When are you applying pre-emergent herbicides?

I’ll apply it in Baton Rouge in mid-February, before crabgrass seed germination which if I recall correctly is a soil temperature of about 55 F.

I personally use Dimension (active ingredient Dithiopyr), many others use Barricade (active ingredient Prodiamine). Both are good. Dimension does have post-emergent control on young crabgrass if you have a crab grass issue and happen to miss the target date of application. I use to have a significant crabgrass issue - no more.

There are different brand names of the products available, just check to make the active ingredient on the label is what you are looking for.

ETA: These are good for St Aug, Centipede, Bermudagrass, Zoysia

How does Southern Lawn Weed Killer Concentrate compare to Dithiopyr? It's active ingredients are Penoxsulam, Sulfentrazone, Dimethylamine salt, Dicamba.

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So if I already have a bunch of lawn weeds right now I should use post emergence? And then do pre emergence next time?

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Yes, anytime weeds are visible in the lawn use post-emergent, you can do that 12 months out the year if you want or need to. Pre-emergement herbicides will not kill weeds that already established - only their seeds as they germinate.

Apply pre-emergent in late winter/early spring (mid-Feb/early March, south Louisiana) to kill spring/summer weeds seeds as they begin germinate with the onset of warm weather. Do the same in early fall, mid-Sept/mid-Oct (south Louisiana) to kill winter weed seeds as they begin to germinate with the onset of cool weather.

Pre-emergent herbicides effectively control (kill weed seeds) for about 2 to 4 months in the Deep South post application.

I put granules down with a spreader and have had good results over the years.