broadleaf weed killer and reseeding lawn

My lawn is fairly weedy. I would like to kill the weeds without harming what grass is left (probably using a 2,4-D type spray) and then reseed in the fall. How long do I need to wait before I reseed?
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Dee wrote:

How long do you need to wait after what? After spraying? You don't need to wait.
Also, with a little TLC, your existing grass will probably fill in without reseeding. I was shocked last year to discover how much damage the violets and creeping charlie had done to a patch of my lawn. I started watering it more and pulling the weeds, and there was very little grass left. But the grass filled in nicely before I got around to reseeding. When I got rid of the creeping charlie and most if the violets, the grass was able to crowd out most of the other weeds (for some reason there were no dandelions here; I woulda spot treated them with 2,4-d if there were.) I think the creeping charlie choked out the dandelions :-/
Bob
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If you are using a broadleaf killer, there's no necessary gap between spraying and seeding. However, there are some other practicalities involved. IME this process works better if you:
1. Make sure the lawn is well-watered and not mowed for about a week; this assures that the weeds are actively growing. 2. Apply herbicide. 3. Mow lawn a week later; resume watering and not-mowing for 2-3 weeks; this gives all the broadleaf seeds an opportunity to germinate and establish. 4. Apply herbicide. 5. Mow lawn a week later; reseed.
You want reseeding to take place after temperatures have begun to ameliorate in the fall, but enough before when the ground freezes that the grass has an opportunity to create vigorous roots and crowns . . . that's about a month. So, what you need to do is find out the average date of ground freezing in your area, and work back by 9-10 weeks to figure the last date on which you could reasonably start this project. The first date will typically be 2-4 weeks prior to that. So, if your ground-frost average date is 15 December; you'd want to start this project between 8 September and 6 October.
An alternative, and somewhat safer option in northern climes is to kill of the weeds this late summer and fall, and reseed in early spring.
Chris Owens
Dee wrote:

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At least 60 days, 90 days is better. Spring is a much better time to treat broadleaf weeds. I would hand pull weeds 90 days prior to overseeding. If you have very little grass, you can use RoundUp over the entire lawn, then reseed in two weeks. If you use the RoundUp method it would be a good time to spread a load of topsoil and compost just before seeding.
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