Pre-emergent is applied in March before weeds begin to germinate. When you see the yellow Forsythia blooming, that's your cue.
Corn gluten has a NPK ratio of 9-1-0. The nitrogen is great for feeding your existing lawn, and it also inhibits seed germination by drying out a seed as soon as it sprouts. It releases nitrogen slowly and feeds your lawn.
Corn gluten needs to be wetted after applying, then allowed to dry for a couple days. Check the forecast for rain.
Do not use in areas where you want to overseed your lawn. Corn gluten will inhibit all seeds from germinating.
One application suppresses weeds for 4-6 weeks.