An Illinois River Almanac

Invasive species are talked about a lot today and some can be controlled more easily than others but invasive species will take over our native areas if we do not all work together to control and remove them.  You may be thinking “Why should I care about invasive species?”  Invasive species have been introduced into an ecosystem where they do not have any natural predators and because they do not have that natural control they will expand and out compete the native species that are important to other native species.  This means that one invasive species can have a huge effect on many different native species.

Garlic Mustard (Alliaria petiolata) is a European biennial herb.  It was first recorded in the US on Long Island, New York around 1868.  Garlic Mustard was likely brought to the US by early settlers for food and medicinal purposes.

Today Garlic Mustard can frequently be found in moist shaded soils.  In Illinois it is found in floodplains, forests, wood edges, and along trails.  Garlic Mustard is a cool season biennial herb.  It has stalked leaf triangular to heart shaped giving off an odor of garlic when crushed.  The first year growth is a rosette up to 4 inches in diameter.  Mature plants flower the following spring and will produce an erect flowering stem that will grow up to 4 feet tall.  Flowers are born in racemes and are white flowers with 4 petals up to half an inch across.  Garlic mustard reproduces exclusively through seed and in central Illinois it is estimated that 15,000 seeds per square meter are produced.

It is important to identify the plant correctly toothworts, sweet cicely and early saxifrage also have white flowers and grow alongside Garlic Mustard.  Garlic Mustard poses a significant threat to both native plants and animals in forest communities.  Forest wild flowers (spring beauty, harbinger of spring, wild ginger, bloodroot, Dutchman’s breeches, and many others) which complete their life cycles in the early spring before the tree canopy closes are being outcompeted by garlic mustard.  As a biennial Garlic Mustard is able to establish itself more quickly than its native neighbors and is therefore able to monopolize the needed resources like light, nutrients, and space.  This loss of native flora deprives a number of native faunal species that are dependent on the nectar, fruit and foliage provided by these early spring plants.

So how do you control this invasive species?  It takes a consistent control method to truly be effective.  The seeds of Garlic Mustard remain viable for up to 5 years in the soil so removal of Garlic Mustard must be done until the seed bank has been depleted.  On the plus side many of the native woodland flowers remain viable for much longer.  Removal of Garlic Mustard can be done in a number of ways.  First is mechanical removal.  This is a labor intensive method and requires the removal of the plant and its roots.  The plant can regenerate from root fragments so it is important to remove the roots and not just the herbaceous portion of the plant.  It is also important to remove the plant before it set seeds and ideally before it has flowered.  This is best achieved in the early spring when the soil is damp.  Grasp the plant near the base and pull the entire plant from the soil.  This method is best used on smaller infestations.  For larger infestations the plant can be cut down as low as possible before it begins to flower.  This is a less effective method than removing the plant because it can still flower from the leaf axils.  This method must be continued until the seed bank has been depleted.  In each of these cases it is best when possible to remove the plant residue from the site entirely.

Chemicals can be used in areas where the infestation is particularly heavy and the risk to native flora is very low.  Fire has also been used to control Garlic Mustard in very large natural areas.  Fire has proven to be an effective control though it does open the understory and exposes soil to allow for germination of the seed bank and as the seed bank remains viable for up to five years the burning regime must be repeated annually for 5 years or until the seed bank has been depleted.

For more information on Garlic Mustard or other invasive species please contact Jason Haupt (jdhaupt@illinois.edu).