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What type of mulch is best and how deep should it be?
Just about any organic (plant-derived) mulch is a great addition to your soil and to protect your plants. Mulch retards soil evaporation. It also acts as an insulator for soil temperature, cooling it in summer and warming it in winter. Mulch helps prevent wet soil from splattering onto plant leaves, which can spread fungal disease. It also reduces weed seed germination.
You can buy mulch by the bag or by the yard. Or, you can use leaves and dried grass clippings (not fresh) from your garden. If you chip your own twigs and branches to use as mulch, let them dry and decay for several weeks before topping your beds with them.
When buying mulch, it doesn't matter if it's cedar or hardwood. But, it's best to select one that's shredded or a small aggregate. Avoid those large bark chips, except for walkways.
Do not use cocoa mulches since they are fatal to dogs.
Plus, it's much better to use local mulches, available at any nursery or box store. Some people like to use pine straw from regional suppliers, especially in vegetable gardens, since it doesn't mat down.
To apply: the general rule is to add mulch about 2-3” deep. But, avoid placing it directly against your plants. Absolutely do not place a volcano mound around your trees. You want to leave the trunks of your trees open to light and air. Instead, create a berm to capture the moisture that falls from the sky or from you. Insulate to protect the soil around your plants, but leave breathing room at their trunks or stems.
Since mulch breaks down quickly in our heat to nourish and aerate our soil, it's important to replenish yearly.
Rubber mulches do get very hot and are challenging to work around if plants need to be replaced. They do insulate your soil, but they don't break down, and add to the soil structure, a big advantage to plant-based mulches. Even a few years of an organic mulch will help open up heavy clay soils and nourish all soils.
You can also use rock as a mulch. It insulates and protects, but doesn't add to the soil structure. Some plants do prefer a rock or decomposed granite mulch, especially succulent plants. It can be hotter, as well, so consider the situation for each plant.
In nature, leaves and small twigs fall as a natural mulch around many plants. In other situations, rocks and gritty soil provide the natural insulation.
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