The recent heavy rains have certainly been a hazard to life and property.  But they also make farming and gardening a challenge.  Recently, a grower at the auction in Jamesport told me that it is turning out to be a difficult year for produce growers.  Hopefully, the weather will straighten out and give us better growing conditions.

While excessive water has direct consequences for plants that are easily seen, sometimes there are effects that may surprise you.  One is sour mulch.

What happens is that large stockpiles of mulch start fermenting.  When the weather warms up, compounds produced during fermentation can cause problems.  If this mulch is applied to your plants, damage may result.

Dr. Chris Starbuck, State Extension Woody Ornamental Specialist has seen this before.  He says, “In years with above normal rainfall, it is fairly common for the leaves and stems of annuals, perennials and small woody plants to be seriously injured by volatile chemical compounds released from fermented mulch.”

Dr. Starbuck says that damage can occur very quickly.  You may see bleaching or silvering of foliage within a few hours of mulch application, especially in hot weather.  Usually this kind of damage is temporary, lasting only 24 to 48 hours, if the concentration of fermentation products is not too high.  After the fermentation products have volatilized, your plants should hopefully recover.

However, damage may be more serious, depending on the concentration, temperature, and other factors.  Chemicals produced during fermentation may include acetic acid, fatty acids, methanol, formaldehyde, ketones and phenols, according to Dr. Starbuck.

If these compounds are present in high amounts, there will be an acrid odor to the mulch.  It may smell like vinegar, for example.

These mulches can cause severe injury to plants. One researcher found that soured Douglas-fir sawdust prevented germination of radish and sunflower seeds.  This same material killed corn and onion seedlings soon after emergence.  When this sour mulch was applied to tomato, pepper, and cabbage plants, they were killed within one week.

Surprisingly, this mulch was found to have a pH of 2.2, which is quite acidic.  No wonder plants were dying!

If you have mulch that has an unpleasant, sour smell, don’t use it right away.  It could kill your plants. Wait until the smell goes away.  If the smell is gone, that means those fermentation products have leached out of the material.  If the mulch is hot, it is going through a composting process, and it’s best to wait until it cools down before using it.

With mulches that have soured, try spreading them out so that potentially toxic compounds can volatilize away.  This may take several days.  Before using it, make sure the sour smell is totally gone. 

To test your mulch, try a little of it on a few plants before you commit your whole garden to treatment with it.  Or, while it is still spread out, place a few tender transplants that are still in their cell packs on top of the mulch to see what happens.  If toxic compounds are still present, these test plants will show if there are any problems.  Look for bleaching, silvering or collapsing of foliage within a few hours on a warm, sunny day.