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Garden Myths Busted: Wood Chips vs. Bark Mulch, Compost Tea,

By Paul Rodman (paulgrowMarch 5, 2013
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Two horticultural professionals took some of the most popular garden myths into the university laboratory to prove or disprove the accuracy of these myths. Dr. Linda Chalker-Scott from Washington State University and Dr. Jeff Gilliam from the University of Minnesota tested these myths under controlled conditions to determine if they really work.

Gardening picture

The Myth
Bark mulch and sawdust are aesthetically preferable to wood chips and they work just as well.

The Facts
Can bark mulch and sawdust can be used in place of arborist wood chips?

The main reason that folks ask this question is aesthetics. People like neat landscapes no weeds, no bugs, and no leaf litter. It's another way for us to separate ourselves from the "messiness" of nature.

We all realize that leaving soil unprotected is not a good management practice; however, bark mulch and sawdust are viewed as acceptable mulches because they are uniform in color and texture.  You can even buy colorized spray products to return weathered mulch to its original appearance!

The "invention" of bark and sawdust mulch was beneficial to both the landscape and timber industries. Prior to this time, the timber industry used these lumber leftovers as hog fuel. Recycling these materials in a more environmentally friendly way theoretically benefits everyone.

There are some potential problems with bark and sawdust mulches that we need to know about.

Bark does not function like wood chips in its water holding capacity. We all know that bark is the outer covering of the tree and contains a material to prevent water loss. Suberin is a waxy substance that will repel water, and in fact helps explain why fresh bark mulch always seems dry.

Wood chips, on the other hand, consist primarily of the inner wood, which is not suberized and has the capacity to absorb and hold moisture.

In the case of sawdust its super fine texture creates an impermeable barrier, which repels rain and irrigation water.

Image Image  Image
 Wood Chips          Bark Mulch             Saw Dust

Bark mulch can be a source of weed infestation.

What is means to the gardener?

  • Bark mulch can be contaminated with salt or weed seeds
  • Bark naturally contains waxes that prevent absorption and release of water in landscapes
  • Sawdust is too fine a material to use as landscape mulch and will prevent water and gas movement as it compacts
  • Softwood bark mulches are often not "gardener friendly" due to the presence of tiny, sharp fibers
  • Arborist wood chips can be finely chopped if this is more aesthetically desirable

The Myth

Compost tea is an effective alternative to traditional pesticides.

The Facts

This myth has been spreading like wild fire on the web as well as a variety of published articles.  The problem is that this theory has very little scientific evidence to establish its effectiveness.

Compost teas and extracts are traditionally used as liquid organic fertilizers, but recently have been touted as powerful antimicrobial agents capable of combating pathogens associated with foliar and fruit diseases.

Image

Controlled, replicable experiments do not support this theory.

A quick search of the Internet revealed that most of the websites containing the phrase "compost tea" are .com sites: most are selling something. The few .edu sites that do exist are cautious in regard to the miraculous properties associated with compost teas.

Nothing ticks me off more than searching for facts on a subject and finding many of the articles are commercials for selling a product.

One of the biggest problems with compost, and extracts, is the high variability among composts from different sources as well as different batches.
Before we can ascertain whether compost tea is an effective pesticide certain facts need to be established.

1. What organic material is in the compost? What are the chemical properties of the compost (%N, pH, etc.)?

2. What are the active ingredients?  Are they chemical agents (allelopathic compounds)? Are they beneficial microbes?

Scientific research is under way to determine if compost tea is indeed effective in controlling pests as well as diseases.

What is means to the gardener?

Properly composted organic material makes wonderful mulch.

Compost teas have not been suitably characterized, nor have their purported benefits as an insecticide or fungicide been validated scientifically.

Compost teas can be overused and potentially contribute to ground water pollution. 
 

Practices need to be validated through the scientific process before they can be recommended.

In closing, I expect some of you might disagree with some of these findings. However remember that this research was done in reputable university labs under controlled conditions. Each of us will draw our own conclusions.


  About Paul Rodman  
Paul RodmanPaul Rodman has been gardening for over 45 years. He is an Advanced Master Gardener, and American Rose Society Consulting Rosarian. He is President Emertius of the Western Wayne County Master Gardener Association in Wayne County, Michigan. He currently serves as the greenhouse chairman of this group. Rodman has amassed over 5500 volunteer hours in the Master Gardener program. Rodman is the garden columnist for The News Herald newspaper, in Southgate, Michigan. He has also written for the Organic Gardening.com web site. He is a certified Master Canner and has taught classes on Home Food Preserving for 7 years. He has lectured on various gardening topics throughout southeastern Michigan. His favorite pastime is teaching children about gardening. For the past several years he has conducted classes for second grade students teaching them about subjects ranging from vermi-composting to propagation.

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Discussion about this article:
SubjectTopic StarterRepliesViewsLast Post
Garden myth or not. juezsmith 0 11 Mar 12, 2013 10:00 AM
A new article and subject! greenbrain 3 57 Mar 5, 2013 7:47 PM
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