Weed seed germination is directly related to soil temperature, not air temperature. If it would make you feel better to go ahead and put down Surflan now, you can do so without any harm; but in my opinion, you're about 2-3 weeks too early. But, then again, I'm not living in SW Florida either, so what do I know, huh? Just hearing that you folks down there are having night time lows in the 20's means that it is just going to take that much longer for the soil temperatures to get warm enough for weed seeds to germinate.

Anyway, when you do apply Surflan, here is some information for you. You will need to apply Surflan 3 times per year (every 90 days) in order to get season-long control. Each time, apply at the rate of 1 oz./1000 square feet. After each application, you must apply a minimum of one-half inch of water (rainfall or irrigation) within 21 days after applying Surflan. Also remember that if you are going to do any type of cultivating (aerating, verticutting, dethatching) it should be done prior to the application of any pre-emerge herbicide. Good luck!

Thank you for your reply back. I put it down, just incase I will not be able to get it down next weekend. This week it is supposed to be mid 70s highs and 50s as the low. So I may have hit it pretty good putting it down early.

I have a similar question regarding Central Texas (just north of Austin). After firing my lawn "care" company last year for substandard results, we were left without any pre-emergent applications in 2010. I'd like to put down something myself this year, and would prefer an organic program as long as it's effective.

So, when do I put something down on my central TX St. Augustine lawn, and, what should I use? THANKS in advance.

Im pretty new to the pre emergents so Im not sure what will work well there. Im trying surflan, if it works well, then Ill keep it. From the reading that I have done, unfortunatly, organic pre-emergents do not work too well.

Here is a list of some pre-emergents.
benefin (Amaze, Balan, Crabgrass Preventer)
bensulide (Betasan)
oryzalin (Surflan)
pendimethalin (Halts)
dithiopyr (Crab-Ex)
prodiamine (Barricade)

The link is the article this list came from.

Here is a link that might be useful: pre-emergents

To jscaldwell: I don't think I'm qualified to talk to you about organics, but dchall is qualified. Hopefully he will get on here soon and respond to your post. He grows a quality St. Augustine lawn in the San Antonio area and is very organic. Good luck!

Firstly, it's soil temp you have to use a meat thermometer or something that goes low enough but you can test the soil temp it's below label directions to check believe somewhere in the low 50s from memory. This year I myself I'm in South Florida never got there so you know what I'm doing I'm not putting down pre-emergent. Do the warmer temperatures it doesn't pay product. You're better off to wait a few weeks and look at the newly emerging Sprint's a selective herbicide like 2 4 D dicamba. Keep a monitor on it next two months in the summer you ready to hit any crabgrass with quinclorac. Remember follow your labels good soil pH I don't over fertilize. Prodiamine 65 for South Florida recent temperatures may not be something we can use unless you want to waste money. I just had this conversation what dr. Pat Williams at Sarasota extension office and he is not a pre-emergent herbicide fan. I'll take the advice of PhD why are we throwing money into a chemical that's polluting the environment eventually and not doing anything that's crazy so don't worry about it.

In my opinion, the only organic material making claims to be a preemergent is corn GLUTEN meal. As far as I can tell, it makes a great fertilizer but doesn't do anything to block weeds. One year I applied it every month from April through October and did not notice any reduction in weeds.