Introduction

Map of Montana with names of counties. There are four burgundy-colored rectangles and two yellow-orange rectangles on the map in about six different counties.

Garlic mustard reports in Montana in the last 5 years (burgundy rectangles) and 10-20 years (yellow-orange rectangles). Map courtesy of Montana Natural Heritage Program, fieldguide.mt.gov.

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Garlic mustard, a biennial mustard native to Europe, was featured in the 2015 June Weed Post. This post revisits the species as it has been increasingly reported in Montana and is now listed in Lewis and Clark County. Efforts in Montana are focused on learning to identify garlic mustard so we can find it and manage it early and proactively.

Identification

First-year rosette leaves are kidney-shaped with scalloped margins. Leaves of second-year plants are heart-shaped at the base of the plant while upper leaves are smaller, triangular, and arranged alternately; leaves have toothed margins and pointed tips. A plant will have single or multiple flowering stems and can range in height from one to four feet. Flowers are white with four petals and occur in button-like clusters. Long, slender seed capsules, called siliques, are green initially then turn brown and curve slightly as they mature. Leaves and stems give off a garlic odor when crushed.

Cluster of three, four-petaled white flowers. Green leaves in background.

Photo: Chris Evans, bugwood.org.

Green, heart-shaped leaf being held between thumb and index finger of a human.

Photo: Jane Mangold, MSU.

Habitat

Garlic mustard forms dense stands, especially in the understory of deciduous forests, and has been especially impactful in New England and the Midwest. Garlic mustard produces phytotoxic chemicals that can inhibit growth of tree seedlings as well as alter soil biological and chemical properties. In Montana, reports of garlic mustard have been small populations in urban settings.

White flowers at the top of a green stem with multiple, long slender seed pods curving upward.

Photo: Michelle Delepine, West Multnomah Soil and Water Conservation District.

Management, including a recent development in biological control

Where garlic mustard is not well-established, early detection and rapid response is the main objective and being confident in garlic mustard identification is critical. Hand-pulling is effective for small infestations, and mowing second-year plants before seed development is also effective. There are several chemical control options, and the best option depends on the situation. In most cases, applications should be applied at the rosette to bolting stage. If plants are found, management should be swift and highly effective to prevent seed production and spread. For places where garlic mustard is well-established, there is good new regarding biological control: the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service (APHIS) issued approval for releasing the root crown weevil Ceutorhynchus scrobicollis to biologically control garlic mustard. The larvae of this tiny weevil eat within the root crown, shoot bases, and leaf stalks (petioles) of garlic mustard. Canada approved the release of C. scrobicollis in June 2018, and results have been encouraging.

Further Information

For more information about this month's weed post, contact Extension Invasive Plant Specialist Jane Mangold. Past posts are available in the Monthly Weed Post Directory.

This weed post is also available as a printable PDF (309 KB).