early afternoon (high sun). Topical weed killers work through photosynthesis (cover the leaves). It also helps to put a drop of dish soap in your sprayer. It will help the chemical stick. If you have a light rain or your sprinklers turn on the soap will help keep the chemical present for another day or so. It may take two applications.

You may have to call a local lawn/pest comany if that does not work. Stronger chemicals require a professional application.