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Invasive Plant: Garlic Mustard

Information
Photo shows green plants with white flowers with a forest in the background.
Brought to Oregon as a garden plant, garlic mustard is now moving into and through our forests. It grows in dense patches that push out native wildflowers and tree seedlings by taking water and nutrients. Ultimately, this can affect whether or not new trees replace old trees that die.
On this page

At a Glance

Photo shows green plant with forked leaves and white flowers.
Garlic mustard roots may produce a chemical that kills some important kinds of soil fungus that tree seedlings depend on. Report sightings of garlic mustard to Environmental Services.
  • Scientific name: Alliaria petiolata
  • Annual, perennial, or biennial: Biennial – Needs two growing seasons to produce seed.
  • Conditions it likes best: Shade or mixed sun and shade
  • How it spreads: Makes a lot of small seeds that can survive several years in the soil before sprouting. Running water (streams or rainwater) can move seeds to new places. People can also move seeds in their boots or tires.
  • How it causes damage: Makes dense patches that take over forested areas and takes the place of other plants. Prevents new trees from growing up. Studies from other parts of the United States suggest that garlic mustard roots may produce a chemical that kills some important kinds of soil fungus that tree seedlings need.  
  • Oregon Department of Agriculture noxious weed rank: B
  • Report patches of this plant? Yes. Please report sightings of this plant to Environmental Services. Find contact information on this page. A photo and location are required.

How to Identify Garlic Mustard

Four photos in a grid show stages of garlic mustard from rosette, bolting, flowering, seeds.
Stages of garlic mustard. Clockwise from top left: Rosette, bolting, flowering, and seed pods

Winter: Rosette. Garlic mustard looks like many other plants, including several kinds of other mustard plants. For many people, its crushed leaves and roots smell like garlic. Mustards are known for having unexpected sizes and for flowering at unexpected times, so the tips listed below may not always describe what happens in the world.

Garlic mustard overwinters as a rosette (flat circle of leaves) with round leaves with scalloped, almost toothed edges. Rosettes are easily confused with several native species (see the look-alikes section below) and can be hard to find under fallen leaves during winter.

Spring. When days get longer and warmer in April, rosettes bolt (grow a stem). Stems are usually 1-4 feet tall, and new leaves change from round to triangular. Plants as short as 1 inch or as tall as 6 feet are not unusual.

In April and May, the bolting plants will open into four-petaled white flowers. Flowers usually last two to three weeks, falling off from the top of the plant to the bottom.  

Summer. By late May or early June, all the flowers have been replaced with tiny seedpods. The seedpods need one to two weeks before seeds become viable (able to create a new plant). Seeds then ripen for another four to six weeks, until the seedpods dry out and turn brown. This happens in late June or early July.

Dry seedpods explode at the slightest touch, scattering seeds all around. When this happens, well-meaning people who are pulling plants can accidentally get seeds in their clothes or shoes and then move seed to unexpected places. For this reason, we don't recommend pulling plants between late June and late October. It is better to have seeds fall where you know they are than for the seeds to go someplace you’re not looking.

Summer and Fall: Seeds Ripe. By October or November, most seeds have been knocked off the plants. Fall and winter are a good time to dig up rosettes but be sure to clean the mud out of your boots. Fall and winter are also good times to accidentally move seed around!

How and When to Remove Garlic Mustard

  • Manual. Digging up garlic mustard on private property can be effective but requires weekly attention to catch new plants. Pulled plants will continue to flower and set seed. Be sure to bag pulled plants and put them in the garbage (not in your yard debris). Be sure to get all the roots. Don't cut or mow. Leftover roots can regrow, and cut plants might still flower and set seed. It is better not to pull garlic mustard on roadsides unless you have “adopted” that roadside.
  • Herbicide. Herbicide treatment is not necessary on small, local patches. The City of Portland uses herbicide mostly along roadsides, where miles of work makes handpulling difficult and dangerous.
  • Check. Monitor the site every two weeks in spring and especially in the second and third year after treatment when surviving seed takes its big opportunity to sprout.
  • Replant. After a year or two of managing this plant, consider planting native or less aggressive non-native plants.

City Assistance and Current Work

If you’ve got garlic mustard on your Portland property and there’s too much to pull, let us know. We may be able to help. Our treatment seasons runs May to June. If the treatment season has ended, we will add your name to our treatment inventory for the following spring.

How to get assistance. Before the end of April, download and send to the Environmental Services Early Detection and Rapid Response team a completed Permit of Entry form (link below). Contact Environmental Services (contact information on this page) to learn more.

Current Work

City employees and licensed contractors continue to manage garlic mustard across Portland.

