Need Help with Plant Identification
My daughter just bought a house in Silver Spring this year, and this plant sprouted like a ground cover in her yard, which is quite shady and moist. We can't find the plant in books and Valley View Farms staff near Baltimore couldn't ID it either. The plants seem to sprout from single stems in the soil and the flowering bodies have thicker stems.
Montgomery County Maryland
Your lawn weed appears to be Carolina geranium. Take a look at the images in a search engine. You'll see that there is a lot of variation in the leaf structure but there are photos that closely match yours.
Carolina geranium is a native wildflower that is used by pollinators and birds, who eat the seeds. No doubt other wildlife uses it also. You can let this annual grow if it grows where you want it. Otherwise, use a broad-leaved herbicide labed for it. If you kill off a large patch, you'll need to reseed immediately or else weeds will simply repopulate the spot.
If you see this plant in a lawn, that suggests the turf is thin and weeds are filling in the bare areas. The lawn needs to be overseeded. The best time for successful grass seeding is early fall--even as early as late August when the evenings begin to get cooler. (Also is this a shady area? Grass can't grow in total shade, but if there is 3-4 hours of sun a day, plant fine fescue--not tall fescue.) See the many fact sheets on lawn care and planting on our Home and Garden Information Center website: http://extension.umd.edu/hgic. One particularly good publication for you may be "HG102: Lawn Establishment, Renovation & Overseeding" : http://extension.umd.edu/sites/default/files/_images/programs/hgic/Publications/HG102%20Lawn%20Estab.