European black alder (Alnus glutinosa)

a branch of Alnus glutinosa with leaves and flowers

 
 

European black alder is a medium-sized tree in the birch family that has been planted as a landscape plant. It is most adapted to moist soils, but can survive in a variety of soil types. It is a concern because it can form dense patches crowding out other species. It can change nitrogen and acidity levels in the soil which can negatively affect native plants.

Description

Appearance

European black alder is a medium-sized tree that can grow to 50-70 feet tall. It has shiny, round leaves with serrations.

Leaves and stem

Dark green leaves are oval to round in shape, have serrations along the edges, and are arranged alternately along the stem (come off the stem one at time at each leaf node). Leaves are 3-5 inches long and 2-3 inches wide. The tip of the leaf is rounded with a slight indentation or notch at the end. There are 7 or fewer pairs of parallel leaf veins.

The trunk is smooth when young.  The bark on mature trees has flattened plates separated by shallow cracks.

Flowers

European black alder has separate male and female flowers although both male and female flowers are present on the same individual. The male and female flower clusters are called “catkins.” The male catkins are cylindrical, often around 1.5 inches long, reddish-brown to yellow-brown in color and are found in clusters of 2-5 male catkins. The female catkins are also found in clusters of 2-5 catkins, but the female catkins look like small (about ¾ inch long) cones.

Seeds

Female European black alder plants produce seeds in structures that look like tiny pine cones. The “cones” release small seeds that are dispersed by the wind or spread by water.

Roots

The root system can be shallow, especially in wet areas. Plants can have root suckers which help plants to form thickets.

Biology

European black alder is a perennial tree. It has separate male and female flowers that are found on the same tree. It reproduces by seed. It prefers moist habitats but can grow in a variety of soil conditions.

Origin and spread

European black alder is native to Europe, western Asia, and northern Africa. It has been planted in the United States as landscape plant.

Refer to EDDMapS Distribution Maps for current distribution.

Don't be fooled by these look-alikes

  • Speckled alder, Alnus incana subsp. rugosa (native) – Speckled alder is generally a shrub with many stems while European black alder is generally a tree with a single trunk. European black alder leaves are more round in shape than speckled alder and the tips of the leaves are more rounded with a notched tip while speckled alder has a more pointed tip.

  • Green alder, Alnus viridis subsp. crispa (native) - Green alder is generally a shrub with many stems while European black alder is generally a tree with a single trunk. European black alder leaves are more round in shape than speckled alder and the tips of the leaves are more rounded with a notched tip while speckled alder has a more pointed tip.

  • Birch trees Betula species (native) – The female flowers European black alder become woody and look like tiny pine cones while the female flowers on birch trees are more elongate and do not become woody. Birch leaves have strongly pointed tips, while the tips of European black alder leaves are more rounded with notches.

 

Regulatory classification

This species is a Minnesota Department of Agriculture Restricted Noxious Weed meaning it is illegal to import, sell, or transport. 

Threat to Minnesota

  • Forms dense stands that can outcompete native species. Noted to be a concern especially along waterways and can grow in sun to shade. Its dense root structure can cause sediment accumulation and increase flooding risk.

  • European black alder can fix soil nitrogen, increasing soil nitrogen levels. It also alters soil acidity levels. Changes to soil can negatively affect native plants.

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. 

PlayCleanGo: Stop Invasive Species in Your Tracks

  • 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 hand pulling small plants. If plants are cut, they are likely to resprout and additional cutting would be needed. 

Herbicide control can be done using systemic herbicides, which are taken up by plants and move within the plant, killing leaves, stems, and roots. If using herbicides near water, use an herbicide labeled for aquatic use.

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) applied directly to the bark of European black alder 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

Reporting

Report new occurrences by submitting a report through EDDMapS, emailing Report A Pest, calling Report A Pest (1-888-545-6684), or contacting your local county agricultural inspector.

Resources

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