Amur cork tree was commonly planted as a street tree or in yards, but has spread into forests and natural areas where it suppresses growth of native tree seedlings. This tree can be identified by its furrowed, corky bark.
Amur cork tree is a 35-50 foot tall tree with spongy bark.
Leaves and stem
This tree has compound leaves that can be 10-15 inches long and are arranged opposite each other on its twigs. Each leaf is made up of 5-11 leaflets. Leaflets are 2-4 inches long, have smooth edges, and smell of citrus or disinfectant when crushed. Leaves turn yellow-bronze in the fall. Mature trunks have gray to brown bark that is deeply furrowed. Branches and bark have a spongy, corky feel. If you cut into the bark, you can see distinctive bright yellow inner bark.
Amur cork tree has male and female flowers, which normally occur on separate plants. Only female flowers produce fruits containing seeds. This tree's green flowers bloom from May to June and are arranged in clusters.
Only female plants produce fruits, each containing one seed. Fruits are round and green, turning black in the fall as they mature. The drooping clusters of fruits stay on the branches into the winter. Birds can spread the seeds beyond where they fall. A single tree can produce thousands of seeds each year. These seeds can be viable in the soil for several years.
Amur cork tree has shallow, spreading, fibrous roots.
Amur cork tree is typically considered to be dioecious, meaning male and female flowers grow on separate plants and only the female plants produce fruits containing seeds. However, some varieties may grow flowers that contain both male and female parts, meaning they are able to produce seed without nearby trees of the opposite sex. Cork trees can grow in sun to shade, and can be found along roadsides, forest edges, forest openings, and unmanaged areas of urban parks.
Origin and spread
Amur cork tree was introduced to the United States in the mid-1800s from its native range in China and Japan. It was commonly planted in yards, parks, and along streets. These urban Amur cork trees produced seeds that spread to natural areas. Some states have banned the sale of all Amur cork trees, while others allow the planting of male plants only.
In Minnesota, Amur cork tree has been reported in isolated populations in natural areas in a handful of counties scattered across the state. Refer to EDDMapS distribution maps for current distribution.
Don't be fooled by these look-alikes
- Tree of heaven, Ailanthus altissima (invasive) – has leaves arranged alternately along the stem, unlike the opposite arrangement of Amur cork tree.
- Black walnut, Juglans nigra (native) - has leaves arranged alternately along the stem, unlike the opposite arrangement of Amur cork tree.
- Kentucky coffee tree, Gymnocladus dioicus (native) - has leaves arranged alternately along the stem, unlike the opposite arrangement of Amur cork tree.
- Smooth sumac, Rhus glabra (native) and staghorn sumac, Rhus typhina (native) – the leaflets of the sumacs are serrated while the leaflets of Amur corktree have smooth edges.
- Regulatory classification
Amur cork tree (Phellodendron amurense) is a Minnesota Department of Agriculture Specially Regulated Plant. Only sales of named male cultivars are permitted. Sales of all other Phellodendron amurense are prohibited. All existing planted and escaped fruit producing trees must be controlled, by tree removal or other means, such that no seed is disseminated.
- Threat to Minnesota
- Amur cork tree can grow in both sun and shade. It can grow densely in forests, limiting growth seedlings of native trees and displacing native shrubs and herbaceous plants.
- It releases chemicals into the soil (allelopathic) that can negatively impact soil microorganisms and nearby plants.
- Amur cork tree berries are less nutritious than nuts produced by native oaks and hickories that it displaces, which can harm native wildlife.
- What you should do
One way that invasive plant seeds and fragments can spread is in soil. Sometimes plants are planted purposefully. You can prevent the spread of invasive plants.
- REMOVE plants, animals and mud from boots, gear, pets and vehicles.
- CLEAN your gear before entering and leaving the recreation site.
- STAY on designated roads and trails.
- PLANT non-invasive species.
- Native substitutes
- Control methods
Mechanical control can be done by digging or pulling small plants. Girdling of trees can also be effective. In this technique, a 3-4 inch strip of bark is removed all the way around the tree in late spring to mid-summer. Mowing or cutting can be used to prevent trees from reaching maturity and producing seeds, but the stumps will continue to resprout.
Herbicide control can be done using systemic herbicides, which are taken up by plants and move within the plant, killing leaves, stems, and roots. Cut plants will resprout if the stump is not treated with herbicide after cutting. Immediately after cutting (within 2 hours), apply an herbicide containing triclopyr (Garlon 3A/Vastlan, Garlon 4, or other brush killers with triclopyr) or glyphosate to the cut stump to prevent re-sprouting. Always follow label instructions for herbicides. Herbicides can be applied to cut stumps with a paintbrush, wick applicator such as a dauber or "buckthorn blaster," or a low volume sprayer. In cases where more than a few plants are treated, add an indicator dye (available where pesticides are sold), such as Mark-It Blue, to the herbicide to mark which cut stumps you have sprayed.
For basal stem treatment, a method that applies chemical through the bark, low volume spray applications can be made with Garlon 4, Pathfinder II and similar oil-based products. This application method uses triclopyr ester mixed with an oil diluent (i.e. Bark Oil Blue, kerosene) applied directly to the bark of Amur cork tree from the root collar up about 12-18 inches. An ultra-low volume spray wand should be used to minimize herbicide use and reduce the potential for non-target injury.
Herbicide treatments are less effective in early spring. Summer and fall are better times for herbicide treatment with water-based herbicides such as glyphosate or triclopyr ester. Oil based herbicide such as triclopyr ester are effective during summer and fall as well as during the winter when temperatures are below freezing.
This species is unregulated, but you can add to the public information about this species by reporting new occurrences through EDDMapS.
- Identification and management of Amur corktree (Minnesota Department of Agriculture)
- Identification and management of Minnesota Noxious Weeds (Minnesota Department of Transportation)
- Identification and management recommendations (Woody Invasives of the Great Lakes Collaborative)
- Identification and natural history of Amur cork tree (University of Minnesota Extension)
- Woody vegetation control (University of Minnesota Extension)