Did you seed with fescue, Kentucky bluegrass, both, or a mix with rye? You might have to find the bag or box of seed to answer this.

If there was any Kentucky bluegrass in the mix, then you will not need to reseed. If it was only rye or fescue, then you might want to reseed. KBG will spread to fill in while the other grasses will not spread.

Here are the basics of lawn care...

Basics of Lawn Care

After reading numerous books and magazines on lawn care, caring for lawns at seven houses in my life, and reading numerous forums where real people write in to discuss their successes and failures, I have decided to side with the real people and dispense with the book and magazine authors. I don't know what star their planet rotates around but it's not mine. With that in mind, here is the collected wisdom of the Internet savvy homeowners and lawn care professionals summarized in a few words. If you follow the advice here you will have conquered at least 50% of all lawn problems. Once you have these three elements mastered, then you can worry about weeds (if you have any), dog spots, and striping your lawn. But if you are not doing these three things, they will be the first three things suggested for you to correct.

Watering
Water deeply and infrequently. Deeply means full inch (as measured by cat food or tuna cans placed around the yard) in every zone, all at once. Infrequently means monthly during the cool months and no more than weekly during the hottest part of summer. Do not spread this out and water for 10 minutes every day. If your grass looks dry before the month/week is up, water longer next time. If that does not work, then you might have to water every 5 days during the summer's hottest period. Deep watering grows deep, drought resistant roots. Infrequent watering allows the top layer of soil to dry completely which kills off many shallow rooted weeds.

You will have to learn to judge when to water your own lawn. If you live in Las Vegas your watering will be different than if you live in New Orleans. Adjust your watering to your type of grass, humidity, wind, and soil type. It is worth noting that this technique is used successfully by professionals in Phoenix, so…just sayin.' The other factors make a difference. If you normally water 1 inch per week and you get 1/2 inch of rain, then adjust and water only 1/2 inch that week.

Mowing
Every week mulch mow at the highest setting on your mower. Most grasses are the most dense when mowed tall. However, bermuda, centipede, and bent grasses will become the most dense when they are mowed at the lowest setting on your mower. In fact there are special mowers that can mow these grasses down to 1/16 inch. Dense grass shades out weeds, keeps the soil cooler, and uses less water than thin grass. Tall grass can feed the deep roots you developed in #1 above. Tall grass does not grow faster than short grass nor does it look shaggy sooner. Once all your grass is at the same height, tall grass just looks plush.

Fertilizing
Fertilize regularly. I fertilize 5 times per year using organic fertilizer. Which fertilizer you use is much less important than numbers 1 and 2 above. Follow the directions on the bag and do not overdo it. Too little is better than too much. At this point you do not have to worry about weed and feed products - remember at this point you are just trying to grow grass, not perfect it. Besides once you are doing these three things correctly, your weed problems should go away without herbicide.

You will not need to fertilize with chemicals again until the grass is up and being mowed for the second time this year. Otherwise, as you say, you are fertilizing the weeds. If you want to use something, use an organic fertilizer like alfalfa pellets (rabbit chow). The application rate is 20 pounds per 1,000 square feet.

Thanks so much! Yes we did use a blend with some Kentucky Bluegrass, but I don't know what the percentage was. It was a sun/shade mix. So I may just wait and see if it spreads a bit this year. I was just afraid that if I left bare patches that I was asking for crabgrass :)

Crabgrass should not be a problem if there is shade. It needs full sun to thrive.

KBG at 10% should fill in nicely if it has enough sun. Mow it at least 2.5 inches high. Taller will work, but shorter will let more weeds in.

My guess is your going to have a fair amount of weeds as soon a the temperature warms up. It is true in dense shade you will have less weeds and little to no crabgrass but if you have open areas something is going to fill in those spots and it's not going to be all kentucky bluegrass, KBG is not the most aggressive grass out there and tall and fine fescue definitely are not. If it were me, I would think about a pre emergent herbicide that is not Preen and as soon as your grass starts to green up some apply a pound of N per 1000 sq.ft. to get whatever KBG is there moving. I would not try seeding in the spring, wait until the fall. Those completely bare spots I would buy a few pieces of tall fescue sod, it will have plenty of KBG in it, and cover those areas. The idea that if you mow high, water deep and infrequently and fertilize regularly and you will never have to apply a herbicide is a great theory, but just not true. This method will encourage perennial grasses, but nature left to it's own devices does not provide a monostand anywhere especially a lawn.