Here are your best two options:
Do you have less than 75% good grass with bare spots larger than a few inches in diameter?
Answer: If lots of larger spots, then seed this spring. Be sure to use a special Starter Lawn Food plus Weed Preventer that is compatible with grass seed (the regular crabgrass preventers will keep grass seed from growing). If you do not prevent weeds when you seed, you are likely to be very disappointed as crabgrass and other weeds will germinate and choke out your good grass before it has a chance to take hold. A machine called a slit-seeder will help make sure the grass seed comes in contact with the soil. You just select the Turf Builder Grass Seed blend that is right for your conditions, such as sun, shade, heat-tolerant, etc. If you only have a few bare spots to take care of, consider Scotts EZ Seed. I think this is the best bare spot repair product we have ever sold! Spread the Starter Lawn Food plus Weed Preventer after you put down the seed. Feed your lawn again in one month after seeding with Turf Builder Lawn Food. Once your new grass has been mowed 4 times, you can kill weeds with Ortho Weed B Gon MAX plus Crabgrass Killer OR Roundup For Lawns.
Do you have more than 75% good grass however your lawn is thin and weedy without widespread bare spots?
Weed, Feed or Seed?
Certain types of weed spray allow for seeding to occur as soon as the next day. Other types may require you to wait a few weeks.
Weed control treatments are tricky. You need the correct product applied at the right time to make it work. The first step in the process involves the assessment of your lawn. The decision to seed first or spray first depends on the result of this.
Post-emergent weed sprays are less likely to disrupt seed germination and grass growth. Selective sprays that target broadleaf weeds are also safer to use since they don’t affect the grass.
Analyze the Current State of the Landscaping
Many species drop seeds that can lie dormant for years. Once the right combination of water, sunlight, and temperatures arrive, they can pop up fast. This is why you may feel like hundreds of weeds have grown overnight.
Overseeding a lawn is often necessary to replenish grass that didn’t survive the harsh summer or winter seasons. Grass seed needs to reach the ground for it to have the best chances of sprouting. Eliminating competition from the weeds helps grass seed to have what it needs to grow.
Certain weed varieties thrive during the winter. Mediterranean grass is an annual grassy type of weed that is commonly found in Arizona. Winter weeds often show up as patches of tall green grass among a sea of brown desirable grass blades that have done dormant.
You might think that the problem is over because the plants die off during the winter. Seeing dead weeds in your yard means that the plants have entered a new growth cycle.