Garlic Mustard
Alliaria petiolata

Name and Family

Garlic mustard, Allaria petiolata, is a member of the mustard family (Brassicaceae).

History

  • Garlic mustard was first recorded in the US in 1868 and in Canada in 1897.
  • It was likely brought to North America as a food cultivar as it was traditionally used in Europe as a culinary and medicinal herb.

Identification Guide

  • First year plants form leafy rosettes close to the ground
    • First year leaves are kidney-shaped with scalloped edges.
  • Second year plants can reach 1m and have a flowering stalk
    • Mature flowering (second year) plants can reach up to 3.5 feet
    • Flowers are small and white with 4 petals, and grow in clusters at the top of leafy stems
    • Second year leaves are arranged oppositely on stalks, coarsely toothed and are triangular to heart-shaped.
  • Leaves give off a garlic or onion odour when crushed.
  • Very tolerant of shade and grows well under forest cover.

What it does in the ecosystem

  • Garlic mustard tolerates shade and grows in rich moist areas, which makes this plant of particular concern since it is commonly found invading woodlands.
    • It forms dense stands that block sunlight and outcompete native species for water and nutrients.
    • It can outcompete native woodland flowering plants like Sweet Cicely, Dutchman’s Breeches and violets.
    • Garlic mustard leaves are known to accelerate the decay of leaf litter on the forest floor, which alters the decomposition cycle and affects both decomposers and species that use the forest floor for foraging and shelter.
      • Native organisms are often unable to effectively adapt to these changes in decomposition. 
    • Fortunately, garlic mustard is not yet widespread on PEI. We hope to keep it that way!
  • Garlic mustard reproduces mainly by seed, with one plant producing thousands of shiny black seeds that can spread several metres from the parent plant.
    • In the case of long distance dispersal, seeds are moved about by animals and humans, and in some cases water.
    • Garlic mustard seeds germinate early in the spring, before most native plant species. 
  • Garlic mustard’s vegetation is unappealing to native herbivores because of toxins produced in the plant’s tissues. 
  • Chemicals released from the plant’s roots negatively affect mycorrhizal fungi growth.
    • Since the fungi grow underground and form mutually beneficial relationships with plant roots, this also negatively impacts the native plant species that depend on it. 
  • In an agricultural setting, garlic mustard can act as an alternate host for several agricultural diseases. 
  • When garlic mustard infests the pasture of dairy cows, cows that eat the plant can start producing garlic-flavoured milk, which is unattractive to buyers.

Management

GENERAL CONSIDERATIONS

  • In the life cycle of garlic mustard, there are two distinct growth forms: the rosette and the adult plant.
    • The rosette form occurs in the first year and the plant reaches maturity in the second year.
      • This is the immature stage. It appears as a low-growing circular arrangement of leaves that does not flower or produce seeds.
    • Flowers and seeds are not produced until the second year.
      • This is the mature stage. The plant gets much taller, producing one or more central stems tipped with white flowers.
    • Both life stages should be removed.
      • If working with the plants after the seeds have been dispersed, focus on the immature rosettes only. This is because garlic mustard is monocarpic, meaning it dies after producing seed. The mature plants that have already dispersed their seeds will die soon and are thus not primary targets for management.
  • Management should ideally take place before seeds are produced, beginning in July.
    • Seeds may persist on the plants until November.
  • If managing the plant after seeds have been produced, care must be taken to avoid unintentional dispersal.
    • The seeds are the perfect size to fit between the treads of footwear. Clean your footwear before leaving the site to prevent dispersal over long distances.
    • If the plant is in seed, you may want to take the time to clip seed heads.
      • Clipping seed heads will prevent seed dispersal, but not eradicate the plant. 
      • Carefully place a plastic bag over the seed head.
      • Cinch the bag around the stem.
      • Clip the stem below the bag.
      • This will ensure that the entire seed head has been deposited into the bag, with little chance for accidental dispersal.
      • If used alone, this will reduce the reproductive expansion of the population but not offer the possibility of eradication. 
      • The plant dies after producing seed, so over time, this practice may cause the population to shrink.
  • As the immature garlic mustard rosettes persist into the winter, it is possible to manage through the winter when the ground is not frozen. This will give you a nice head start on management the following spring. 

