Invasive plants and animals are an expensive problem in the United States. Federal agencies spent more than $104 million last year to control them. But a study on the garlic mustard plant shows that it might be better to leave some invasives alone. Garlic mustard is a forest plant with heart-like leaves and clusters of white flowers. It can grow up to about four feet tall and is often the first green plant you’ll see in the spring. Europeans settlers brought it to the United States in the 1800s