A mulch is basically anything that you lay over the top of the soil with the intention of suppressing weeds or reducing soil moisture evaporation. The usual options include bark, wood, rocks, plastic sheets, compost, landscape fabric, rubber pieces and paper products.

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Unfortunately, most people prefer FINE MULCH, or "medium" fine mulch. Any mulch with 'fine' in it is botaniclly worse for your soil & plants than no mulch at all. This is because it typically blocks water & air from reaching your soil.

 

Overhead water cakes fine mulch, resulting in water runoff and nearly no gas exchange. Unless and until fine mulch becomes super-saturated it won't begin to be available for the soil below. Meanwhile, that moisture evaporates into the air. Finally, fine mulch does not prevent newly added weed seeds from germinating.

 

A BULKY MULCH (large shredded pieces or chips of bark or wood) works so much better. Air & water readily reach the soil. Old & new seeds are prevented from germinating because sunlight is restricted from the soil and the roots cannot penetrate the large pieces. Greatly shaded from sun and wind, soil moisture evaporation is greatly reduced. It also reduces soil compaction. An invisible enemy of roots.

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Bulky bark lasts longer than bulky wood, but it does nothing to feed the soil.​ Bulky wood breaks down faster, but it has some (limited) abiity to feed the soil. I consider bulky bark & wood to be respectively the 3rd & 2nd best mulching options; however, very distant 3rd & 2nd options to my push in this writeup: arborist chips.

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ARBORIST CHIPS are a bulky mulch derived from all the arborist trucks running about town shredding whole trees & bushes. This is by far (f-a-r) the best mulch ever. ​It contains green material, tender new shoots & branchlets and bulky bark & wood. Arborist chips not only reduce soil moisture evaporation & prevent weeds,

it...

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 - adds organic matter to the soil

 - prevents further soil compaction

 - (slowly) reduces existing soil compaction

 - feeds the micro & macro fauna...no more fertilizer ammendments

 - reduces soil temperature fluctuations

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The ​CONS to arborist chips?  They're long proven to be false.

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 - invites termines....nope

 - infects your soil & plants with pathogens from other peoples' yards...nope
 - acidifies your soil if it's full of conifers. Conifers don't make soil acidic. Rain does.

 - black walnut bits harm plants. That's not a thing.

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COMPOST is used far too much. Used when your soil's organic matter is too low it's fantastic stuff. I use it to refresh existing container media or to make my own. Used as most people seem to use it, every year or an inch or more at a time, it's detrimental to the health of your soil.

 

Compost releases a lot of nutrients. Too much compost releases too many nutrients and too fast. Worse, it introduces far too much phosphorous into your soil, which I all but guarantee you that your soil does-not-need. Phosphorous is used in tiny amounts and is stable in your soil. It builds up until it creates a nutrient toxicity, resulting in your plants' reduced ability to take up other nutrients. Phosphorous soil toxicity also largely eliminates mycorrhizal fungi in your soil. Used too much or too often also introduces far too much organic matter into your soil. That's a real thing.

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A little compost goes a l-o-n-g way. Ease up on compost.

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We know PLASTIC SHEETING restricts water & air from entering the soil. It also heats up the soil, making seeds & roots "uncomfortable". Micro & macro fauna die off or move away. That's how plastic sheeting works. However, it harms or kills the good roots below that plastic, too. That said, if you have a large patch of bamboo, two years under a heavy, transparent or black plastic sheet eliminates that problem.

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PAPER PRODUCTS (incl. sheet mulching) work very much like plastic sheeting. How can cardboard & newspaper be used to kill lawns, but also be good for sheet mulching? Even "temporarily until it decomposes" doesnt' make sense. Why would you radically harm the soil & good roots underneath like that? You don't normally see worms above mulch. The worms you see above the paper or cardboard moved from under the paper product because there was less & less water & air below the cardboard. Good roots and good micro & macro fauna are dying off or moving out.

 

Worse, modern newspaper & cardboard are full of waxes & plastics expressly to slow down water-degredation. Don't use paper products; even if you remove tape & decals from it.

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RUBBER MULCHES (& repurposed tire soaker hoses) contain heavy metals and toxins, such as carcinogens & mutagenics.  The heavy metals don't break down and now they're in your soil...forever.

 

LANDSCAPE FABRIC or weed barrier. Most any fabric you & I purchase restricts water and air from reaching the soil underneath. Elsewise is a complete myth

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However, if you look carefully, you can find "double-punched, geotextile, non-woven" fabric. THIS fabric does allow water and air to pass. However, it does not stop weeds. It's the only fabric you should use with French drains, too, by the way.

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Both categories of 'landscape fabric' trap all the dust on top of it, making it a fantastic place for new seeds. Aggressive roots, good and bad, go right through them, meaning it's doing nothing for those weeds and when you reailze this and pull it out you tear up good roots.

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ROCKS. I think there's legitimate use of rocks in the ornamental garden, but there are caveats to consider. People often place landscape fabric underneath. See 'Landscape Fabric' above. Rocks trap dirt & plant material, which breaks down and is a fantastic medium for weeds. You can reduce this with frequent, powerful leaf blowing, but that means more maintenance and it doesn't prevent the inevitable. Rocks are also highly reflective and soak up the heat for release later, possibly harming (or helping) adjacent plant material.

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For narrow rock pathways I might use fabric anyway. This is so I don't have to constantly add rocks that are pushed further & further under the soil; or to make it easier to recover the rocks when you finally have to remove them when the buildup of dirt & plant material becomes too much.​​​​​​​