PULLING 

  • Pulling should take place before the second-year plants have gone to seed. 
  • Loosen the soil around the plant using a digging tool to get up the whole root.
  • Pull on the day after a rain, as the soil will be softer and the plants easier to pull.
  • Pulling is fairly simple, as the plants usually have shallow roots with little branching
    • Roots often curve in an S-shape, which offers some resistance to pulling.
  • Pulling can be simplified when done in combination with stirrup hoeing.
    • This technique is only effective for immature rosettes. Larger plants may have roots that are too developed to effectively uproot using a stirrup hoe. 
    • Using a stirrup hoe to loosen the soil around the plants.
    • Pick the plants out of the loosened soil and collect them for disposal.
  • Sites should be monitored every growing season thereafter. This is because garlic mustard seeds can remain viable in the soil for at least 10 years.
    • If no plants are allowed to go to seed, there should be a steady decrease in population size until eventually the seed bank is depleted. 

CUTTING

  • Cutting immature garlic mustard rosettes rarely leads to mortality because they are capable of producing new shoots.
    • The immature plants grow so low that many mowers may not be able to clip them.
  • Cutting adult plants at ground level has been shown to be effective in killing the plant.
    • Re-sprouting shoots will die in a few weeks if the plant has been cut to the ground. 
  • As cutting does not affect rosettes, these can be hoed using a stirrup hoe as above. Monitor the site in subsequent years. If no plants go to seed, there should be a steady decrease in population size until eventually the seed bank is drained.

BURNING

  • Burning is not an effective control method for garlic mustard. 
  • Habitat and surrounding vegetation often present inappropriate conditions for burning.
    • This is especially true when it invades forested places and urban areas.
  • The roots of the burnt plant are not affected and can re-sprout the following year. 
  • Garlic mustard is quick to dominate a disturbed area and burning may improve conditions for this invader by inhibiting native species. 

CHEMICAL CONTROL 

  • The PEI Invasive Species Council does not offer advice on chemical control measures at this time. 
  • Herbicide application may negatively impact native species in any environment, and the use of herbicides near watercourses on PEI is restricted. 
  • Herbicide application as a method of control must be carefully considered and should be used as a last resort.
  • If using chemical controls, it is imperative that all local legislation and manufacturer’s instructions be followed.

FOLLOW-UP AND DISPOSAL

  • As is the case with most invasive plants, return visits to the site after management and in subsequent growing seasons will be required to completely eradicate garlic mustard. 
  • Repeat management as regrowth appears for several years after initial removal efforts or for as long as regrowth is seen. 
  • This is because the plant may regrow from leftover root fragments and seeds, which can remain dormant in the soil for four years. 
  • Each time you manage the site, collect, bag, mark, and dispose of all plant parts properly.
    • Bag the plant material in clear plastic bags.
    • Mark the bags boldly with a permanent marker. Write “INVASIVE PLANT” or “GARLIC MUSTARD” on the bags. 
    • If you have a place away from foot and wildlife traffic, you can dry out the material, making it much easier to move.
      • Leave the bags open and in the sun for a week to dry. 
      • Only do this if you can guarantee that the plant will not be spread from where it is drying
    • If the bags are thin, double bag them to prevent accidental dispersal later on.
    • Tie the bags tightly and place them in your usual residential waste collection (THE BLACK BIN)
      • Never dispose of invasive plants in the compost. This may allow them to become established once they reach the heap. 

Map

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Garlic mustard
First year leaf
Garlic mustard stalks
Second year leaf

References

THE BIOLOGY OF CANADIAN WEEDS.: 35. Alliaria petiolata (M. Bieb.) Cavara and Grande (cdnsciencepub.com)

About Garlic Mustard – Integrated Pest Management (msu.edu)

Garlic Mustard – Profile and Resources | Invasive Species Centre

Garlic Mustard – Ontario Invasvie Plant Council BMP

Garlic mustard identification and control: Alliaria petiolata – King County