what is this weed
This weed is growing in my yard. It started out last year on a narrow strip of grass. Now it has spread to an enormous area of the lawn. I have attached 3 photos. The first shows the seed head at the top of the weed. Number 2 and 3 show the weed in full. It is perhaps 10 inches tall at present, but that is only because I cut the lawn 12 days ago. I presume it would be taller, if I had not cut the grass. Can you ID this please? Thanks.
It looks like you are dealing with crabgrass. Take a look at the following web page for information on this weed and options for controlling it in lawns: https://extension.umd.edu/hgic/crabgrass
I am not an expert, which is why I asked you the question. However, my sense is that it is not crabgrass. Here's why. I applied pre-emergent crabgrass killer both last year and this year to the entire lawn. I used dithiopyr. I have no crabgrass anywhere in the lawn except where this weed is growing. After seeing the weed last year, I concluded that it might be hairy crabgrass, and therefore this year I was particularly careful to make sure I put down enough preventer in that area in particular. I used a split dose at the proper time in the spring. I know that in some years crabgrass preventer does not work well, because there is too much rain or too little rain. However, I was particularly careful to put down the preventer this year just before .5 inches of rain fell. My understanding is that that amount of rain is sufficient to activate the preventer, but not so much that it washes all the preventer too deep in the soil.
Given all the care I have given to this problem this year, I'm a bit surprised that I could have even a worse problem than I did last year in this part of the lawn. I have an immense area of this weed in the same area where it was last year, but have no crabgrass anywhere else in the lawn. Could you please take a look at my 3 photos again, and see whether you think it might be any other weed rather than crabgrass. Thank you.
We had one of our county agricultural agents review your photos. He identified this as crabgrass. Timing is everything when it comes to controlling it, even with dithiopyr. This product is a pre-post emergent, but we warmed up in some areas as early as late February.
Chuck Schuster at the Montgomery County Extension office would be glad to work with you on this if you have further questions. You may contact him directly at the number/email provided below.