Preventing wild onion growth in your lawn. First encourage your lawn grass to compete with weeds by applying 1-2 lbs of actual Nitrogen fertilizer in Mid-August through September. Also applying a good soil herbicide like 2,4-D in late fall will reduce the number of onion plants that you have to deal with next spring. You need to do this while the onion plants are actively growing, but late enough that you get most of the emerged plants, usually around early November. Your next control will be to spray the onion plants that show up in the spring again with a 2,4-D product. You need to avoid getting 2,4-D on sensitive broadleaf annuals and perennials in your lawn.