I see many lawns with crabgrass and other weeds in seed now. These seeds will be problems for years to come. The following is more information.
Farmers say, “One years’ seeding means 7 years weeding.” This adage indicates how long weed seeds remain viable in the soil. For example, if a soil contains 10,000,000 (ten million) weed seeds per acre, 50% will typically decay per year. Assuming no replenishment through reseeding, at the end of 7 years the population will be reduced to approximately 100,000. This is a lower number, but still significant. Also realize that the weeds that emerge each year often account for less than 5% of the soil’s weed seed population.
Information from the September 3, 2009 edition of the Plant & Pest Advisory, Landscape, Nursery & Turf Edition from Rutgers University http://njaes.rutgers.edu/pubs/plantandpestadvisory/2009/ln090309.pdf