I live in Maryland, Zone 7, and my lawn seems to be the dream environment for chickweed. The stuff grows 12 months a year; certainly not as vigorously over winter, but it does grow and pop up in the winter.

"Conventional wisdom" says to apply pre emergent killers for this stuff in the Fall in September. But I wonder: if new plants are sprouting ALL THE TIME, why wouldn't a pre emergent be effective if laid down now --- for those future infestations that I know will come with the warmer weather?

I understand the pre emergent will not kill existing weeds. But I don't have the time, money, or patience to be spot treating this stuff every week until September.

Common chickweed is a winter annual, is this the kind of chickweed you have? There are several weeds called chickweed in Maryland:

https://extension.umd.edu/hgic...

Is your lawn shady/thin, and mowed short?

Zone 9a here. Our local Lawn "Recipe" includes pre-emergents 3x/year, Fall. Early Spring and Spring before it hits 80º.

Here's a pic.

Speedzone is effective against this weed, although once it flowers, the effectiveness is spotty for some reason.

Yes, the weed does take advantage of existing bare spots (but isn't confined to those areas).

I mowed for the last two seasons at what my mowers say are "3 1/2", but this season I'm just putting both at the top setting.

I good part of one section of the turf is a mix of what is known locally as "wire grass" and tall fescue. The wire grass doesn't grow high enough to "shade out" weeds (the conventional wisdom with fescue and other tall season grasses).

I have been fighting that stuff now for at least five years.
I put some STRONG weed killer on a big patch, that was not near any thing else, it died and then came back late fall.
I do use pre-emergent but mostly use dedicated killer now.
Some areas it just does not go away, it dies and and then comes back.

It is mostly in my garden area but I have a second variety in my one part of lawn that is just as hard to get rid of.
High grass does nothing to get rid of eiher variety once it is established. Where it is growing on the edge of the thickest grass lawn, it gets fertilizer run-off from the garden , you think it is gone then one day while mowing , there it is, again.
There are spots under my Bridal Veil that I spray, and spray, and spray , and spray with killer.
As I said it dies, and then come back.

I have dug, not pulled , dug it out of a few areas and that does seem to work but in the lawn only works if you dig outside the area , down four to five inches , put new dirt in and reseed.
In the long run , it is far, far, far more miserable to deal with than Crab grass or even Canadian Thistle.

Thanks for sharing your frustration!! I'm hoping not for a cure from pre emergent but at least a diminution in the amount of new growth one sees. It sounds as if your experience validates my observation that it really does not matter when you apply the pre emergent: the stuff is coming up 365 days a year.