Garlic Mustard
Background
Garlic mustard (Alliaria petiolata) is an herbaceous member of the mustard family (Brassicaceae) brought over by early European settlers. First documented in New York in 1868, it was used as a source of food and medicine. This plant's biennial life cycle consists of a ground-level, or "basal," year and a reproductive, or "bolt," year. Garlic mustard's vigorous reproduction has enabled it to spread from coast to coast, where it blankets habitats with moist, rich soils. A prolific seeder with allelopathic properties, it forms dense monocultures, leaving little room for native plants.
Description
Year 1 - basal year
Size: Less than 8 inches tall
Leaves: Kidney-shaped, rounded teeth, and highly variable in size, each leaf is usually less than 4 inches across. The leaves form a basal rosette, meaning all the leaves emerge around a central, underground stem. produces a garlic odor when crushed.
Year 2 - bolt year
Size: Highly variable, from a few inches up to 3 feet tall.
Leaves: Heart-shaped, each 2 to 4 inches across, with pointed, irregular teeth.
Flowers: Beginning in early spring (March or early April), clusters of white, four-petaled flowers emerge at the uppermost growing tip. Flowering time is often staggered within a population, so individual plants will start flowering at different times, starting in early spring and continuing through early summer.Â
Fruit: By early- to mid-summer (depending on flowering time), the flowers are gradually replaced by branched stems bearing the seed pods, called "siliques." At first green, they become brown and brittle when ripe, a stage referred to as "seed shatter." There is wide variability in silique size and exact number of pods. The individual seeds are tiny and brown, each less than ¼ inch long.
Look-alikes
Many native members of the mustard family, such as cut-leaf toothwort (Cardamine concatenata), also have cross-shaped white flowers with four petals. However, garlic mustard leaves are unique with their simple, kidney- or heart-shaped leaves in contrast to the compound leaves of the native species.
Dispersal
As an herbaceous biennial, it propagates solely through seed. In the spring or early summer of their second year, garlic mustard rosettes rapidly elongate their stems and produce a flowering head. Each plant will release many, sometimes thousands, of highly mobile seeds. Light enough to be carried by wind, they can also travel in water or by soil movement. The seeds also remain viable for fairly long periods—over five years in optimal conditions.
Site
Suited to a wide range of habitat types, garlic mustard thrives especially well in areas with a disturbed overstory, rich, moist soil, and basic soil pH. It is shade-tolerant and will often spread from forest edges and openings to mature forest understories.
Control
Garlic mustard has a taproot, and unlike some invasive herbaceous perennials, it does not regenerate from root fragments. Therefore, this is one of the few invasive plant species that can be controlled manually by pulling. Manual operations that completely remove shoot tissue will prevent regrowth. Ideally, plants should be pulled before siliques are developed, as studies have found that plants bearing siliques when pulled, even unripe ones, can still develop viable seed. For this reason, it is best practice to bag and remove pulled plants from the site, as even early pulling treatments probably include some plants that have, or will develop, viable seed.
Similarly, herbicide treatments should be timed before a significant number of plants have flowered and started development of siliques.Â
Management Calendar
The management calendar for garlic mustard emphasizes treatment prior to seed shatter. The rosette stage represents a year-long window for herbicide treatment.
