In North America, garlic mustard is highly invasive. Something about how the plants’ roots secrete stuff into the soil that dissuades other plants from growing there. It prefers to grow on the forest floor, and generally where I find some of it I find quite a lot of it. When harvesting for the greens, just make sure they’re tender. As garlic mustard matures it not only becomes substantially more fibrous, but also much more bitter. However since it grows on the forest floor and gets relatively little sunlight, I’ve had luck harvesting the spring plants well into May. Don’t harvest the plants that have bolted because they always taste gross.