Answer: What you describe sounds like black medic. Black medic is normally a summer annual, but can act as a perennial in some conditions . It has a tap root, and spreads low to the ground, but it does not root from nodes on the stems. Black medic is more active on soils low in nitrogen fertility, but with all the weeds in the neighborhood, it's probably just blowing in and taking advantage of your hospitality.
The leaf of Black medic is similar to clover and other legumes, having three leaflets. Black medic's center leaflet is on a separate petiole.
The flower of black medic is a compressed cluster of bright yellow flowers in the shape of a globular spike on short branches. The seed pod will turn black at maturity. Black medic produces viable seed under normal mowing conditions.
Hand digging is the most environmentally friendly way to deal with Black medic (and other clovers, as well). Because of the root systems, broadleaf killers such as 2,4-D are usually not effective. You can use a premix such as Trimec that contains dicamba, MCPP, and 2,4-D. Another good option for control is to carefully apply glyphosate (e.g. Roundup), because this herbicide will move into the root system. Glyphosate is relatively safe for humans and the environment and it breaks down rapidly. Remember that even though glyphosate is relatively benign in the environment, it will kill any plant it touches, including grasses and trees, so apply it very carefully. One method of application is to wear rubber gloves and apply it with a sponge or with a cloth glove worn over the rubber glove and dipped directly into the solution. If glyphosate is used, select the concentrate formulation rather than the ready-to-use products, which are often less effective. Mix the concentrate according to label directions. Herbicides containing triclopyr can also be useful. When using any pesticide, be sure to read all label instructions and follow them carefully.
Best wishes with your lawn!
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