There are several different kinds of buttercup weeds, including tall buttercup, creeping buttercup, and Bermuda buttercup. All of them have several things in common besides being a problem. They steal nutrients, water, and sunlight from desirable plants and grasses, grow where they're unwanted, and look unsightly. There are several things you can do to remove these aggressive weeds, before they wreak havoc on your garden or lawn.
Dig up buttercup weeds with a sharp trowel or hoe. It is best to do this between fall and spring, when soil is moist and roots are less likely to break apart.
Spread a clear polythene sheet over the area. Make sure the sheet is pulled taut and held down at the edges by something heavy. Leave the sheet for 4 to 6 weeks. This will kill any remaining buttercup weeds and unsprouted seeds. Note that this will also kill any grass in the area, thus, if there is healthy grass in the area that you want to leave unharmed, skip this step.
Fertilise the soil. Fertiliser helps promote healthy grass. Be sure to fertilise at peak growth times for the particular grass you are growing.
Add lime to the grass. The best time to add lime is before you plant grass. However, it can be added in the fall or spring if the grass has already been planted. Lime also helps promote healthy grass growth and prevent buttercup weed from re-establishing itself.
As an alternative, you can apply a selective herbicide. Herbicides can be spread over the entire area or painted onto the buttercup weeds with a brush, to avoid harming surrounding grass and plants. Be sure to apply the herbicide as directed on the package. Herbicides contain toxic chemicals and can be dangerous if used incorrectly.