Make sure you have cool season grass first. If you are down south, this isn't an option. The product is called siduron or tupersan - usually on fertilizer you will see it called siduron.

The pre-emergents are a useful product, but they have their limitations. First, they have a window of opportunity. That might or might not be the 'useful life' listed on the label. That depends, obviously, on the conditions under which the product was put down. So let's assume you spray, and it's supposed to be good for three months. It might even stretch to four, but you can't count on it. Well, poa annua germinates in the fall. Those seeds become viable in the fall, winter over, and sprout in the spring. Crabgrass germinates in late spring, AND LATER. One would think that one could simply overdose the lawn, and spray every thirty days for twelve months, and that would do it. It might, by the way. I've been sorely tempted. But size of lawn, and expense, have kept me from it. Other than that, you've got the concept of re-seeding. The pre-emergents aren't a WEED (with a W) killer, they are a SEED (with an S) killer. The theory of a pre emergent is that 'no seed will germinate'. That's how you stop the cycle of poa, or crabgrass, continuing to grow from seeds from prior crops. But that means that grass seed won't germinate either, doesn't it? If your lawn is primarily Kentucky Bluegrass, that's less of a problem, as you can, with fertilization, get the existing plants to spread out, and fill in bare or thin spots. That's not possible with the ryes, and fescues. They don't spread much, if at all. Striking some sort of balance, between the need to re-seed, and putting down pre-emergent, is more art than science. Sometimes you get lucky. Sometimes you don't. But that really is the nub of the problem. But needing to put down multiple applications of pre emergent, over the course of a year, is fairly common. Having to do it for a two year or three year period, is fairly common. You've got to break that cycle of weeds-to-seeds-back-to-weeds-again. And it ain't easy.

My understanding is that pre-emergent keeps seeds from germinating. Therefore, if you are planting seedlings which are already sprouted young plants, it should not harm the. I don't know how long that herbicide stays effective, if you would put seeds down, but I don't think very long, becoz folks that use it reapply each year. I think you will be ok. Now that you own the place, I do encourage you and others to investigate and implement organic lawn and gardening practices. In my opinion, a less than perfect lawn (keep it mowed and it looks good anyway) is preferable to toxic chemicals. Not that you asked but I just slipped that in for good measure. Happy gardening. Laurie

Postpone the seeding and concentrate on the crabgrass first. Crabgrass only germinates in the spring, but you can grow grass from seed any time. Most pre-emergents for weeds will also prevent grass seed from germinating. http://www.garden-counselor-lawn-care.com/pre-emergent.html