Shredded bark mulch
Shredded bark mulches are available as: new or aged, single shredded or twice shredded.
Shredded bark mulches have various amounts of wood debris as part of their composition.
Single shredded new bark mulch is bark as it stripped off logs during the milling process. Hardwood bark mulches, such as from oaks, are better and longer lasting than are soft wood barks from aspen, poplar, birch, etc. New, fresh milled bark should not be immediately used as it may still contain toxic compounds, diseases or pests. Studies at the University of MN http://www.extension.umn.edu/info-u/environment/BD273.html show that aging it a couple of months kill most any insects and diseases. Turning, mixing or aerating the mulch a few times before using it allows for aerobic respiration and negates the effects of anaerobic bacteria activity. If you smell a strong vinegar odor coming from the mulch, it should be aerated 2-3 times before using it. Otherwise, the chemicals emitting from the mulch will damage or even kill the plants you are mulching.
Twice shredded new bark is bark that has been run through the milling machine an additional time to break up the large pieces of bark. Occasionally some larger pieces make it through the second milling process.

Twice shredded aged bark is our preferred mulch
Twice shredded aged bark mulch is the best of the group as it is remilled and allowed to age for a year or so before being sold. Ours is principally composed of oak with occasionally some other hardwoods.

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A Commentary on Various Landscape Mulches

I guess it had to happen sooner or later, Designer Mulches for the landscape.

Besides gravels and glass in every color of the rainbow, you now may find artificially colored chipped wood mulches in a wide array of colors.
And, if wood mulches don’t turn your fancy, you can always get recycled synthetic rubber in Caribbean Blue and Black Pearl among many other colors.
Many folks confuse bark and wood.  Bark is the ‘skin’ of the tree. Wood is the ‘bones’ of the tree. They are quite different. Building your home out of bark would be ill advised. Wood is the home building material of choice.

Chipped bark mulch
Bark mulch is derived from the BARK or the tree. It is generated as a byproduct from the lumbering industry. Chipped bark decomposes fairly slowly, depending upon what species of tree the bark come from. Pine (Pinus), fir (Abies & Pseudostuga), hemlock (Tsuga), cypress (Cupressus), redwood (Sequoia and Sequoiadendron arerelatively unavailable outside the Pacific Northwest), red cedar (Juniperus), white cedar (Thuja and Chamaecyparis), and pacific cedar (Thuja and Chamaecyparis, true Cedrus-Cedarmulch is rarely available in this country) are some of the bark chip mulches that you may come across. Many of the lesser quality bagged & bulk bark mulches have a very high percentage of wood in them. White Cedar, AKA Arborvitae bagged bark chips is often derived from the milling process and formation of split rail fencing. It contains a great deal of wood chips. Carefully check the quality and fertilize accordingly with slow release nitrogen. Most chipped barks are typically acidic and make wonderful substitutes for peat moss. Fir bark is commonly used in Orchid mediums. My favorites are Fir and Hemlock bark mulches.  Some very good research may be found at: http://www.fpl.fs.fed.us/documnts/usda/agib666/aib66604.pdf

Pine needle mulch
Pine needles have a pleasing appearance and acidify the soil around acid-loving plants. They are occasionally available commercially but may be raked up from around pine plantings. It is so distressing to see piles of pine needles raked up and waiting to be picked up by the municipalities.
Pine needles decompose slowly, are resistant to compaction, and are easy to work with. They provide excellent protection around newly set or tender ornamental plants. If left on year-round, pine needles should be renewed annually.

Shredded leaf mulch
Leaves that have been shredded with a composting mower make wonderful mulch. If not shredded finely enough, however, some leaves tend to mat together and form a barrier that blocks free water and oxygen movement into the soil.
However, woodland plants have evolved with natural leaf litter and many of them rely on them for humus and nutrients. In a naturalized area, there is no ecological reason to remove the normal leaf litter. If you pile it up 1-2 feet thick, you will kill everything it covers and often stunt the bushes growing in the immediate area.  No one has leaves that pile up naturally 2 feet thick in a natural forest environment. You might have an area around your home where the wind piles up leaves, but your home is not part of a natural forest environment. I have used fresh leaves in my garden for years with no ill effects.
For best results, allow leaves to partially compost before using them as a 2-3” mulch. They will finish decomposing in place, contributing humus, nitrogen, and other nutrients to the soil. Once composted, I have found no ill effect from using any leaves, including walnut, oak and boxelder. The City of Madison furnishes wonderful finished sifted composted leaf mulch. We have been using this leaf mulch for many years at our nursery as part of our growing mediums.

