I’ve saved one of the best for last because this is something that I think too few folks understand. I even saw a garden center recommending that we apply pre-emergents to our garden beds now without any recommendation that the beds be weed free first (or snow free, for that matter–at the time I read the blog post, 6″ of snow still covered most of my beds and the garden center wasn’t too far from me!)
Pre-emergents are just that–for weeds that have not yet come out. If there are weeds already there, forget about it. If you have perennial weeds, forget about it.
What do I mean by perennial weeds? Well, dandelions are perennials. Did you get all those out last year? I know that sadly, most of my weeds are perennial or I wouldn’t be back there going over the same thing year after year after year. And if they’re not perennial, they’re bi-enniel, like garlic mustard, for example. No pre-emergent is going to stop that from coming back.
Worse yet, the advice was simply to apply a pre-emergent with no mention of how to do it correctly. In order for the darn things to work (and please keep in mind that unless you’re applying corn gluten meal, all pre-emergent are not organic) you must water them in. So perhaps that’s why the garden center made no recommendation about how to do it–because they knew it was impossible to water right now.
But without water, the pre-emergent doesn’t work properly so all you’re doing is basically applying a chemical to your garden that will not control even annual weeds–so why are you doing that? Don’t go there.
If you want to control weeds and don’t have heavy clay soil, you’re much better off mulching. If you do have heavy clay, try what I saw one garden writer describe as a “living mulch”–otherwise known as plants or a low groundcover
But these pre-emergents are way over-sold for what they can do. And if you cannot apply them properly and water them in, and if your area is not weed free to begin with, do not expect good results.