Management Notes: Techniques for Controlling Invasive Plant Species
Donald Geiger, Restorationist and Michele Banker, Land Resource Manager
Marianist Environmental Education Center
THIS IS A WORK IN PROGRESS - A Complete manual is in the works.
Garlic Mustard Alliaria petiolata (Bieb.) Cavera & Grande.
Descriptive introduction- Garlic mustard is a cool season biennial herb with stalked, triangular to heart-shaped, coarsely toothed leaves and white flowers with four petals in the shape of a cross (Cruciferae, Mustard Family). The leaves give off an odor of garlic when crushed and have a slightly hot, mustard taste. The plant is native to Europe and was introduced to the United States for culinary, herbal and medicinal purposes. It has invaded much of the U.S. and Canada and can be found throughout Ohio. The plant generally prefers some shade and can be found in a range of habitats from upland sites to flood plain forests, occasionally in full sun. It is more common in mesic lowland semi-shaded forest edges than in dry mature forest interiors but can invade either (Cavers et al. 1979, Carlson and Gorchov 2004).
The plant has a biennial life cycle that begins with the production of seeds in May and June (See the life cycle diagram below). Following the required winter exposure to cold many seeds germinate the following spring between late March and early April, offspring, season 1, forming small seedlings with two cotyledonary leaves. Light availability is a critical factor in the survival of first year garlic mustard seedlings. Subsequent leaves form a rosette that grows slowly during the summer. In unmanipulated areas mortality of the small 1st-year rosette plants is 92 to 98% in contrast to the low mortality in the second year (Anderson et al 1996). This genetic selection produces a surviving population well adapted to its micro-environment that develops through the 2nd season. Because of the biennial garlic mustard life cycle, two generations of plants can be seen in spring-summer, small 1st-summer rosettes, offspring, season 1 and larger 2nd-summer rosettes, offspring, season 2. During the 2nd winter and into spring the rosettes grow toward maturity. In March/April they begin to bolt and by May they form a flower stalk 2 to 3½ feet high. After pollination, each flower forms a slender silique filled with seeds. Upon drying, this fruit springs open, launching a large number of seeds that are scattered over the ground. Annual seed rains of over 9,500 to over 100,000 per m2 have been reported (Cavers et al. 1979). These germinate and, if on bare ground, produce a colony of seedlings that are able to smother the competition.
Because garlic mustard has an early phenology second year plants negatively impact native spring ephemerals and are important competitors of first year garlic mustard plants (Herold et al. 2011). This latter effect likely is responsible for the alternating dominance between first and second year plants and the need for several years of removal of second year plants to achieve effective control. Continued management for several years is also needed because some seeds may not germinate in the first year and are reported to remain dormant and viable in the soil for up to seven years. Understanding the life cycle provides opportunities to engage in effective control measures as discussed below.
Biennial Life Cycle
2nd-summer rosettes, offspring, season 2. During the 2nd winter and into spring the rosettes grow toward maturity. In March/April they begin to bolt and by May they form a flower stalk 2 to 3½ feet high. After pollination, each flower forms a slender silique filled with seeds. Upon drying, this fruit springs open, launching a large number of seeds that are scattered over the ground. Annual seed rains of over 9,500 to over 100,000 per m2 have been reported (Cavers et al. 1979). These germinate and, if on bare ground, produce a colony of seedlings that are able to smother the competition.
Because garlic mustard has an early phenology second year plants negatively impact native spring ephemerals and are important competitors of first year garlic mustard plants (Herold et al. 2011). This latter effect likely is responsible for the alternating dominance between first and second year plants and the need for several years of removal of second year plants to achieve effective control.
Factors favoring invasion- Garlic mustard is a pioneer that thrives where the soil has been disturbed. The plant is common along paths and other patches of exposed soil such as the areas left after bush honeysuckle and other invasive plants have been removed. The disruption of protective ground cover and disturbance of soil that is created by pulling of dense stands of garlic mustard and by the associated foot traffic can cause the buried seeds to germinate. Invasion also is promoted by the plant’s huge seed production that supplies the seed rain generated by the vigorous dispersal of seeds when drying siliques open. Seeds from improperly discarded plants can mature and can germinate.
Negative impacts of invasion- Garlic mustard aggressively out-competes native wildflowers and native seedlings in the understory of forests and woodlands. High plant density and the flush of garlic mustard growth produce a significant impact on native plant populations especially in spring (Carlson and Gorchov 2004). Garlic mustard plants are known to produce allelopathic chemicals that affect plants and their arbuscular mycorrhizal fungi (Roberts and Anderson 2001). Recent studies from the Harvard Forest in Massachusetts (Stinson et al. 2006) have established that a substance released into the soil by garlic mustard kills soil mycorrhizal fungi, thus damaging plants dependent on these organisms. The process directly suppresses the growth of native species, including hardwood trees, which have a mutual relationship with the fungi.
