Answer: Grass seed can give uneven results for many different reasons. It can be related to the specific type of seed and viability of the seed you planted, the even-ness with which it was spread, the amount of water each area received, and how the soil was prepared, for example. It is not too unusual to need to patch some areas or overseed later. The best time of year for starting lawn from seed is in late summer to early fall so you have another opportunity coming up soon.
Mowing at three inches is a good practice. Make sure you mow often enough that you never remove more than one third the grass height at a time. Correct mowing will help your grass shade out some of the weeds and encourage the grass to grow denser.
With a new lawn, it is important to keep the soil evenly moist while it is becoming established. With the heat of the summer upon us, it is really important to water as needed. Your goal in watering is to keep the soil evenly moist like a wrung out sponge, not sopping wet and not dried out. To know if you need to water, dig into the soil with your finger. If it is still damp, do not water yet. When you do water, apply it to the soil surface and water thoroughly and slowly so it soaks down to the deeper roots -- it should be damp down about six inches to encourage the grass to grow its roots that deep. After watering, wait a few hours and then dig down to see how far the water soaked in; it can be surprising.
There is no set schedule for watering, it depends on your soil type and on the weather.
As far as the weeds, whenever you disturb the soil you bring additional weed seeds to the surface where they will germinate; weeds seeds also blow in on the wind and will germinate right next to your desired grass seed germinating. And, some perennial weeds may return from pieces of root left in the soil during the preparation phase. For now, the grass will not be able to handle herbicide applications. You can spot treat perennial weeds with an herbicide containing glyphosate. Annual weeds will die out at the end of the season. As your lawn establishes the weed problem should diminish.
To help the lawn along, make sure to run some basic soil tests to check fertility and pH. Apply fertilizer and lime as indicated by the tests -- correct pH can really make a difference in the health of hte lawn. Your local county extension should be able to help you with the testing and interpreting the results. Good luck with your new lawn!
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