Nitrogen Fixer Fixin’ For Trouble

Contact: Francis Smith, Maryland Department of Natural Resources- Forest Service | francis.smith@maryland.gov

Leaf top and underside. James H. Miller, USDA Forest Service, Bugwood.org
Berry clusters. Leslie J. Mehrhoff, University of Connecticut, Bugwood.org

ANNAPOLIS MD (August 1, 2018) – Prolific seed production. Rapid re-sprouting. Highly adaptable. Three traits of a very successful plant, and also what makes autumn olive an ecological nightmare. As summer winds down and fall creeps in, the red berries of this invasive shrub will begin to ripen, so it seems fitting to feature it as the Maryland Invasive Species Council’s August Invader of the Month.

The shiny silver-white underside of autumn olive leaves can be spotted in a range of growing conditions across the state, from streamside forests in Western Maryland to hedge rows lining farms along the Eastern Shore’s Route 213. Also known as Japanese silverberry or oleaster, it was introduced from Asia in 1830 as an ornamental plant and cultivated for wildlife habitat, farm windbreaks, erosion control, and restoring degraded lands. To date, it has been found in 32 states as well as Canada – stretching from Ontario south to Florida, along the Gulf Coast to Arkansas and Louisiana, and west to Wisconsin to Kansas. Whether floodplain, farm field, or roadside vacant lot, the list of habitats this pervasive invasive can colonize is extensive. It is tolerant of drought and able to grow in poor nutrient-depleted soils, making it even more of a biological menace.

Autumn olive can grow as a shrub or a small tree, reaching up to 20 feet tall and 30 feet wide. Its leaves are oval shaped with a smooth, often wavy edge, and are arranged alternately along the stem. Leaf size varies, but they can be 3 inches long and over an inch wide. Stems are thorny with a speckled silvery or golden brown appearance. Aside from the dark, dull green upper side of the leaves, silvery to rusty colored scales adorn the stems, buds, and undersides of leaves. Clusters of 5-10 aromatic cream to yellow colored flowers bloom in spring, each with 4 petals pointed at the tips. The flowers will be replaced by small brown-dotted red berries from September into November. This one-seeded fruit is produced after 3 years, with a mature autumn olive capable of producing anywhere from 8 to 30 pounds of fruit annually.  

Known as drupes, the abundant olive berries are a form of seed dispersal for the plant, as birds and some mammals will feast on the juicy red treats and spread seeds throughout the landscape. With already high germination rates, the plant can also spread quite rapidly via vegetative propagation, meaning it readily re-sprouts after being cut or burned.

Autumn olive forms a very dense understory and outcompetes native species, while hindering germination of native plant seeds. Autumn olive leafs out earlier in the growing season than most native plants, and holds onto its leaves later into the fall, giving it yet another advantage. A long-term impact of this invasive is its roots’ ability to pull nitrogen from the air into the soil, known as nitrogen fixation. Due to these nitrogen-fixing capabilities, autumn olive can alter soil chemistry and the nitrogen cycle in a local ecosystem while limiting the native plant community and reducing wildlife cover diversity over time. As streamside forests are established to limit nutrients like nitrogen from washing into our waters, colonization of these woodlands by our Invader of the Month could become a new source of nutrient runoff within these riparian forest buffers.

The most effective treatment for autumn olive removal is applying glyphosate to a cut stump in late summer. Foliar and basal spray methods are also available, but using a non-selective herbicide like glyphosate puts other surrounding vegetation at risk. Triclopyr can be considered to avoid non-target impacts. Young seedlings may be hand pulled only when soil is moist and all root fragments can be removed. An organic approach to consider for controlling the spread of autumn olive is to eat the vitamin-packed berries. Numerous recipes can be found online, and we can improve our health while improving our environment at the same time.

One silver lining to this silvery shrub invader: lycopene. Lycopene is found in autumn olive berries at almost 20 times the amount that’s in a tomato. This antioxidant is thought to have potential for repairing our damaged cells, possibly protecting against certain cancers and heart disease. Health studies looking at diets rich in red fruits like tomatoes and watermelon have had mixed results, but considering the exponential difference in the amount of lycopene found in autumn olive berries, this problematic invasive may one day be a medical solution.      

Full shrub view. Chris Evans, University of Illinois, Bugwood.org
Flower clusters. Leslie J. Mehrhoff, University of Connecticut, Bugwood.org

For more information:

USDA National Invasive Species Information Center

Penn State Extension, Autumn Olive

Autumn olive: Weed or new cash crop?

Autumn Olive Jam Recipe