In order to establish itself in your lawn, a weed needs the same three things your lawn does: contact with the soil, water, and sunlight.
For this reason, weeds are most likely to sprout up in the bare or thin spots in your lawn. So the best defense against weeds is to maintain a healthy, lush, dense canopy of grass that outcompetes a weed’s ability to establish itself.
Lawns with open canopies (St Augustine grass) or those that are slow to recover from damage (centipede grass and some tall fescues and bluegrasses) exposes themselves to weed establishment more than those with thicker canopies (zoysiagrasses) or those with more rapid recover from damage (bermudagrasses).
If you pay the neighborhood boy to mow your lawn or hire a professional, keep in mind that your lawn is being mowed with the same equipment that just mowed another type of lawn and is very likely dispensing small plantlets of another grass type into your lawn.
So if you have a brand new zoysiagrass lawn, and Johnny mows your lawn just after mowing the bermudagrass lawn of Mrs. Smith, then you will probably have bermudagrass in your zoysiagrass before too long.
The great thing about mowing your lawn (assuming you have a walk behind mower) is that you get to walk every foot of your lawn pretty regularly.
Mowing a weed before it puts up a seed head will eliminate that weed’s ability to spread seeds and propagate the next generation, so mowing alone can be one form of week control.
If your lawn has relatively few weeds, you should consider just hand-pulling them (be sure to pull out the root if you can). Its good exercise and its 100% organic.
Herbicides can be selective or non-selective. Selective herbicides kill some plants but not others. For example, a selective herbicide might kill dandelions in your lawn but not your lawn.
A non-selective herbicide kills all plants. So unless you want to kill your lawn to replace it, you will only want to use selective herbicides that specifically state on the label that they will not kill your lawn type.
Herbicides control weeds one of two ways, before they germinate or after. A pre-emergent herbicide is used preventatively to keep weeds from popping up in the first place. A post emergent herbicide is used after a weed has already established itself in your lawn.
A pre-emergent contains a chemical that prevents a weed from putting down roots. Because it is a root inhibitor, it can also negatively affect your new lawn, which is trying to establish itself by putting down roots.
If you successfully killed and removed your old lawn, including all the weeds in it, your newly sodded lawn should have relatively few weeds. It is suggested that you hand pick weeds out of your newly sodded lawn for the first season.