This should be done a few hours before installation of the sod so the soil will be firm and not difficult to walk on. Sod will normally knit with the underlying soil in about two weeks but it will take a month before the roots have extended deep enough for the grass to survive any extended period without water. You can survive some mistakes in sod handling, installation and growing, but if sod heats up before installation or drys out after it will die. Weed seeds can lie dormant in the soil for many years. When favorable conditions develop such as high acidity, low fertility, excessive moisture, compaction or light intensity at the soil surface, seeds can germinate.
Most soils contain many kinds of dormant weed seeds.
The same can be said of insects and disease organisms. Sod producers try to maintain an environment that prevents the development of these pests. Sometimes they resorts to the use of pesticides to achieve this. In many cases the certified sod you purchase has already been treated for crabgrass control for the year of purchase. Certified sod in Delaware will contain two or three varieties of Kentucky.