Tender Plants Injured
by 'Sour' or 'Toxic' Mulch
Gail Ruhl, Senior Plant Disease Diagnostician, Department
of Botany & Plant Pathology, Purdue University
Burn-like symptoms that appear on plant tissue shortly
after applying a
hardwood bark mulch around plants may be due to a problem known
as "sour" or"toxic" mulch.
Hardwood bark mulch that has been stacked in tall
piles for long periods may
begin to compost anaerobically (without air). The term 'sour' or
refers to the extreme harmful acidity (pH of 1.8-4.0) that can
such conditions (in contrast to a "normal" mulch pH around
reports indicate that pine bark does not sour.
Hardwood mulch tends to heat as it breaks down and
steam may be seen
escaping from the pile when it is disturbed. This heat, by itself,
directly injure plants if the mulch is not allowed to cool first.
the more serious problem is that chemicals such as ammonia, alcohol,
methanol, acetic acid or hydrogen sulfide, produced by fermenting
in the mulch pile under oxygen-deficient conditions can cause significant
injury on annual and perennial landscape plants.
Injury to young, tender plants is swift; usually
within one day of mulch
application. Symptoms may include yellowing, bleaching or blackening
foliage and leaf drop. Depending on the extent of the injury, plants
are able to recover. Thorough watering especially during hot, dry
will help prevent further stress.
The best course of action is to prevent 'sour' mulch
injury in the first
place. If hardwood bark must be stacked in large piles, turn piles
periodically to allow some aeration. Alternatively, hardwood bark
has already soured can be allowed to mellow before use by spreading
mulch in shallow layers and allowing the mulch to air out for at
least a few
days, preferably a week, before using around young, tender plants.
rain has occurred, watering the mulch may also help wash away toxic
Mulch - Michigan State University
Figure 1 - http://hortweb.cas.psu.edu/
Figure 2 - http://www.oznet.k-state.edu/