Mulch is a given for most experienced vegetable gardeners. It cools the soil, retains moisture, smothers weeds, and when it breaks down, adds organic matter to the soil. I’ve tried a few different types in my day, and wanted to spout some old guy opinions.
Hay: Love the smell and the ability to cover a large area with an armload. With hay straight from the bale, normally the individual pieces are so long that after time, the more persistent weeds will find their way into the light. If I use hay, I’ve learned to run over it a few times with a mower. A shredder would do the same or better. Of course that makes the pieces smaller and gives better coverage, but also dramatically increases the amount of hay I need. It’s not free in my case. That being said, shredded hay, when it’s free, makes a beautiful mulch.
Straw: Not at all my favorite mulch when used as-is from the bale. The stems are hollow and woody with less nutrient value than hay. Weeds poke through even the heavier applications of straw eventually (in my experience). It’s tough to nestle small seedlings into whole straw. But as with hay, shredded straw is a different matter, and it actually makes a pretty good vegetable garden mulch. Again, I don’t have free straw. I realize that many really good gardeners mulch with hay and straw. They’re just not for me unless I can find some freebies and do the shredding.
Wood chips: Some gardeners swear by wood chip mulch. Also not for me except in paths. The stuff I get free from the county isn’t screened and there are just too many large pieces for anything other than paths (which I do cover with wood chips when I can). Even bagged chips seem iffy to me, because since my crops are rotated, there could be small seeds to plant in any given bed any given year. If the large chips have worked into the soil, that could pose a problem with germination of tiny seeds. Shredded bark is probably an excellent mulch, but way to expensive for my tastes.
Leaves: Whole leaves aren’t a great mulch. They eventually sog down and mat together, not a good environment for most vegetables. But shredded leaves are a whole nuther matter. You’re probably noticing a pattern here. When in doubt, shred. It makes all the difference. More surface area, more air space, better water flow. Free.
Grass clippings: Probably my favorite mulch. When applied correctly (maybe 4″ deep, depending on the plant size, dried for a bit in the sun) they’re awesome. Weeds won’t normally pop through a good mulch of grass clippings, and they return organic matter and nitrogen to the soil. My neighbor sometimes lets his field grass get pretty tall before mowing and lets me help myself to the clippings. They’re easy to pick up, but face the same drawback as hay; too much length to effectively keep weeds from popping through. Shred. The only problem that I’ve ever had with grass clippings applied correctly is getting enough of them. I’ll shamelessly stop the car at a neighbor’s curb and scarf up their brown Ace bags, but that can be a mixed lot. Sometimes even nauseating.
My opinions only. Not scientific, not thorough. A permaculture system would negate most everything I just said. But no matter what the material, your garden needs mulch.