The early green in Minnesota woodlands is a welcome sign of spring for many people, but not for John Peterson, who owns 25 acres of forest in Delano.
Peterson knows that much of the green on his forest floor is garlic mustard, and he’s been waging an all-out war on the invasive plant for the past five years.
“I never thought a little biennial plant would be such trouble and so hard to get rid of,” Peterson said. “It’s really murder.”
New garlic-mustard patches pop up in late April and early May. It is one of the state’s most noxious weeds, and it forms a dense carpet that crowds out tree seedlings and native wildflowers such as bloodroot, lady’s slipper, wild ginger and trillium.
Janet Van Sloun, restoration specialist for the city of Minnetonka’s natural resources department, said garlic mustard is such a problem because it came from Europe and has no natural predators in the United States to keep it in check.
“Nothing eats it here, no animals, no insects, and it gets into a woodland and it just takes over and spreads incredibly quickly,” she said.
Since 2007, Van Sloun has offered spring workshops and trained more than 600 Minnetonka residents in how to identify and remove the plant from their yards. One workshop was held last week, and two more are slated for May 23 and June 12. Van Sloun also works with volunteer groups and individuals to yank the prolific plant at 15 of the city’s parks.
Plant hogs light, nutrients
Eli Sagor, University of Minnesota Extension educator, said garlic mustard is widespread in the southern half of the state and is spreading north, especially in the shady, moist conditions of hardwood forests and other places with trees: flood plains, trails and creek corridors. Garlic mustard and buckthorn — a non-native woody invader — are probably the two biggest plants jeopardizing healthy forests, he said.
The damage to native wildflowers occurs because garlic mustard creates a thick layer on the forest floor that monopolizes light, moisture, soil and nutrients, Sagor said.
During its first year, the plant looks a lot like creeping charlie, he said. Each plant forms a rosette of three or four scallop-edged dark-green leaves that grow low to the ground and smell like onion or garlic when crushed. In the second year, Sagor said, those plants sprout stems that typically grow 2 to 3 feet high, produce small white flowers, and then form pods with hundreds of seeds.
Finding a plan of attack
Van Sloun said the key to controlling the spread is to prevent the seed pods from being formed.
“The number one way to control garlic mustard is to hand-pull it at the proper time,” she said. That means from mid-May to early June, when the stems and flowers of the mature plants are visible and the moist soil makes the roots easier to dislodge.
Other methods of control are cutting down the stems before they flower or applying appropriate herbicides.
Sagor said the plant produces so many seeds that some remain in the soil in a seed bank for several years without sprouting. That means getting rid of a garlic mustard patch can take five to 10 years, he said. “You have to be in it for the long haul,” Sagor said. “You’re not going to go out once and hit it and then be done.”
Peterson said he has learned that in his woods. “After about two years of working on this, it appeared we were getting nowhere,” he said. “I’d kill every plant, and next spring there would be a million more of them.”
The retired engineer fine-tuned his approach and became more methodical. He has an aerial map marked with about 80 garlic mustard patches on his property. He sprays the patches with an herbicide each spring, re-checks each site several times, pulls plants if he finds any that are flowering, and uses a spreadsheet to keep track of it all.
Now that he’s been at it for five years, Peterson said he’s beginning to see success. Wildflowers and tree saplings are returning, and garlic mustard patches are spotty. “I’ve got the upper hand on it now, and I’m going to kill every one of them,” he said.