I can tell you, wood mulch on top of black landscape fabric makes for a very good worm casting production system. When I pull back my mulch after about a year, I’ll have ½” of pure castings on top of the fabric. Easy harvesting for seed starting, and much of it will wash through the fabric and infiltrate the soil.

If you want to use this system to collect castings, it’s important to irrigate beneath the fabric. The irrigation drives the worms to the top to deposit their castings between the fabric and the mulch. If not the castings will still be there, but mixed throughout the soil, and thus not harvestable.

I’m USDA Zone 4, and have just the opposite problem Jack mentioned with heat. Rather than gaining too much heat, exposed landscape fabric in my area loses heat too quickly. Either way, the mulch insulates.

Mulch and landscape fabric can both help suppress weeds (though the fabric is much more effective). The mulch has other benefits the fabric does not. Using both systems increases the weed barrier beyond what either could accomplish on it’s own, and conveys those additional benefits of mulching.

Consider mulch a moderator. It moderates temperatures, preventing drastic swings. It moderates moisture, absorbing excess during flooding, while allowing most of it to pass through during light rain events in a drought. If you use chemical fertilizers, or even blood meal, the wood takes up excess nitrogen and keeps the roots from burning. In infertile soils, it breaks down and adds fertility.

In my experience, plants always want to grow. It’s us gardeners who screw things up, either through ignorance, neglect or being over zealous in our efforts to “help”. Adding mulch increases our margin for error in many areas. I’m humble enough to assume I’m going to make some mistakes every year. Thankfully, most of those mistakes are minor, and wood mulch is so often a part of the solution, it makes sense to do it on everything I plant right from the start.

As for the weed block, I often use cardboard, and old t-shirts. Some retail liquidators will even sell king size bed sheets for only a couple of dollars, often making that a cheaper alternative to landscape fabric. I don’t know who would own a king size mattress and still want spiderman sheets, but thanks to such marketing blunders, my garden is pretty much weed free for very little cost, and fitted sheets work really well in raised beds.

The mulch in this case also covers up spiderman, lending to the aesthetics of the garden. My garden looks a little more like something you would see in a home magazine, and less like a trash heap in the front yard of a meth lab. I don’t really care that much about the aesthetics really, but I do try not to piss off the neighbors by turning my home into an eyesore in the community. 🙂