Don't worry, your new lawn can be salvaged.
Where do weeds in new lawn come from? It's important to understand this process first. In a new lawn, weeds are a result of the soil, not the seed blend. Weed seeds can lie dormant for long periods of time, waiting for their perfect opportunity to germinate under conditions where soil has been tilled and exposed to water and light.
It usually takes about two years to establish a solid lawn. In the first year, grass coverage is roughly 50%, consisting of single, straight blades. The second year will yield about 90% coverage, as rhizomes will fill in the remaining voids to form a solid, dense lawn so long as proper care has been provided.
To manage weeds in new grass, avoid utilizing broadleaf weed killer until after the fourth or fifth mowing. Spraying weed killer too early on immature grass can result in injury. Note that broadleaf weed control is most effective when applied in the fall.
Many common weeds, like velvet leaf and lambsquarters, will be eliminated once mowing begins and by following up with chemical control as needed. If annual grasses, like foxtail and crabgrass, germinate with new seeding, wait until the following spring to apply a pre-emergent product. This will help to prevent the gemination of new weed seedlings.
Weeds like barnyard grass that germinate and grow aggressively during hot periods will be killed by the first fall frost. During the fall and subsequent spring, the dead material will decompose, allowing new desirable grass to fill in the area. Treat with fertilizer to encourage the healthy growth of new grass for the season.