Steward – Jan ’14 Real Dirt

Much About Mulch

by Cheryl Gross

There is much to consider about mulch.  Curious about mulch?  Maybe not.  Perhaps you think of it as the icing on the cake or window dressing.  A freshly mulched landscape can have that ‘finished look‘.  But, is that all there is?  Is mulch only about appearance?  Mulch decisions can be made based upon a design element in landscaping, such as stone mulch in a rock garden, or as a tool in weed suppression.  Can it do more?  Can the material used as mulch harm the plants?  What about mulch throughout the season?  Is there a good mulch for spring that might harm the same bed in summer?  Oh, my.  Too many questions.

According to Brian Zimmerman, of Four Season Nursery, prior to the 1970’s, it was common to cultivate garden beds to remove weeds and mulch foundation plantings with black plastic and stone.  That was it.  Spreading mulch was not yet in vogue.  By the mid-70’s cedar bark was given away in our region by mills as it was viewed as a waste product.  While the cedar bark was far more reasonable than stone, it did require ‘freshening’ annually.  Finally, the movement toward wood chips as mulch appeared on the scene in the late 70’s.  Any wood scraps from the mills could be turned into a product and sold.  Municipalities began giving away wood chips and shredded wood from tree trimming to residents.  Zimmerman clearly distinguishes between bark and wood chips, recommending bark but never using wood chips.

It is a widely held tenant that the primary purpose of mulch is to retain moisture around the plant during dry spells and to shade roots from hot summer sun.  Secondarily, mulch will suppress weeds.  Pile it on deeply enough and banked seeds will not see the light of day, so to speak.  Another important virtue of mulch is moderating soil temperature.  A good mulch layer will keep soils cooler in the summer and reduce the freeze/thaw cycle in winter.  However, that same mulch can slow the soil warming process in spring.

Some mulch products will ‘knit’ together over time and form a solid barrier.  This barrier, according to Zimmerman, is to be avoided.  It prevents water and oxygen from reaching the soil and creating habitat for the billions of soil microbes in healthy soil.  Watch your mulch for this tendency and break it up if needed.  Mulches with knitting tendencies are hardwood chips and cedar.  Non-knitting mulches that allow for free air and oxygen transfer include pine bark mulch, compost, straw, and pine needles.

While there is a host of mulch materials from which to choose, there is no ‘perfect’ mulch.   To begin, mulches can be either organic or inorganic.  Organic mulch will decompose, inorganic mulch will not.  Organic mulch may enhance the quality of the soil or may absorb nutrients from the soil underneath.  Inorganic mulches stay in place and may not decompose, but some stone can leach alkaline minerals into the soil and, if spread too heavily, can compact and damage roots. Review the information below and consider your past mulching practices/experiences.

 Mulches to consider:

       Organic materials

Straw:  Good for winter coverage to moderate soil temperatures.  Good to use in vegetable beds year round.  Need to watch for seeds in the product (do not mistake hay for straw).  May house rodents.  Needs to be removed in spring in landscaped beds.  Rarely compacts.

Wood Chips: Hardwood chips spread in the spring can last for a couple of seasons.  However, the color will change to a gray or silver color.  Piling more fresh mulch over dull old mulch may create air and moisture barriers to the soil.  Chips require nitrogen from the soil to decompose therefore, a nitrogen fertilizer is recommended under wood chip mulch.  The old layer should be cultivated and broken up before a fresh layer is laid.  Caution:  wood mulch can host shotgun or artillery fungus (Sphaerobolus).  This fungus shoots spores into the air that land on plants, house siding and cars.  The tiny black dots can be very hard to remove.

Shredded leaves or leaf mold:  Fall leaves can be advantageous in the garden.  It is recommended that they be shredded first, or even allowed to begin decomposition before being used.  Full-sized leaves limit water and air circulation to the soil.  Shredded leaves can form a nice barrier, if not shredded too finely.  Leaves that have begun to compost can be used to amend the soil, however, watch that a crust does not form.

 Bark:  Bark is generally viewed as a suitable mulch.  It can come in largish chunks or shredded.  It may float away in a heavy water situation and may attract wood-eating insects, such as termites.

Pine needles:  If you can collect them, pine needles work nicely around acid loving plants.  Generally not sold in bulk or bags.

Compost:  a well processed black compost will feed the roots below as well as warm the soil in the spring and retain moisture.  Beware of weeds seeding in the compost;  a weed-preventer might be helpful to create the weed barrier.

      Inorganic materials

Gravel, pebbles and stone:  Best used in permanent landscaped beds.  Do not use around acid loving plants.  Use about a one inch layer to prevent soil compaction.

Black plastic:  Good a weed prevention.  Good for moisture retention.  If soil is wet, it will not dry well when covered in plastic which can add to root diseases.  Hot in summer, plastic breaks down quickly if not protected from sunlight.  Not attractive.

Landscape cloth (various):  Allows air and water movement to and from the soil.  Controls weeds well, but grasses may survive.  Not attractive.

Recycled rubber:  Recycled rubber is the newest mulching material and can most often be seen on playgrounds.  Will not decompose.  Effectiveness it not yet known.

While there may be no perfect mulch, there may be a suitable one for your yard and your plants.

-Mulching around trees to create a barrier between the lawn mower and the trunk is a very good idea.  Be sure to allow space between the mulch and the trunk.

– Use black plastic to warm the vegetable garden soil in the spring and, when removed, the warmed soil can jump start your plants.

-Adding pine needles around acid loving plants can add a kick to the ph.

-Shredding and bagging fall leaves can be a real budget saver come spring.

There is much to know about mulch.  You can read more about it from the following websites.


Savvy Gardener.Com.  “All About Mulch,”

Cornell University, Department of Horticulture.  “Home Gardening, Mulches for Landscaping,”