Almost everywhere, garlic mustard plants are solidly in the "seeds forming phase."  Like any fruit, the pods still need several weeks to ripen the seeds inside.  So, there is still a small window for some herbicide treatments, likely through the end of May. 

If you are pulling plants by hand, it is still important to get all the root to prevent regrowth.  That will also probably be still be true through the end of May.

Last update: May 21, 2024 

Areas managed (2024)Treatment Date
SW Custer/Bertha @ Beav-Hillsdale4/23/24
NW Thorburn/SW Sam Jackson/Terwilliger 4/24/24
SE Foster/Barbara Welch/162nd5/1/24
Rocky Butte/SE Flavel5/3/24
NW Skyline/Germantown/Thompson5/8/24
SW Macadam/Hamilton/Shattuck/Patton5/9/24
NW Cornell/Greenleaf5/10/24
SW 45th/Multnomah/Boundary/39th5/11/24
SW Taylors Ferry/Capitol Hwy/Pasadena/55th/Vermont5/13/24
NW Royal/Burnside/Barnes5/14/24
Burnside/SW Humphrey5/15/24
SE Boones Ferry/Fairmount (E)5/16/24
N Portland Rd/NE Fremont @ 102nd5/17/24
Linnton/Springville Rd/NW Fairfax5/20/24
  
Bold indicates spray treatments in the last 2 weeks. 
Current Phase 
(in bold)
Description of Garlic Mustard In Each Phase
Rosette

Like many plants, garlic mustard changes form several times over its lifetime. A typical garlic mustard plant forms as a seedling in the spring and overwinters as a rosette: a low-growing cluster of leaves typically one to six inches tall. The rosette form is easily confused with several native species, and is often hard to find under winter's leaf litter.

Bolting

With longer, warmer days in March and April, garlic mustard rosettes undergo a change called bolting. The stems lengthen and leaves go from round to triangular.  Buds form at the top of the rising cluster of leaves.

Flowering

After enough warm, light days in April or May, the bolting plants will open into garlic mustard's characteristic four-petaled white flowers. The flowering stage of garlic mustard typically lasts 2-3 weeks. Herbicide treatment is most effective during this phase. Roots not removed during pulling will likely form a new flowering plant in a few weeks.

Seeds Forming

After all the flowers are gone, the seedpods need 1-2 weeks to "fill in," typically by late May or early June in the Portland area. Careful herbicide treatment can halt seed growth in this phase. Pulling efforts should still attempt to remove all roots. All pulled plants should be bagged and put in the trash to prevent spread in yard debris.

Seeds Ripening

Mature seeds ripen for four to six weeks until the plants begin to dry out, typically in early to mid-July. Pulling can be done in this phase, without regard for root re-growth, but MUST stop when plants begin drying. Herbicide treatment will have no effect during this phase and should not be attempted.

Seeds Ripe Don't pull!

Newly-dry seedpods will explode at the slightest touch, depositing seeds in clothes and hair and making the well-meaning puller an ideal vector for infesting new areas.

Prevention is Best Practice

Clean your boots, pets, and maybe even your tires when you finish a hike or trail ride in Pacific Northwest natural areas, or if you have invasive plants on your own property. Cleaning boots and pets keeps invasive plants from spreading to new places. Be careful when trading plants with neighbors and fellow gardeners to make sure you’re not trading this or any other plant of concern.

Be Aware of Look-alikes

These plants can look like garlic mustard. Here are some tips to tell the difference.

  • Fringecup (Tellima grandiflora). This plant has hairy leaves and stems. Garlic mustard doesn’t.
  • Piggy-back plant (Tolmiea menziesii). This plant has also hairy leaves and stems. Garlic mustard doesn’t.
  • Ground ivy (Glecoma hederacea). Ground ivy leaves have the same shape as garlic mustard seedlings and rosettes, but ground ivy grows along the ground. Garlic mustard grows straight up.  
  • Hairy bittercress (Cardamine hirsuta). These mustard “cousins” have deeply-lobed leaves, compared to garlic mustard’s simple round or triangular leaves. Hairy bitter-cress is also usually shorter (6-8 inches high) and flowers February through March.  
  • Black mustard (Brassica nigra). These mustard “cousins” have longer pods than garlic mustard with obvious bumps where the seeds are.

Properly Dispose of Invasive Plants

To prevent spreading invasive plants:

  • Never place plants or seeds in your compost or yard debris.
  • Never throw pulled plants on the ground or into the street or sidewalk. 
  • Put all pulled plants, bulbs, or seeds into a plastic bag and put the bag in the trash.
  • Wash all garden tools and gloves.

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