| B. Rosie Lerner
Sour Mulch "Burns" Tender
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