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About Hardwood Mulch

Consist of shredded and composted bark from hardwood trees.  Sawmills remove this bark before cutting the timber into lumber.  The bark is obtained from the mill, composted for at least 6 months, then shredded and screened.  The longer the bark is aged, the darker it gets.  Most bark mulch producers only shred and screen the bark once.  The screen size determines how fine or coarse it will be.  “Double” and “Triple Ground” are terms used by consumer and do not represent how mulch is really made.  Some producers add shredded pallets, yard waste and sewage to hardwood bark mulches to increase profits.  Indiana Mulch produces and sells 100% pure hardwood bark mulches to ensure the health of your landscape.

Qualities
Proper composting and aging produces mulch rich in nutrients with a dark brown to black color. Coarse hardwood mulch will help prevent erosion, while the finer well-aged hardwood bark gives your landscape a nice clean rich look.  Because of its density, hardwood bark insulates and protects your plants from extreme temperatures better than most mulches.

Concerns
Though it looks great, it can be a high maintenance mulch. Hardwood mulch does compact causing a dry condition under the mulch. When this occurs, your landscape will be starved of water and air. This will promote the growth of fungus and will raise soil temperatures through the summer months.  Occasional aerating, cultivating (raking) and watering will greatly decrease the chance of fungus and disease. The finer and dryer the mulch, the more prone it is to this problem.

Beware of hardwood mulch that may have a vinegar or ammonia smell, it could be toxic.  This is a result from a mulch producer which has improperly composted the mulch. It will kill annuals and perennials over night and can cause damage to your shrubs and trees. Do not use it! If it has already been applied, water the mulch and plants thoroughly.

MAY
2004

By
B. Rosie Lerner

Extension Consumer Horticulturist
Purdue University

Sour Mulch "Burns" Tender Plants

Although the benefits of mulching garden plants are many, wood mulch that has been improperly stockpiled can lead to plant injury or even death. Young herbaceous plants are the most susceptible to such injury, which becomes obvious shortly after applying a hardwood bark mulch. Symptoms appear as if the plants have been burned with fertilizer or pesticides, or possibly, like severe water stress. All of the above could potentially be a problem, but, apparently, we must now add "sour mulch" to the list of suspects.

When hardwood bark mulch is stacked in tall piles and allowed to stand for long periods, the material in the center of the pile begins to compost anaerobically (without air) and may sour. The term "sour" refers to the extreme acidity that occurs under such conditions (reported to be pH 1.8 - 3.6, while that of "normal" mulch is generally close to 7.0). Some reports indicate that pine bark does not sour.

The mulch tends to heat as it breaks down, and steam may be seen escaping from the pile when it is finally disturbed. This heat can directly injure plants, if the mulch is not allowed to cool first. However, the more serious problem is that anaerobic composting of hardwood leads to the production of several plant-damaging components, possibly including methane, alcohol, ammonia and hydrogen sulfide.

Injury to young, tender plants is swift, usually within one day of mulch application. Symptoms include yellowing or blackening of foliage and leaf drop. Depending on the extent of the injury, plants often are able to recover. Thorough watering, especially during hot, dry weather, will help prevent further stress. Do not apply fertilizer to plants injured by sour mulch; such materials can cause further injury. If plants recover and seem to be lacking in vigor later, a side dressing of nitrogen may be helpful in mid-summer.

The best course of action is to prevent sour mulch injury in the first place. Stockpiles should be no higher than 4 feet, if possible, and, if necessary to stack taller, turn them periodically to allow some aeration. Alternatively, sour mulch can be made fit for use by spreading in shallow layers and allowing it to air out for a few days, preferably a week, before using around young, tender plants. If no rain has occurred, watering the mulch also may help wash away toxic accumulations.

5-6-04