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Invasive Plant Management

Invasive plants threaten native habitat, biodiversity, and eco-system health. Invasive plants are non-native plants that have been introduced from other eco-systems, become weedy, and spread out of control. Invasive plants often lack natural enemies and can flourish in areas disturbed by human activity, including trail-sides, road-sides, and logging cuts.

Garlic Mustard Rosette - Chris Evans
Garlic Mustard Rosette - Chris Evans

Invasive plants threaten native habitat, biodiversity and ecosystem health. Invasive plants are non-native plants that have been introduced from other ecosystems, become weedy, and spread out of control. They often lack natural enemies and can flourish in areas disturbed by human activity, including trail-sides, road-sides and logging cuts.

Once introduced, invasive plants can grow into monocultures, crowding out diverse native plants. This reduction in plant variety means less food, less often for native wildlife. Cover and nesting habitat can be lost as well. Invasive plants often have shallow root structures, which can contribute to erosion and degraded water quality in streams and rivers.

To learn more about invasives, come to our monthly work party: First Saturday of each month at 10 a.m. Check in: Portland Audubon parking lot (5151 NW Cornell Rd, Portland, 97210). Our efforts generally focus on controlling English Ivy, Garlic Mustard, English Holly, Japanese Knowtweed, Himalayan Blackberry, and several invasive geranium species.

You can also learn more about invasive plants through the following links:

On the Lookout for Garlic Mustard

By Tom Costello

In the past two years we have made significant strides in our efforts to remove invasive plants form our sanctuaries. Grants from Metro and the Bureau of Environmental Services allowed us to bring in AmeriCorps field teams for 45 days, many of which were spent removing English Ivy and Himalayan Blackberry. Our new management partnership with Metro for the Collins Sanctuary has brought in contract crews to perform several complete sweeps for Ivy, Clematis, Laurel, and Holly. And three consecutive years of funding from National Audubon’s Together Green program has allowed us to host large-scale work parties per year to primarily target invasive species removal.

This influx of resources has allowed us to get ahead of long-standing infestations, but our efforts have been distracted by the arrival of a new and insidious invasive, Garlic Mustard (Alliaria petiolata). Garlic Mustard was first seen in the United States in 1868 in Long Island, New York, and infestations have primarily been limited to the northeastern part of the country. Unfortunately, about 20 years ago an infestation managed to pop up in east Multnomah County, at the western end of the Columbia Gorge. It has only been in the past 5 years that populations have been reported throughout the Portland area, and we are now starting to see infestations throughout the state.

While natural area managers and public agencies have been on high alert for the past three years, Garlic Mustard is continuing to spread. Even with monthly sweeps along the Cornell Road corridor and annual spraying by the city and county, this plant continues to be a problem.

Garlic Mustard Flower - Linda Haugen
Garlic Mustard Flower - Linda Haugen
What does it look like?

Garlic Mustard (Alliaria petiolata) is a biennial, meaning each plant lives its life over two growing seasons. Seedlings emerge in early March, forming a rosette of leaves the first year. The leaves are alternate, triangular to heart shaped, have scalloped edges and give off an odor of garlic when crushed. The odor can be used to distinguish garlic mustard from native plants like Evergreen Violet (Viola sempervirens), Piggy-back Plant (Tolmiea menziesii), Fringecup (Tellima grandiflora) and non-native plants such as silver Dollar Plant (Lunaria annua). Garlic Mustard also has a distinct "s" shaped curve at the base of the stem. Garlic Mustard flowers during the second year of growth. In March and April of the second year, plants send up a flower stalk from 12 to 48 inches tall, topped with a cluster of white, four petaled flowers. The seeds form in narrow, green seed pods that originate from the center of the flowers and turn brown as the seed matures. The plant dies after producing seed and the brown, dried out stem with the brown seed pods remain through winter.

Why is it so hard to eliminate?

Garlic Mustard Seed - Chris Evans
Garlic Mustard Seed - Chris Evans

Garlic Mustard has several mechanisms which make it hard to control. Each individual plant produces over 5,000 seeds, and those seeds remain viable for at least 5 years. Missing one plant when removing an infestation can cause a significant headache in the following years. Furthermore, second year plants can continue to produce flowers and seeds even after being picked. For that reason plants must be bagged and thrown away with the garbage when picked. Since plants can re-sprout from root fragments and bloom over several months, repeated sweeps are necessary to ensure complete removal. The seeds are very small and are often spread on the boots (or paws) of hikers. Many roadsides in our area are infested, and if not treated the plant will begin to find its way deeper into the forest. The plant is able to exist in full sun or full shade, providing another advantage over competing native plants. If this all isn’t bad enough, its roots exude a chemical which is toxic to soil organisms that are essential to the health of our native woodlands.

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