other organisms to eliminate or reduce the problem species population.

Buckthorn Removal

Here are the basic steps for removing buckthorn on your property:

1. Cut down all buckthorn trees greater than 3 inches in diameter. Buckthorns of this size produce berries. A handsaw works well, but a chain saw is more efficient if you have many trees to remove.

When cutting trees, remember to keep track of the stumps. If stumps are not treated or removed, they will resprout vigorously. Within four years a stump sprout will grow big enough to flower and produce fruits.

2. Dig out the stumps or treat them with an herbicide. Of the several chemicals that work well, the two most common ones are glyphosate (Roundup) and triclopyr (Brush-B-Gon). Be sure to use chemicals properly, following all label instructions. If possible, look for an appropriate chemical product that comes with a spray nozzle. Wear protective clothing, such as rubber gloves and rubber boots.

If you don't want to use chemicals, you can dig the stumps out with a shovel or grub ax. I had to remove about two dozen trees to clear our garden. This can lead to some really tough calluses and is much more aerobic than spraying.

You can quickly dig out stumps of medium-sized trees (1 to 3 inches in diameter), but you can easily miss this size tree when cleaning out a large infested area. At home I wander around with my loppers and spray bottle a couple of times a year to catch any I have missed.

3. Pull up small trees (less than 1 inch) by hand. They do not have strong roots. Because small trees are also easily overlooked, they are a recurring problem, but one you can control with a little effort.

4. Remove seedlings with a hoe. By cutting down the seed-producing trees, you will eventually eliminate this problem.

5. Tell your neighbors about buckthorn. One of the best control methods is education. Remember, your neighbors' buckthorns will add seed to your side of the fence. Encourage your neighbors to remove their buckthorn, maybe offer to help, and share this Volunteer article; but do not remove anything without permission.

6. Decide what to do with buckthorn debris after removal. A 6-inch diameter buckthorn can be more than 20 feet tall. After removing trees, you could end up with a large pile of brush. If you have the space, you might want to make brush piles to attract a number of wildlife species. Another option is chipping the trees. I have used several large chip piles for my garden and trails. Many communities have brush pickup days: Contact your city to get more information.

7. Replace buckthorn with native species (see "Landscaping Without Buckthorn"). With buckthorns cut down, the character of the landscape will be changed. The understory will be much more open. Privacy hedges will have gaps.

Effective buckthorn control takes a lot of effort. But if many of us work on small areas, we can help keep the problem from spreading.

To learn more about using native trees and shrubs, consult a good book such as Landscaping for Wildlife by Carrol Henderson, available by calling 800-657-3757. Many garden centers offer advice on selection. Ask if the species spreads easily; aggressive plants can end up being a problem. For free fact sheets on growing native plants, contact the DNR Information Center.

John J. Moriarty is a wildlife biologist with Hennepin Parks. He has been battling buckthorn at work and at his home in the Twin Cities for several years. He wrote about moles and shrews in the January-February 1998 issue.

How Big Is Your Back Yard?

The big impact of a little buckthorn

Control of invasive species such as buckthorn costs Minnesota state parks, scientific and natural areas, and other natural lands tens of thousands of dollars every year. For example, in two years Myre-Big Island State Park spent more than $30,000 to control buckthorn, honeysuckle, and Siberian elm.

Does it make a difference at Afton State Park, say, if an invasive exotic such as European buckthorn is planted in your back yard? Yes. No park is an island (well, so to speak anyway). Along with the wind, birds are the primary transporters of berries and seed. Because of buckthorn's cathartic effect, a bird picking up a berry in LeRoy would not make it a hundred miles away to Afton before expelling that berry, but it could certainly deposit it a mile away in Lake Louise State Park.

European buckthorn infests most state parks and natural areas in or near cities. It was, and still is, planted as hedgerows and ornamental trees. Selecting plant materials native to your landscape region helps ensure that what gets picked up and carried away by a bird, an animal, or the wind will not result in the loss of a natural community down the road or across the state.

To volunteer to help with control efforts, or to get a copy of fact sheets on exotic species, call the DNR Information Center or visit the DNR web site.

--Kathryn E. Bolin

DNR Resource Management Specialist

Landscaping Without Buckthorn

Native shrubs add more color and beauty throughout the seasons

Red Cedar

An evergreen with dense foliage and sharp needles, red cedar has several different growth forms, from columnar to low and spreading. This tree can exceed 30 feet, but is normally less than 20. It does best in dry sites.

Northern white cedar

Also known as arborvitae, this evergreen has dense foliage. Many horticultural varieties of arborvitae are available, including columnar forms. The size depends on the variety. This tree grows well in wet or dry habitats.

Speckled alder

This multistemmed, upright species grows well in wet or dry areas, occasionally reaching a height of more than 20 feet. It is one of the few deciduous trees that has cones. Its cones and catkins add visual interest. Unfortunately, nurseries do not commonly stock this tree.

Dogwood

Most of the many species are multistemmed and good berry producers for wildlife. Red osier dogwood has bright red stems, which add color to the landscape in summer and winter.

Serviceberry. Also called Juneberry and amelanchier, this tree will grow larger than buckthorn and comes in single or multistemmed forms. A multistemmed tree can be pruned to form a dense hedge. Serviceberry has showy flowers and edible fruit, resembling blueberries.

High-bush cranberry

This is another multistemmed bush with lovely spring flowers and persistent red berries. Left unpruned, most varieties grow tall, but it can be pruned into a dense hedge. Several horticultural varieties are small and dense.

American hazelnut

This multistemmed bush can grow to be 8 feet tall and almost as wide. Its fruits are hazelnuts, also called filberts. They are edible, if you can beat the squirrels to them. The fruit husks are large and decorative.

Viburnums

There are a number of species and varieties, including nannyberry, arrowwood, and blackhaw. These range from bushy shrubs to small trees. All have white spring flowers and produce plentiful berries.

-- John J. Moriarty