Question
Hey Floyd,

I live in the Washington, D.C. area(Herndon,VA)I have about 1/8 of an acre of lawn. Over the past 2 years I have reseeded the entire lawn b/c each year the weeds and heat kill my lawn.

Questions:
-What weed/crabgrass preventer should I use?
-How do I know if I am using the proper amt. (Scott's website says 'apply at regular rate' what is that?
While using all of this stuff, when is the best time to seed new grass?

Thanks

Answer
Good Morning Steve:
If you cannot control the annual grass weeds by providing a dense turfgrass canopy, the next best way to stop annual grass weeds from establishing themselves in your lawn is through the use of preemergent herbicides. These herbicides control annual grass weeds by inhibiting cell division in the young root system. The failure of the root system to develop results in the death of the young weed seedling shortly after germination. Remember that dense, high-quality lawns may not need yearly applications since crabgrass only occasionally establishes itself in lawns with good density.
A number of preemergent herbicides are available to homeowners for annual grass weed control.
Consult the package directions to see if they can be safely used on Kentucky bluegrass, perennial ryegrass, tall fescue and most fine fescue cultivars.
If preventive annual grass weed control is desired in newly seeded lawns or where young, desirable seedling grasses are developing, a preemergent herbicide containing Siduron is the only herbicide that will provide control but not injure the development of  seedling turfgrasses. In fact Siduron may be applied at the time of seeding.
Preemergent herbicides are generally only effective if applied before the annual grass weeds emerge. Therefore, early spring applications are essential if satisfactory weed control is to be achieved.  Herbicide applications should be completed and watered-in at least 7 days prior to the initial germination date to allow time for the herbicide barrier to be established in the soil. I would suggest that you contact your local Cooperative Extension office to find out what the germination date range would be in your area.
The regular rate of fertilization has been established at one pound of nitrogen for each 1000 square feet of lawn area applied three times a year. When you observe a bag of fertilizer, the first number on the bag indicates the percent of nitrogen in that bag. If you had a 50 pound bag of fertilizer and the numbers say 10-6-4 the bag contains 10 percent nitrogen or five pounds. That would cover 5000 square feet of lawn area.  Most fertilizer bags will tell you the amount of square foot coverage based on the nitrogen content.

I have made it easy for you by giving you the fertilizer calculator below. Copy and paste to your browser and hit "go"

Fertilizer Calculator:
http://aggie-turf.tamu.edu/aggieturf2/calculators/fertsheet.html

After calculating how much to apply, when to apply is the other important decision. Early fall (September 1-15) is a key time for fertilizing lawns in your area. The schedule below outlines when to apply based on how many applications are to be made annually and desired lawn quality. Use controlled-release nitrogen fertilizers in May and September and a fast-release source in late fall. Consider Mother's Day, Labor Day and Halloween as reminders or favorable times to fertilize lawns.
If you desire a moderate quality lawn I would suggest you apply two applications. One in early May and one in early September.
If you desire a moderate to high quality lawn I would suggest you apply three applications. One in early May, one in early September and one in late October or early November using a winterizer fertilizer. A good winterizer fertilizer would be a 24-3-12.
The best time to seed new grass in your area would be late August and early September. The next best time would be very early spring. The reason you should use the August / September time is that the new seedlings will be establishing themselves during the cool season of fall. Use the web sites below as a guide for the best times to do your lawn maintenance in your area.

Lawn Care Calendar for w. Virginia:
http://www.wvu.edu/~agexten/hortcult/turf/calendar.htm

General guide for lawns in your area:
http://iaa.umd.edu/umturf/Cultural/Guidelines%20for%20Lawn%20Maintenance

If you need more help let me know.
Have a good lawn:
Floyd McMahon