In 1990's, Dr. Nick Christians discovered corn meal could inhibit the growth of weed when the seeds are germinating and recommended CGM as a pre-emergent herbicide. This method is most applicable to crabgrass and dandelions, but the timing of application is very important. CGM is not effective for grown weeds, in contrary it would make the problem worse as CGM is a good fertilizer for most plants.
For a larger area, it is more efficient using commercial weed-killer.
To use less harmful chemicals, first use an electric or gas-powered grass trimmer to cut off the grass stems.
Set the trimmer line as low as possible to the surface. You can make a homemade weed-killer by mixing vinegar, dish soap with tap water (see House Cleaners).
Spray the solution to the grass roots. The grass should die out within 1 week and do not re-grow from the roots.
Warning: The chemical solutions will affect the chemistry of the soils and is not suitable for other areas. For lawn, sprinkle a little bit of fertilizers at the spots of weeds. It might kill the weeds and encourage the good grass to crowd out them.
It is not an easy question for watering plants - if over watered, the roots will be suffocated and if under watered the plants will wither. Generally, watering once or twice a week is sufficient for most plants in temperate zones, and if the soil is moist at the surface, you may not need to water the plant. To be sure, you may find a reliable moisture meter to check the soil or use your own finger to feel the moisture content.