Killing actual dandelion plants is one tactic in the war on this weed. Another is creating an environment where dandelion seeds can’t successfully germinate. To do this, use a pre-emergent herbicide like corn gluten meal or Preen. This type of weedkiller interferes with seed germination, which means seeds can’t produce a plant. Use corn gluten meal in fall and early spring (about the time forsythia flowers). Another technique to make your yard unfriendly to dandelion seeds is to mulch planting beds, and don’t cut your lawn shorter than 2 to 3 inches. Taller grass grows thicker, shading soil so dandelion seeds can’t sprout.