Wood mulch
Wood mulches are made from: saw dust, wood milling debris, reprocessed chipped wood pallets & crating, and non-usable wood construction materials. Treated, painted, glued, bonded or stained wood products should not be part of the recycled wood mulches.  Most of these decompose fairly slowly, depending upon what type of trees they are derived from. When working around any wood products use gloves to and knee pads to avoid slivers. As wood decomposes, it ties up the Nitrogen in the soil and is therefore unavailable to plants. Increased Nitrogen fertilizer will be required to compensate for the lack of Nitrogen availability when using wood mulches.  Wood mulches are commonly available in different grades and numerous stained colors. The stains may contain: iron oxide, acrylic resins, magnesium silicates, vegetable oils and ammonia. Studies at the University of Georgia indicated no detrimental runoff or residual chemicals in quality wood mulches. http://pubs.caes.uga.edu/caespubs/pubs/PDF/B1294.pdf

Synthetic Rubber mulch
This type of mulch decomposes very slowly, gradually breaking down into carbon, zinc and metal products along with the other materials from which it was recycled, principally old tires. Most of it is in darker colors. Some colors are topically applied during the milling processes. You may experience black powdery residues on your hands, clothing and tools once the rubber begins to age. Also, some discussions on the breakdown products are mentioned in the literature. Some of the better brands make sure no metals are in their rubber mulches, making them one of the ideal mulches for children’s play ground environments.

Glass mulch
Glass mulches are typically made from recycled glass materials. Due to its high reflectivity and typical color clarity, glass mulch gives a very contemporary appearance.  During the recycling process it is tumbled and or heated to help prevent sharp cutting edges. Since it is a tough durable natural product, there is little concern of environmental chemical breakdown. In environments with high amounts of litter, it is difficult to keep clean. Also, you may experience burn marks on some of your plants due to the occasional concentrated sun beams.

Gravel mulch 
See: http://www.landscapedesigns.bz/archives/V17-GravelMulches.htm

Corn cob mulch
Occasionally you may come across milled corn cobs for sale. This is the reprocessed cobs from our corn industry. Since it so heavily used in many other industries, it is uncommonly offered for landscape mulch.

Nut Shell mulch
Even though they are not nuts, peanut shells are not recommended. Other nuts shells may occasionally be found. Many nut shells are fairly sharp and may cut you as you work the soil. Use gloves. Pecan shells make nice mulch. Many nut shell mulches float quite easily.

Rice hull mulch  
Treated rice hulls are not only replacing some of the peat moss use in the horticultural industry, but it is also a very nice soil conditioner in the garden. It does blow away fairly easily so I recommend as a conditioner only.  Also, it can produce a near impervious layer to water penetration if applied to thickly.

Cocoa bean hull mulch
This is nice mulch with an attractive texture and dark color, and to some folks, a pleasant chocolate odor. However, apply it over other mulches as a decorative veneer. If it is applied to thickly, it molds quite rapidly and forms a nearly impervious layer to water penetration. If you like to apply it thickly, work it regularly to avoid these conditions. If you are allergic to molds, wear a mask when working with it. Cocoa bean hulls are somewhat prone to washing out after heavy rains. Also they have high potassium content and may contain chemical that are toxic to some plants, especially when applied too thickly. Save a bag for occasion use in the garden before a dinner party. Then toss some lightly here and there for a bit of chocolate aroma.

Straw mulch
Straw from wheat, timothy, oats, rye, or barley is widely available and comparatively inexpensive. It is used as winter mulch around tree or shrub roots and as summer mulch in vegetable gardens and strawberry plantings.
Straw has some potential problems that must be recognized before it is used: it is highly flammable; it contains grain seeds that can germinate; it harbors rodents; it lowers the soil nitrogen supply as it decomposes; and it must be renewed annually. Additionally, it is easily blown by wind and lacks the attractiveness of other mulches.
On the other hand, it is cheap, effectively suppresses certain weed seeds and reduces soil moisture losses. As winter mulch, it protects tender roots from cold temperature injury. Careful when using it as winter mulch, as it also makes a wonder cozy home for unwanted vermin.