If you've weeded thoroughly, including removing the roots of any persistant perennial type weeds, then a pre-emergent is unnecessary. The layer of mulch will supress new weed development.

Most pre-emergents have a specific timeline of effectiveness - they should be applied 2-3 weeks before the germination of weed seeds and most are only effective for 6-8 weeks. If you've already weeded, then I assume most of the fall season weeds have already germinated - I know they have here.......so applying a pre-emergent now is just an unnecessary and non-productive expense.

My weeding job will be in January.

In the past, when I've thoroughly weeded -- thinking I've pulled all of the weeds (roots and all) -- and applied Preen weed-preventer (per its instructions), new weeds appear within one to three weeks.

My goal this time is to apply a more-effective weed preventer right after weeding then immediately apply the mulch.

What type of mulch are you using and how thickly is it applied? Typically, 3-4 inches of a wood-based mulch applied after a thorough weeding will suppress weeds as effectively as using any type of pre-emergent. And with significantly less collateral damage to the soil biology than most commercial pre-emergent products. A lot of pre-emergents are not restricted simply to weed control. Depending on active ingredient, many of these products can also affect growing plants - trees, shrubs and perennials - and should be used with caution.

and applied Preen weed-preventer (per its instructions), new weeds appear within one to three weeks.

My only response to this is that the product must not have been applied correctly. Preen (trifluralin) is perhaps one of the most effective pre-emergents on the market with the least amount of non-target side effects. But the timing of its application must coincide with the start of the weed season to be most effective. And it has a limited effective time frame as well.

FWIW, I use compost as a mulch - a pretty fertile seed base on its own - but still have great results with its effectiveness at weed supression without resorting to any chemical products. Usually just a periodic raking to loosen the surface and disrupt anything that has seeded in will suffice for weed control for the season. Bark or wood chips would offer even greater control.

And any pre-emergent has limited ability to control perennial weeds....most are restricted in their effectiveness to annual weeds.

Your weeding happens in January but you just weeded your garden? Sounds like you let a bunch of weeds go to seed. Preemergent can work pretty well, but it isn't magic. The way you get rid of a weed problem is to make sure they never go to seed. Then you just have to worry about some windblown stuff and bird crap which are easily manageable.

When I said, "In the past...", I meant "In past years....".

I'll do the thorough weeding & mulching in January 2015.

My city offers FREE mulch consisting of "recycled green waste". I plan to apply a 3-4 inches of this mulch on top of the soil.

If a pre emergent "weed" suppressor is applied and new "weed" growth appears in one to three weeks then either the "weed" suppressor was not applied properly or the product was defective.
I have not had a problem with unwanted plant growth coming up through 3 to 4 inches of shredded leaf or wood chip mulch although there have been times when new unwanted plants germinated and grew in the mulch. I find newspaper or cardboard, under the mulch, is quite effective in suppressing the unwanted plants without needing to use any growth suppressing material, and much less expensive.

I contacted Lebanon Seaboard Corp. about the rapid reappearance of weeds in my garden.

When I told them what weeds were in my garden and I described my weeding technique, how I applied Preen Garden Weed Preventer and how I watered, I also gave them the date the Preen was manufactured.

They confirmed that my weeds were listed on the "Weeds Controlled" list and they said I was doing everything correctly so they offered me a refund (which I took).

Is there a more-effective pre-emergent that I can use in January?

cardboard.

Cardboard or newspaper will keep many unwanted plants from growing with roots in the soil that is covered, something pre emergent will not do, and if the unwanted plants have seeds in the soil will stop most of those as well. A mulch that is 3 to 4 inches thick will do much the same thing.

In mid-January, I plan to:

1. Remove all gazanias.
2. Thoroughly weed
3. Plant new roses and relocate several existing shrubs
4. Apply 3-4 inches of mulch

Really, they are not joking. Cardboard is great stuff for the garden! I go to a local appliance store and they let me load up my station wagon. (For free - otherwise, they would have to dispose of it.) You just rip off any plastic or metal bits, rip the boxes open (thicker ones might need a box cutter) and lay them down on the ground. You can take smaller portions and place them around the base of shrubs. No problem if in some places the cardboard doubles up. Then you just put your mulch on top, in a thick layer like normal. You won't be able to see the cardboard at all.

