Ruth Stout said you should lay the mulch down first, then pull it away in spots to plant your seeds, so that they can get sun as soon as they sprout. Don't cover your desired seeds with mulch or you will stop them from growing, just like the weeds. Stout expressed irritation that people said, wrongly, that she said you should plant your seeds and then mulch over them.
If the soil is full of weed seeds and you are planning to direct seed all your vegetables, you might have to do something more to control weeds the first year. An example would be to smooth the soil surface, lay clear plastic over it, let a lot of weeds sprout, then remove the plastic and gently pull all the weeds without stirring up soil from below. After this, if you plant you vegetable seeds there should be less weed seeds than before. Another way is to smooth the soil, lay black plastic over it for a month or few months until the weeds below are all killed, then remove the plastic and plant without disturbing the soil much.
Another popular method is to mulch over the whole area with something that decomposes, but weeds really can't get through, like cardboard or thick layers of newspaper. Water it well, then stab holes in specific places and plant your seeds. You can mulch over the top of the paper or cardboard with other natural mulch. It will all decompose down and then next year natural mulch alone might be enough to control weeds.
Works at a residential alternative high school in the Himalayas SECMOL.org . "Back home" is Cape Cod, E Coast USA.
If the vegetables can germinate and grow through the mulch it is very likely the weeds will also come through. Most weeds are much more resilient than our vegetables. I would mulch the whole area and then just pull back the mulch where you want to plant. As your plants come up you can thin them to the desired spacing and slowly add mulch around the individual plants as they grow tall enough. But you will likely need to do some weeding around your plants until they get tall enough.
My family has been gardening this way for over 30 years with good success but it does not eliminate having to weed it just reduces it greatly. An added benefit is it can help improve your soil as the mulch breaks down and it can reduce your watering needs. If you are using a drip water system we found placing the drip system under the mulch works best.
A couple downsides to this approach is that if you are in a cool area it can take longer for the soil to warm up and the heavy mulch can attract slugs and other potentially harmful critters. But from a permies perspective that just means you need to encourage something that will eat the slugs and other pests. In my area garter snakes love to eat slugs so I'm planning on creating habitat for them next to each of my planting beds - rock piles and other such habitat features. I also found that when I grew white clover between my vegetable plants that the slugs really preferred the clover and generally left my vegetables alone. I would go out at night and the clover would be covered with slugs but the vegetables would be mostly ignored. This was in a heavily planted and heavily mulched bed. But with the white clover I had to be careful that it did not overwhelm my vegetables.
Hope that helps and good luck!
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