Thanks for providing a photo. The weed grass is the only thing I see from the picture and is what I call 'winter grass'. It is an annual, which means it grows from seed each year. It will die when the hot weather sets in, but only after it scatters it's seeds for next year. In the meantime it will have shaded out what St. Augustine grass there is and it will take a while to fill back in. Plus you will still have the dead clumps to deal with. Pulling it out clump by clump is what I did, but if you don't have the time and inclination to pull it out you can try keeping it mowed very close to keep it from going to seed, but that will not work 100% of the time because it seems the seeds will just grow horizonally below mower height and then they would still have to be pulled. It's a tenacious little bugger to get rid of. Little clumps will keep coming up and going to seed. You really have to keep after it for a couple of years. This coming fall you can spread some pre-emergent weed killer at the proper time, around the end of September I think, and this will help to keep the seeds from germinating, but you'll still have to pull a few clumps as soon as you see them. Now is the time to spread compost and Fertile Garden on Leslie Rd. is the recommended place to get it. They have several compost mixtures for grass. You can talk to them and see what is recommended for your conditions. If you have pretty good soil already you can probably get by with spreading an organic fertilizer such as fifty pounds of soy bean meal per 1000 sq. feet of lawn. Looks like you have the makings there for a beautiful expansive lawn. Hope this give you a few ideas on how to proceed. If you post your pictures and questions on the 'discussions' forum, which is more widely read, you will get some more answers. Scroll around on the link below for 'Turf Grass' for additional information. Here is a link that might be useful: Plant Answers ...

...seems I saw this picture on the other forum and answered there... Anyway, this is Parthenocissus quinquefolia, aka virginai creeper.

Ranunculus ficaria -- it definitely is the culprit -- and it looks horrible. I can vouch that it's nearly impossible to dig up all the underground parts of a single plant, much less an infestation. I am now going to have to deliberate: Use Roundup / glyphosate even though I have avoided chemicals for years, or spend an eternity digging and probably not succeed. This link sent by kalmia10 says lesser celandine spreads mostly by the underground tubers, but here at my house it seems to pop up FAR from existing colonies, meaning 15 feet from nearest plant, as if it spread by airborne seeds. Is it just inadvertant movement of tuber segments??? or could it also spread by seed?

Hard to tell what it is without a picture of some sort -- can you post one? Most grasses don't have big, tuberous roots, but I could be mistaken, there are all kinds of things growing in Brooklyn! One definition of a weed is a plant in the wrong place -- is this taking up space you want to plant right now, or can you wait a little more for flowers and a more positive ID?