When an upright plant with clusters of small white flowers begins to bloom in April, it’s not good news.
If the leaves have a distinctive onionlike odor when crushed, it’s garlic mustard, an invasive plant that is a serious problem not only in natural areas but in shady areas in many yards.
“All those little white flowers will turn into tiny seeds that are spread by the wind,” said Sharon Yiesla, plant knowledge specialist in the Plant Clinic at The Morton Arboretum in Lisle. “It’s very important to pull up the plants before they set seed.”
Garlic mustard (Alliaria petiolata) is a special problem in wooded areas or under trees because it is extremely shade tolerant. It spreads aggressively into large stands that shade out other plants such as native wildflowers and tree seedlings. The plant also produces chemicals in the soil that prevent the seeds of other plants from germinating.
Invasive plants are nonnative species that disrupt ecosystems by outcompeting native plants. Most have escaped from gardens or farms where they were deliberately planted. For example, garlic mustard was brought from Europe in the mid-19th century for medicinal and cooking purposes.
It is one of the invasive plants that most frequently needs to be removed in restoring Midwestern natural areas such as those at The Morton Arboretum.
The clusters of small, four-petaled flowers make garlic mustard easy to spot in April. The flower clusters are held on stalks that can be 12 to 48 inches tall. The leaves are generally triangular with deep teeth around the edges. “If you’re not sure, the smell is what really clinches it,” Yiesla said. “It’s like a cross between onions and garlic.”
The plant is a biennial; in the first year it has a low rosette of foliage, and the second year it grows a stalk, blooms and forms small pods full of tiny seeds.
“If you spot garlic mustard on your property, the best thing to do is to pull it up,” she said. “You’ll have to be persistent because seeds can remain in the soil and you may miss first-year plants.”
Once you know of a spot where garlic mustard grows, check back every spring and remove any plants you see. Even if you’ve planted something else there, be alert for garlic mustard seedlings.
Dispose of the plants in the garbage or landscape waste, not in your compost pile or anywhere on your property. “You want to avoid the risk of spreading any of the tiny seeds to other areas of your yard,” Yiesla said.
Learn more about invasive plants from the Midwest Invasive Plant Network (mipn.org).
For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or plantclinic@mortonarb.org). Beth Botts is a staff writer at the Arboretum.
