As always with pesticides, read the directions on the label.

I assume you applied the preemergent in spring. Were there many weeds this summer season? If not you might try and skip next spring. Then if it becomes a problem next year you could either pull them before they go to seed or spray with glyphosate. Then reconsider the next spring....maybe you need to apply preemergent on your lawn at the right time so seed don’t blow to your beds. Just some thoughts!

As always with pesticides, read the directions on the label.

I do! And no else either could find this answer if they are killed or just paused sort of. Had a feeling they were forever killed but wanted to make sure as mentioned will probably skip the re-app of PreEm for the sake of having less chemicals but accepting that there might be some more weeds (which can be blow torched or something or just weedwacked to keep in check, or glyphosated which as bad the rep it gets, it probably dissipates/neutralizes faster than a preEm made to stick around for months and months) but not nearly as many weeds as before PreEm was used.

They are only killed if the seed start to sprout and they preemergent is there to affect them. As I understand it, the chemical makes a “blanket” over the area. If the area is disturbed, seeds may sprout there and not be killed. So you don’t want to move the soil or mulch where it is placed. Weeding or digging, etc would disrupt the “blanket” .

I gave you a rather curt answer because it was very early and I was on my way to work--and due in early for a Safety meeting at 6 AM. I'm off today, so you get the book.

Glyphosate binds into the soil very, very quickly (pretty much on contact, but let's call it a short period of time just to be safe). Bacteria then break it down over a given period, which is highly variable depending on soil moisture, temperature, and soil type. That's why, when you spray, you don't get a huge plume of dying plants downstream after the next rainfall...but you can get a small area of additional ones if you get a deluge shortly thereafter!

Your pre-em pretty much does the same. It binds very quickly and firmly, and only digging or other disturbance (worms aren't enough disturbance to count) will break the barrier. The time limit is due to bacterial action that breaks down the barrier--to them, this is a light lunch.

The pre-em layer acts as a desiccant to baby roots. It's weak, so only impacts roots that are very young, and it's a shallow layer near the surface, where baby roots are. More adult plants don't have water-gathering roots that close to the surface and wouldn't be significantly impacted even if they did. They can tap plenty from down below, enough to overcome a minor bit of desiccation from a surface layer.

It's not a hundred percent effective--pouring rain, minor breaks in the layer, and so on, can overcome it. But it's pretty good.

As always, check the labels--as noted--but these do tend to be a lot less dangerous for us and for other species than most pesticides. Preventing the problem causes fewer problems than spewing around glyphosate, and nobody gets lymphoma. :-) Although that's not much of an issue for you unless you're an ag worker, either--the danger is, frankly, overstated.

Having tapped your seed bank for a year, it'll help. But you may find that you have about 70% of the same problem next year. You can skip it to start and see how it goes--but if you find you start to have an issue, kill out what you have and then apply your pre-emergent at that point. It'll pick up coverage at that moment and take over.

It took me about ten years to sufficiently tap out my soil's bank to get things to stabilize. As of now, I only apply a pre-emergent to keep the P. annua sprout at bay in fall. Regrettably, surrounding lawns are infested with that, and there's no way for me to stop the incursion. Even so, spotty Tenacity applications are still required in spring on the two or three tufts that move in.

If you skip some years and your neighbors skip the years you apply you're all going to keep having crabgrass. As I understand it the application will prevent any seed from germinating including grass and flower seeds.