Question
Northeast ohio area.My existing lawn has much spotty yellowing, and different grasses.  I have an irrigation system, and a lawn fertilization service. It has been de-thatched and aerated but the spotty yellowing persists.  How do I install a new lawn over that which I have now? Round-up, then overseeding?  How about Round-up and hydo-seeding? After the round-up, does the dead grass have to be removed?

Answer
I recommend that you spray round-up over the entire lawn and wait a few days to a week to make sure that everything has been eliminated. Re-spray anything you missed.

Then mow the lawn really low (on the lowest setting) and bag the trash.

Rent a slice seeder (slit seeder). it is a de-thatcheer with an attachment which deposit seeds in the grooves which is left by the dethatcher. I recommend running the dethatcher over the lawn first without grass seeds in the sowing compartment and then raking away debris. Then re-run with seeds. Run the slit seeder two directions (north/south plus east/west) for best coverage.

Keep the seedbed slightly damp until germination. This is best done by watering 1-2 times daily for 10-15 minutes each time to keep the top 1" of soil consistently hydrated. The existing (dead) vegetation will help keep the soil surface hydrated.

I would recommend that you start eradicating the lawn in mid august, and by late august to very early septmeber that you seed.

Use a starter fertilizer at time of seeding and then apply another application of fertilizer 4-6 weeks later just as the new grass is thickening up.

Start to mow the new grass when it is 3-4" tall. mow it back to 2.5-3.5" and keep mowing at this height. do not mow too low.

Once the grass has been mowed once, gradually increase the time between watering and the amount of water you apply. the goal is to water infrequently but deeply (such as once per week for 2 hours) 4-6 weeks after germinating (sprouting). THe infrequent but deep watering pattern is how a regular (mature) lawn should be watered. The frequent, shallow applications are used for the initial stages until the grass germinates, but then gradually transition to a deep, infrequent watering pattern.

Good luck.