On the Lookout for Garlic Mustard

On the Lookout for Garlic Mustard
By Tom Costello

Garlic Mustard Rosette - Chris Evans

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 was 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 3 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.

The 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 3 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.

What does it look like?

Garlic Mustard Flower - Linda Haugen

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 has several mechanisms which make it harder 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 fin 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 which are essential to the health of our native woodlands.