1. "Mulch" is not a product. It's what you do with a product -- putting it on top of the ground; for many purposes including to suprress weeds.

2. A worthy mulch for suppressing weeds is at least 4 to 6 inches thick. This blocks out light, cuts down on carbon dioxide and sometimes offers some allelopathic effects. The thinner the mulch, the less weed suppressing characteristics. 25 bags of mulch (assuming 2 cubic feet each) would cover 75 square feet with 4 inches of mulch.

3. It would be rare for a product sold as "mulch" to have weeds in it. Most organic products sold for such purposes (as well as for amending) are composted "forest byproducts". Usually it means they sweep up the floor of the lumber mill, usually the bark, grind it up and compost it naturally or with chemicals. No weed seed in bark. On the other hand, if you bought some cheap product and it was cow manure, you'd be lucky if you didn't have any weeds. Or it could be you bought one of those "decorative nugget mulches" (redwood or cedar) and applied it just enough to make the ground look nice.

4. A mulch applied too thinly often actually encourages more weeds than you had before. It provides just enough darkness for weed seeds to germinate and maintains just enough moisture for weed seedlings to grow nicely.

5. Last note/advice: mulch is great in working gardens such as vegetable gardens and orchards and it's important to mulch newly plants plants (except most natives). But mulch is not a design concept; it's never necessary to mulch an entire garden. One should plant every square inch of one's garden (or put in hardscape) so that the garden is covered with plants so densely, weeds have no place to grow.

Joe

How thick was the mulch? You need several inches to really stop weed seed germination.

So Cal got over 2" of rain in December, that's plenty to get weed seeds sprouting.

I'm assuming this was bark mulch? It would be unusual, but not impossible, for bagged mulch to contain seeds.

I usually spread newspapers down and pile the mulch on top of that, which keeps the weeds down better than bark mulch alone. I'll do this when preparing a new garden bed or replanting an area. It works very well.

I get my mulch from the town recycling and I think it does sometimes have seeds - but it's free :-)