Treatment and Timing
| Treatment | Timing | Herbicide | Product Rate | Comments |
|---|---|---|---|---|
| Pulling | Before flowering/silique development or before seed shatter | N/A | N/A | Small infestations are effectively controlled by hand pulling. Grasp the leaves and stem, remove the entire taproot, bag, and destroy. This can be done anytime before the seed has ripened and dried, called the seed shatter stage. Plants pulled prior to silique development can be left on site. |
| Selective Postemergence | Before flowering | Garlon 3A or Vastlan (triclopyr salt) plus Freelexx (2,4-D salt) |
1.5 quarts/acre or 1 quart/acre plus 68 fluid ounces/acre |
Postemergence applications of water-soluble triclopyr and 2,4-D formulations are useful when target plants are growing among desirable grasses or other monocots. This broad-spectrum combination will control most broadleaf herbaceous species while leaving most grasses unharmed and has little soil activity. A surfactant (e.g., Alligare 90) must be added. |
| Non-selective Postemergence | Before flowering |
AquaNeat (glyphosate) or AquaNeat (glyphosate)Â |
3 quarts/acre or 3 quarts/acre |
Glyphosate has been shown to be effective on garlic mustard even in the dormant season, as long as temperatures are above freezing. It would be effective when there is no advantage to using the selective mixture described above or when you are also treating invasive grasses. A surfactant (e.g., Alligare 90) needs to be added to AquaNeat. If using a different glyphosate product, check the label to see if a surfactant should be added; some come premixed. Glyphosate plus triclopyr is a non-selective combination that controls a broader spectrum of plants and various woody species. This mix will effectively control most any species you encounter during the operation. |
| Selective Preemergence | March through May | Pendulum AquaCap (pendimethalin) | 96 to 128 fluid ounces/acre | Preemergence-only herbicides, like pendimethalin, only affect germinating seed, so this can be useful for treating annuals/biennials among established desirable perennials. This herbicide can help reduce the amount of garlic mustard plants that emerge, but will not control a population when used alone, due to the staggered germination. |
| Pre- and Post-emergence | March through September | Oust XP (sulfometuron-methyl) | 1 - 2 ounces/acre | Sulfometuron will control existing garlic mustard as well as germinating seed for multiple months. One application will likely not fully control a population due to the longevity of the seed bank and extended germination, but it can help set it back. Sulfometuron will control most herbaceous plants. |
Pendimethalin can reduce the amount of garlic mustard that emerges, but will not control a population because the germination window will likely exceed the longevity of the herbicide. Also, the herbicide will not control existing garlic mustard plants. Glyphosate could be included with the pendimethalin to control existing plants, but reduces selectivity if green desirable plants are present. Treating earlier in the spring will reduce impacts to nontarget plants. Sulfometuron will control both existing plants and germinating seed for a longer period of time, but will also eliminate most desirable herbaceous plants that are present.
Postemergence applications during the rosette stage with a spot foliar spray is a fairly targeted approach and can be done anytime during the year. Foliar applications can be applied anytime the temperature is above freezing. Treating rosettes in the late fall through winter and early spring avoids accidental overspray on nontarget species. Again, this will control existing plants but not seed in the soil, so spot treatment or hand-pulling will need to take place during the spring and summer as well.
A selective mix useful for treating biennials is water-soluble triclopyr plus 2,4-D. This combination does not injure grasses, sedges, or most other monocots. Preserving native groundcover is desirable, as the garlic mustard seed bank will quickly take advantage of open space. This mix is useful for most broadleaved herbaceous plants, but may not be as effective on woody species at the rates listed in the table. For better woody plant control, increase the Garlon 3A or Vastlan rate to 2 quarts or 1.5 quarts per acre, respectively.
If targeting both grasses and woody species, such as various invasive shrubs, in addition to garlic mustard on the same site, a nonselective herbicide mix is advised. A mixture of glyphosate plus triclopyr (2:1 ratio) will be effective against almost any species you encounter, whether grass, forb, or woody. The application rate provided in the table is more concentrated than needed to kill garlic mustard. To retain the ability to control difficult species and apply an appropriate dose to less resilient targets, dilute the mixture by half and adjust your spray pattern. Apply as normal to garlic mustard, and twice as heavy for more difficult targets, such as invasive shrubs. In this manner, you can use one mix to treat many species, and dose each as needed.
The approach to treating garlic mustard is to target it early in its cycle and as selectively as is practical. By targeting these plants selectively, either through pulling or spot herbicide treatment and possibly combined with a selective preemergence application, the greatest amount of nontarget plants can be preserved to prevent reinfestation by garlic mustard. Though individual plants are easy to control, due to their huge seed banks and season-long germination period, clearing one flush of garlic mustard may simply invite another. Persistence and a long-term approach to control is key with this species.
Prepared by Skylure Templeton, Art Gover, Dave Jackson, and Sarah Wurzbacher. Reviewed by Norris Muth, Amy Jewitt, and Andrew Rohrbaugh. Updated by Emily Rojik.
