Typically a new lawn installation is replacing an old tired weed-infested one and like in surgery, pre-op, or in this case, “sod-prep” is really important. The last thing you want to happen is have your new lawn become a hot spot of weeds or your old turf taking over the new.
A trick of the trade is to fertilize and green up your existing turf, then kill it. Start off by feeding your weeds with liquid plant food or granular fertilizer, water and wait three or four weeks. Make it as green as possible but don’t fall in love with it because it’s still that old nasty lawn you want to replace. Oh, don’t use a fertilizer that has a pre or post-emergent herbicide as that’ll kill the sod.
In about 4-6 weeks spray an effective weed killer, like glyphosate, to all of the stuff you just greened up. You see, the weed killer translocates or moves through fleshy green weeds into the root system finding the weed seed, snuffing it out, so it doesn’t pop through after you’ve sodded your lawn.