Black medic Medicago lupulina is an annual weed that plagues the lawn every year. Right now you can go out in most open areas and you will be greeted with bright yellow pin size yellow flowers. Black medic is a member of the bean family so it will add nitrogen back into the soil. We often encounter it in lawns that have low nitrogen fertility. The seed head will turn almost black when ripen.
The plant can be a challenge to manage in the lawn because of the way it grows. It creeps low to the ground spreading out from a central tap root and winding its way through the lawn. Because it grows close to the ground it isn’t managed by the mower. However, you can hand pull it. Another method of managing the plant is to cut your lawn at 3”-3.5” tall. This will help shade out the plant naturally. Other options include post-emergent combination products or some newer chemistries such as Mesotrione. Pre-emergence herbicides such as Prodiamine and Dimension would also be good options to help prevent the seeds from germinating.
One of the interesting things about this plant it is recognized as the true shamrock in Ireland. But I think it is safe to say that most of us do not want to have it growing in our lawns.