Control measures- Control of the abundant 1st year rosettes is not worth the effort since over 90% of these seedlings will die without treatment (Anderson et al 1996). The garlic mustard rosettes which survive the winter will flower and produce seed in the 2nd season if not controlled. On warm days in February or early March, with temperatures of 50oF or above, foliar spraying of the actively growing 2nd year rosettes with 1% Roundup® can kill and prevent them from forming flowers in May. A downside of this early treatment is the fact that early removal of the bolting second year garlic mustard rosette plants favors increased survival of first year seedlings in these open soil patches.
Timing of control and removal can be used to our advantage. Seed dispersal by the annual seed rain needs to be prevented to achieve control. Studies of garlic mustard control methods at MEEC demonstrate that it is possible to control garlic mustard plants mechanically late in the flowering season but prior to maturation of viable seeds. To verify this condition Inspection should verify that only immature non-viable seeds are present in the oldest siliques at the base of the flowering stalks. Sizeable patches of garlic mustard can be controlled effectively at a late stage of flowering by sequentially removing flower heads with a string weed cutter or a weed saw. Generally this stage occurs during mid May in the Dayton area. The method is particularly helpful for controlling dense patches of garlic mustard that develop in sites following removal of bush honeysuckle.
A sturdy string weed cutter or a steel brush saw is used to cut the stems of the garlic mustard plants. First, the top portion of the flower head (approximately 6-8 inches) is trimmed off at stage when the last flower buds at the tip are nearing maturity. A second cut for flower removal may be required for taller and fuller plants. Then the rest of the body of the plant is removed at ground level by the following cut. In dense stands one pass can cut the heads from several plants. To prevent maturation of seeds after the stalk is cut, only part of the flower head should be removed so that cells and sap present in the thick stalk are not sufficient to supply nutrients that can enable seeds to mature. A final cut at the base of the plant takes the plant to the ground. This procedure generally slows and often prevents production of a second group of flowers– a potential problem when plants are cut instead of pulled. Tests for seed viability conducted in 2003 after the seed heads removed in this manner had dried for 8 months showed that viable seeds were not matured. This method of controlling garlic mustard is most effective in woods and in shaded areas where growth is less vigorous. Once stands are greatly reduced the remaining plants can be removed by hand pulling from moist soil during the flowering season in succeeding years. These plants are disposed of in a black plastic trash bag that then is placed in the sun to kill the plants. Advantages of the mechanical method include avoidance of herbicide use, a low impact on the native herbaceous layer and minimal disturbance of the soil. Protection from re-invasion can be achieved by care to avoid seed dispersal and disturbance of the woodland floor and by measures to promote a vigorous native herbaceous layer and ample leaf litter cover.
Practical management goals- Because garlic mustard is so prolific and spreads rapidly into disturbed areas, it should be removed without significant disturbance of the soil before if dominates a habitat. The persistent seed bank makes it necessary to continue vigilant control over a number of years. A practical goal is to thin the garlic mustard coverage to enhance the growth of the herbaceous layer to a point where shading the soil greatly reduces reinvasion. Our experience is that there is often enough of a seed bank for establishment of a layer of shade-producing plants such clearweed (Pilea pumila), white snakeroot (Eupatorium rugosum), lopseed (Phryma leptostasha), enchanter’s nightshade (Circaea quadrisulcata), avens (Geum sp.) and sanicle (Sanicula sp.). Seeds of native species collected from an active herbaceous layer in fall can be distributed to augment coverage.
Research and references-
▪ Invasive Plants of Ohio Fact Sheet 3 www.dnr.state.oh.us/tabid/2005/default.aspx
▪ USDA Natural Resources Conservation Service, Connecticut, Invasive Species Identification Sheet. http://www.ct.nrcs.usda.gov/invas-factsheets.html
▪ Anderson, RC, SS Dhillion and TM Kelley. 1996 Aspects of the ecology of an invasive plant, garlic mustard (Alliaria petiolata), in central Illinois. Restoration Ecology 4:181-191.
▪ Carlson, AM and DI Gorchov. 2004 Effects of herbicide on the invasive biennial Alliaria petiolata (garlic mustard)
▪ Cavers, PB, MI Heagy,and RF Kokron. 1978 The biology of Canadian weeds, 35: Alliaria petiolata (M. Bieb.) Cavera and Grande, Canadian Journal of Plant Sciences 59: 217-229.
▪ Herold, J, MR Anderson, JT Bauer, V Borowicz and RC Anderson. 2011 Comparison of the effect of early and late removal of second-year garlic mustard (Alliaria petiolata) on first-year plants and deciduous forest spring and summer dominant herbvaceous groundlayter species in central Illinois, USA. Ecological Restoration 29(3): 225-233.
▪ Roberts, KJ and RC Anderson. 2001 Effects of garlic mustard (Alliaria petiolata) (M. Bieb. Cavera and Grande) extracts on plants and arbuscular mycorrhizal (AM) fungi. American Midland Naturalist 146: 146-152.
▪ Stinson, KA, SA Campbell, JR Powell, BE Wolfe, RM Callaway, GC Thelen, SG Hallett, D Prati, and JN Klironomos. 2006. Invasive plant suppresses the growth of native tree seedlings by disrupting belowground mutualisms. PLoS Biology 4: 727-731.