I don't use chemicals so I can't compare, but cardboard does a number on weed growth in my experience. It will also break down and improve your soil, so your shrubs will be healthier. Pre-emergents can't do that. So I'd add step 3.5 - lay down a layer of cardboard.

Good luck!

Ok, you say that you have weeds appearing in a bed where you have weeded properly, applied Preen correctly (after carefully considering the side effects of using a toxic product, I would hope), and then applied free compost as a mulch provided by your city.

How deep are you mulching? I also use compost to mulch about 3"-4" deep (or more) and don't have any weeds come through it for months. If you are putting on less than that, however, weeds could come through it. In addition, it won't last very long and weeds will start to appear in a relatively short time, as it wears away.

However, if you are mulching correctly, it seems to me that the weed seeds must be coming from someplace other than underneath the mulch. Have you tested the mulch you are using to see if your free mulch has weed seeds in it? From reports on this forum, it seems a lot of cities' free compost is not top quality, but rather a 'you get what you pay for' sort of deal. Maybe it has weed seeds in it that have survived their composting process. You could easily test it by spreading out a few square feet of it and placing a cold frame over it (or make a Remay or clear plastic tent over it), wait 3 weeks and see what sprouts.

Alternatively, you could just buy some bark, compost, wood chips, whatever you want to mulch with, that you can be sure is not contaminated.

Or maybe the weed seeds are arriving by some other means. Are they being brought in by birds or other creatures? Are they blowing in? Are there other weedy areas nearby? On a neighbor's property perhaps? If so, those weed seeds are in the air and they will land on top of your mulch and sprout.

You aren't going to get a lot of support here for dumping chemicals on your soil to try to solve problems. Most of us here know that that at best that is short sighted and tends to create more problems. The best solution could very well be less work and less expensive than applying another chemical, but you'll have to use your noggin to figure it out.

None, please, there are enough chemicals in the world.

I am another fan of cardboard under the mulch, and I also wonder about the weed seeds that might have survived your city's composting. I put down coardboard and then several inches of wood shavings and have had not problems with seeds sprouting or with plants coming up from leftover roots.

I truly appreciate all of the comments (especially against using chemicals).

I've decided to put cardboard over the thoroughly-weeded soil then to apply 3 to 4 inches of clean mulch -- NOT the city's "free recycled green mulch" (whatever that is) -- on top of the cardboard.

Question: Will weeds grow in the mulch?

There may be weeds in the mulch, but the better you treat your soil biosphere (i.e. no chemicals), the better it will treat you. The weeds should pull out very easy if the soil is well fed by the mulch/compost. Cheers!

You'll always have a few weeds popping up where you don't want them. A good mulch will make them much more manageable, but nothing totally eliminates them.

I mentioned nearby weedy areas above because that can really cause problems. I had some neighbors who let their yard go to weeds, waist high, for two growing seasons, and that brought a huge number of both weeds and destructive insects to my garden. It's been a couple of years now since they landscaped, but I am still dealing with the effects.

Another way that I think people have problems with weeds is if they don't monitor the situation closely enough. I don't know if this applies to you, but some folks have the mindset of 'once and done'. They mulch and then don't think about it for 6 months or so, and then they have a weedy mess. It's much better to walk by the bed and pull any weeds you see sprouting as often as you can. It's the old 'shadow of the gardener' effect. If you take care of them before they become a problem, they won't ever be a problem. :)

I'm glad to hear you want to give a non-chem approach a try. Good luck!

"Question: Will weeds grow in the mulch?"
Yes, unwanted plants, aka "weeds", can and do grow in mulches. Seeds from those types of plants arrive in mulches through movement by wind and bird droppings. However, removal is quite easy and often needs little more than simply stirring the mulch a bit to uproot any unwanted plants that start to grow.