Fall invasive species management in on-farm woody areas

UNIVERSITY OF MINNESOTA EXTENSION - www.extension.umn.edu

Most farmers are still busy harvesting, cleaning fields, and preparing for next year. However, as your workload begins to slow down, consider taking some time to monitor for invasive species on your farm. While this article is about woodland invasive plants, we are posting it here because many fruit and vegetable farms in Minnesota have wood lots or small forested patches. Two plants in particular should be actively managed in these plots in order to maintain a diverse and healthy woodland ecosystem: Buckthorn and Garlic Mustard. Both plants are most effectively managed in the fall and in early spring.

Buckthorn

Common buckthorn (Rhamnus cathartica) or glossy buckthorn (Frangula alnus) arrived in Minnesota in the 1850s; it was brought from Europe as a landscaping plant. Indeed, in its first few years of growth, it forms a dense patch of shrubs with nice dark green leaves that remain on the plant much longer than most other deciduous trees. However, it quickly takes over the landscape and crowds out native vegetation. Our friends on the crops team wrote up an excellent and comprehensive overview of buckthorn management on their page: https://blog-crop-news.extension.umn.edu/2020/07/controlling-buckthorn-who-what-where.html

A patch of buckthorn on a field margin in Dakota County, Photo NH

Garlic mustard

Garlic mustard (Alliaria petiolata) is one such invasive that can be devastating to Midwestern forests. Originally from Europe, humans have eaten garlic mustard for at least the past 6,000 years. In the 1800’s, this species was introduced into North America by European immigrants. Despite its use as an edible herb, garlic mustard is a restricted noxious weed, meaning it cannot be transported, introduced, or sold in Minnesota.
 

Garlic mustard growing on the forest floor. Photo credit: Shane Bugeja

Their seeds are not commonly spread by wind or animals, but rather through water and/or mud. These seeds can stay alive in the soil for a long time, causing headaches every spring and fall. While it may seem like a hassle, removing soil from your boots before and after you leave forests is a great way to avoid introducing invasive species, including garlic mustard.

One challenging aspect of garlic mustard, aside from how it spreads, is its effects on other plants. Studies suggest garlic mustard is allelopathic, which means it sends out chemicals that hurt the growth of its neighbors. The chemicals garlic mustard releases are called glucosinolates. These give it a spicy taste but also harm beneficial soil fungi called mycorrhizae. These fungi help provide important nutrients to plants in exchange for energy. However, like many members of its plant family Brassicaceae, garlic mustard does not have this fungal relationship. Over time, a large garlic mustard patch can severely damage native plant populations that otherwise would thrive in the area.

How to identify garlic mustard

Leaves of garlic mustard and its lookalikes. From left to right: Creeping charlie, garlic mustard, and a violet species. Photo by: Shane Bugeja

Because garlic mustard is a biennial weed, the 1st year of its life is spent as a rosette low in the forest floor. Often, it is confused with either wild ginger (Asarum species), creeping Charlie (Glechoma hederacea), or violets (Viola species) due to its kidney bean shaped leaves.

One trick to identify garlic mustard is to scout in the late fall, as garlic mustard is one of the last green plants in the woods. Its first year leaf edges also are “scalloped”—with blunt, shallow teeth on its sides. In the spring and early summer, garlic mustard’s second year leaves look more triangular further up the stem. However, they will still have scalloped edges.

Garlic mustard also tends to be true to its name, as crushing the leaves can give off a mustardy, garlic odor.

• Violets will not have a strong smell.
• Creeping Charlie will have a minty, herbal aroma.
• Wild ginger’s stems, if broken, will release a smell similar to ginger.

Unlike creeping Charlie and wild ginger, garlic mustard does not spread as a vine. If you pull garlic mustard plants you will not find runners or rhizomes, but a single crown with an S-shaped root attached.
 

A fully uprooted garlic mustard in its first year. Notice the lack of a vine structure. Photo by: Shane Bugeja

Controlling garlic mustard

If the area is small, hand removal of the plant and most of its root system is an option.

For larger sites, herbicide applications are generally the favored technique. If you decide to go this route, a labeled herbicide that contains the active ingredient triclopyr (Garlon) or glyphosate (RoundUp) can be effective.

Be aware that glyphosate products are non-selective, and will harm most actively growing plants if sprayed. Triclopyr normally does not hurt grasses and sedges, as it is more targeted toward broadleaves. Always read and follow the label before applying any herbicide.

Sources:

• Cipollini, D., Rigsby, C. M., & Barto, E. K. (2012). Microbes as targets and mediators of allelopathy in plants. Journal of Chemical Ecology, 38(6), 714-727.
• Cosme, M., Fernández, I., Van der Heijden, M. G., & Pieterse, C. M. (2018). Non-mycorrhizal plants: the exceptions that prove the rule. Trends in plant science, 23(7), 577-587.
• Durka, W., Bossdorf, O., Prati, D., & Auge, H. (2005). Molecular evidence for multiple introductions of garlic mustard (Alliaria petiolata, Brassicaceae) to North America. Molecular ecology, 14(6), 1697-1706.
• Michigan Department of Natural Resources. (2018). Garlic Mustard. Michigan Natural Features Inventory. https://mnfi.anr.msu.edu/invasive-species/GarlicMustardBCP.pdf
• Prati, D., & Bossdorf, O. (2004). Allelopathic inhibition of germination by Alliaria petiolata (Brassicaceae). American Journal of Botany, 91(2), 285-288.
• Saul, H., Madella, M., Fischer, A., Glykou, A., Hartz, S., & Craig, O. E. (2013). Phytoliths in pottery reveal the use of spice in European prehistoric cuisine. PLoS One, 8(8), e70583.