Types of Mulch

Straw

Straw, commonly used in the vegetable garden, is one of the most effective mulches because:

It traps air easily, effectively moderating soil temperatures.
It is extremely porous.
A bale can be easily broken into "flakes" which can be easily moved.
A bale of straw is so compressed, that it covers a surprisingly large area.
It is relatively inexpensive.
It can serve as an undercoat beneath a more attractive and costly mulch.
Worms love it!

There are different kinds of straw. Rice straw, for example, is usually available in the late summer and early fall after rice is harvested in the Central Valley. Because it is weed free, it can be used immediately. But it deteriorates quickly.

Wheat and oat straw are alternatives. They last longer than rice straw and they have a better consistency. But, neither can be used until the seeds are killed. Soak the bales with water until they are covered with 1-2" of sprouted grass, or leave the bales exposed to rain for a month during the winter. But use the straw quickly at that point, or the bales will rot completely, and remember to place the bales where you want them to be before they get wet - they're hard to move when soaked.

Alfalfa Hay/Pellets

Though more expensive, alfalfa offers all the advantages of straw plus the added boost of more nutrition. Rich in nitrogen, alfalfa is used as animal feed. It can give your plants an added boost, too. It is seedless so can be used immediately, without the soaking treatment described above, and it has a longer life than rice straw.

Pellets are merely compressed alfalfa in small chunks. Sold in bags, they are more easily transportable if you don't have a truck. Whether in bale or pellet form, alfalfa and the other straws are available at most feed stores.

Barks and hulls

Barks and hulls can usually be purchased in bags. They are attractive, easy to transport, and they are useful for small areas. But a bag does not cover many square feet, and costs can quickly add up.

There are quite a few different types:

Cocoa bean hulls are appealing because initially they smell like chocolate. If applied too thickly, they can get gummy after a while. Consequently, they lose their porosity.
Barks have the advantage of being long lasting, attractive and allow water to pass easily to the soil. But they take many months, often years, to decompose. As a result, they protect soil, but they do not enrich as other mulches can.
Redwood "hair" works effectively, but can look artificial. Like bark, it takes a long time to decompose compared to other mulches.

Barks and hulls are available in bags at most hardware and garden centers and in bulk at landscaping materials suppliers such as Wheeler/Zamaroni or Grab N' Grow.

Yard Waste (including grass clippings, garden clippings and dried leaves)

There's nothing as satisfying or cost effective as using materials that your own property (or a neighbor's) generates. And this way, you are retaining all of the nutrients that those plants took out of your soil!

Fallen leaves are great! If you rake them on to your beds in the fall, they will soften the heavy rains' effects on your soil, and they will protect your plants during freezing temperatures. Before using them, toss them in a shredder or a run a lawn mower over them, to prevent them from clumping. If they are dry even crumbling them with your hands as you spread them around is effective. While not as attractive as barks, they are porous and decompose quickly, enriching the soil.
Grass clippings are better consigned to the compost pile because they tend to clump unless they are spread very sparingly or mixed thoroughly with other materials.
Clippings from flower beds and shrubs, if chopped up, can be used to cover the back portions of beds. They are not attractive, but they can suppress weeds and they are an effective way of dealing with erosion.

Municipal garden waste/Chipped trees/Re-cycled wood

Our county has a number of sources which provide re-cycled materials, ground up and sorted according to size. These are worth their weight in gold. And they have the added advantage of blending easily with fallen leaves and other debris. Usually sold by the yard, they are relatively inexpensive, though sometimes transportation costs can equal the materials if you arrange for a large delivery.

Getting the right consistency of material is very important. Chunky is better than thready, to ensure good water absorption. Also, consider the source of the material and use it in your garden accordingly. Re-cycled lumber products are better used for pathways and ornamental areas than for vegetable gardens since they may be impregnated with unknown chemicals.

Sonoma Compost at the Sonoma County Landfill and the various local tree companies such as Atlas Tree are potential sources of this type of mulch. There are also companies such as Affordable Landscape Materials that sell attractive chipped and screened lumber mulches.

Remember, compost and mulch are not the same. Compost is made up of fully decomposed plant materials and is an asset when added to your soil, as the microorganisms can use it immediately. Mulch has not yet decomposed, and only becomes compost over time. Mulch should be used as a top-dressing only: never mix raw mulch in with your garden soil, as it will deplete the Nitrogen level in your soil as it decomposes, and rob the soil of a key ingredient for plant growth

Gravel

Fine or coarse gravel or crushed decorative rock can act as both mulch and decorative hardscape element in parts of the garden.
Gravel is long lasting, attractive and permeable, but, like some of the barks, will provide no ultimate nutrients to the soil.