Sour Mulch

 

by Karen Weiland, Advanced Master Gardener

 

 

 

Wood chips and bark nuggets have been used by gardeners for years as mulch around landscape plants.  Mulch is very beneficial to plants in that it prevents weed growth, evaporation of moisture in the soil, keeps the roots cool during the hot summer months and enhances the look of a landscape.  Mulch can however turn sour if it is stockpiled too high (taller than 5 to 6 feet) for a long period of time. This mulch is known as sour mulch or mulch with wood alcohol syndrome.

Mulch becomes sour when lack of oxygen and high moisture towards the center and bottom of the pile starts a process known as anaerobic fermentation.  This process produces by-products such as ammonia, alcohol and organic acids that can harm plants.  Large piles, such as those accumulated at lumber mills or companies that manufacture mulch can develop huge piles that can lead to sour mulch.  Pine bark is not as prone to forming toxic by-products as hardwood bark is.

Once sour mulch is spread, plants usually show symptoms within a day with some recovering and some not.  Many plants can be affected, from herbaceous perennials and annuals to newly planted trees and shrubs.  Plants may show some wilting and the some of the foliage may turn white or light tan as if bleach had been thrown on it. The turf next to sour mulch may turn a light yellow color.  Do not apply fertilizer to plants that have been injured by sour mulch.  If plants recover, but show some lack of vigor, a side dressing of nitrogen can be applied mid summer.  Avoid fertilizing during hot, dry weather.

Sour mulch will have a vinegery, rotten eggs or ammonia like smell which will go away quickly once it is spread out.  It can also feel hot.  If stockpiles of mulch are suspected of being sour, the pH should be checked.  Sour mulch pH will be very low, ranging from 2 to 3.5.  Properly composted organic material will have a pH of 6.0 to 7.2.   If it is found to be sour, the pile should be turned to provide aeration, have lots of water applied to it to leach out the toxins and have an ample amount of lime applied to it to bring up the pH.  The mulch should be safe to use after a few weeks.

In a nutshell, never use mulch around your plants that is hot or smells nasty!

 

As always, Happy Gardening!

 

More information about gardening and related subjects can be found online at www.hort.purdue.edu/ext/garden_pubs   The Purdue University Cooperative Extension Service can be reached at 499-6334 in LaGrange Co., 636-2111 in noble Co., 925-2562 in DeKalb Co. and 668-1000 in Steuben Co.

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