Japanese stiltgrass has invaded my lawn as well as plant beds. Last year I did my best to weed the beds and keep the lawn mowed. What would be the best practice to control it in the lawn? Would a preemergent weed and feed application touch it? I bought a bag of 18-0-4 fertilizer that contains .38 prodiamine for weed control but wonder if that is a suitable product or if you would recommend something different. Also, when should something be applied...spring or fall or both? Is temperature a factor? I appreciate your suggestions because I have large patches that are crowding out fescue grass.
Japanese stiltgrass is an annual weed, so it dies back in the winter and seeds germinate to grow anew in the spring.
So yes, a spring application of a preemergent can help, and the timing is now.
We suggest finding a preemergent that is NOT part of a 'weed-n-feed' fertilizer, as fall is a better time to fertilize lawns here. You can find the preemergent herbicides like pendimethalin or prodiamine on their own, with maybe a very small amount of 'carrier' nitrogen. (like Halts).
Here is our page on Stiltgrass and it's control: http://www.extension.umd.edu/hgic/lawns/lawn-weed-identification
Using what you have there could be used in the fall, and would prevent weeds that are winter annuals like hairy bittercress (see our lawn weed gallery here: http://www.extension.umd.edu/hgic/lawns/lawn-weed-identification) and others that germinate in the fall.