I wonder where these ideas generate from. You have a mix of things going on.

The time to apply any preemergent is prior to your yard reacting to the spring time weather. The forsythia is usually the first indication of the events of spring. Thus the earliest blooming of the earliest forsythia is the trigger. You are already late for this year. The "good news" is that crabgrass will germinate under proper watering and sun conditions all summer.

Chemical fertilizer should not go down until you mow real grass twice. By waiting you will ensure that you are fertilizing your grass and not your weeds. When you apply has nothing to do with root or leaf growth.

I'll tell you what has everything to do with leaf AND root growth, AND it controls your weeds to boot, is setting your mower to the highest setting. By growing longer leaves you will grow deeper roots. Deeper roots can get water deeper in the soil, so you can water more deeply and less frequently. Tall grass shades the soil keeping it cooler and less prone to evapotranspiration of the moisture, so you water less.

You don't have to use a weed-n-feed. You don't have to do anything until you mow (tall) twice. Then use regular fertilizer. By then some of the winter annual weeds will be dead anyway. By letting your grass grow up tall, weeds like dandelions will die out. Why? Because the surrounding grass will force the broad dandelion leaves to point up and they don't get enough sunshine. The plant will die. Any seeds that blow off the dandelion will not be able to get down to the surface through the tall grass so no new ones will crop up. And if they did hit the surface, and you did water them daily so they would sprout, the tall grass will provide enough shade that the seedling dandelion will not get enough sunlight to take root.

If you fertilize in a few weeks, many of the weeds will be gone by then. Then in two more weeks, a few more weeds will be gone but the remaining weeds will be healthy enough to absorb a spot spray like weed-b-gone. This is a less expensive idea than the weed-n-feed and seems to work better.

But if you water infrequently and deeply, and mulch mow at your mower's highest setting, you should not have crabgrass this year regardless of your use of preemergents. Mother Nature can throw you a curve and apply enough water to germinate the crabgrass, but that is not the end of the world. The tall grass will severely curtail any spread and may choke it out. It needs plenty of sun to thrive.

DC,

Thanks for your reply. I've been having my lawn cut to the highest setting for ~2 years now (I have someone cut my lawn, but I take care of the fertilizing, aerating, etc). Also, I always water deeply and infrequently.

So, I guess my thought that my grass is not yet growing is not related to lack of fertilizer. This seems confirmed now that we've gotten a good day and a half of rain and I can already see the grass greening-up and growing (I also aerated ~9 days ago).

I crabgrass can potentially germinate all summer, isn't there still benefit to using the pre-em to control crabgrass to prevent later-germination?

Thanks for your reply. I've been having my lawn cut to the highest setting for ~2 years now (I have someone cut my lawn, but I take care of the fertilizing, aerating, etc). Also, I always water deeply and infrequently.

EXCELLENT!

I crabgrass can potentially germinate all summer, isn't there still benefit to using the pre-em to control crabgrass to prevent later-germination?

Probably. You could use it every 6 weeks if you wanted to.

Now getting back to your excellent watering and mowing practices, are you saying you still had crabgrass issues? What kind of grass do you have and is there any Kentucky bluegrass and/or bentgrass in it?