Originally Posted by Think Green
I wasn't trying to be vague with my response.
Several variables can create issues that Riggle and Deukster pointed out.
I can't recall ever applying a preemerge herbicide that wasn't watered in for two weeks in a row without some sort of precipitation. Since I am in the transition zone of Arkansas, the dew-frosts seem to deliver moisture........just a little extra kick to the 3 gallons of water per k that I apply.
I can assure that the density of the turf canopy, the thatch layer or rhizome layer can reduce the product from making it to the surface of the soil unless it is watered in. The labels usually call for .5 inches of rainfall or irrigation, otherwise the product is subject to light deterioration. If the product just lays there on the surface of the grass, thatch, then what else is it supposed to do. It must be water carried into the surface of the soil. We are in the cooler temperature days, and sometimes in the 60's but not hot enough to deplete the product too much. It is the over watering of the product that reduces the effectiveness..........therefore, no water reduces the effectiveness.
Since you didn't directly focus on a certain pre herbicide..........I am directing my answer to my use of Simazine, Prodiamine, Pendimethalin.
I'm not sure that your 3 gal/M carrier volume does much to move any product into the soil. To cover an acre in a half-inch of water takes 13,577 gallons. Assuming equal amounts of solids and voids in soil (not always an accurate assumption, I know), it would take 6,788.5 gallons of water per acre to move your PRE treatment a half-inch into the soil. Your carrier volume applies only 130.68 gal/A moves the product only 0.005 inches into the soil. Knowing that a heavy dew only adds a gallon to two to an acre, we can see that neither carrier volume nor dew contribute in any meaningful way to moving the PRE into the soil.
All that aside, I don't think that not getting water on this application is going to make it a total disaster. I've had many customers over the years that didn't water in their app and didn't get any rain, but still had fewer weeds than their neighbors. Sure, getting it watered in is the best scenario, and I think it's not too late to begin watering now.
You'll notice better control in the areas that don't have much weed pressure (thick, healthy lawn, not too wet nor too dry, right amount of sun or shade for what you're growing), but you'll have some weed breakthroughs (maybe not bad, but I think there would be some) in the areas that are more prone to weed problems (thin grass, overly shady or sunny, excessively wet or dry).