Georgia Gardener Newsletter

Georgia Gardener Newsletter Design Tip: October 16, 2008

Mulch in the Garden
(Updated and Revised from June 29, 2006)

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Mulch near a tree

Mulch is one of those items in the garden that is often misunderstood, commonly misused, sometimes missed and occasionally maligned. I would even dare to say that an entire industry has grown out of the mis-information about mulch.

When I visit someone's garden, one of the first things that I will notice is the mulch. What type of mulch do they have, how deep is it and where is it being used are the questions that immediately come to mind. I'm disappointed to say that in more than 90% of the gardens I see there is either no mulch or an inadequate amount. Mulch is absolutely critical to the garden and without it your plants will suffer and weeds will go wild.

What is Mulch?

A widely used definition is any loose material that is placed over the soil to conserve moisture and control weeds. While partially true, this definition overlooks some of the more critical functions of mulch.

I define mulch as any undecomposed plant material that forms a protective and nutritive layer over the soil. This can be leaves (shredded or not), pine or wheat straw, wood chips, shredded wood and even grass clippings. While most people understand the protective aspects (moisture retention, temperature regulation, erosion control and weed prevention), many people overlook the nutritive importance of mulch. In the garden mulch:

1. Helps to regulate soil temperature and moisture.
2. Reduces erosion.
3. Helps control weeds.
4. Serves as a slow release fertilizer for plants as it decomposes.
5. Promotes the presence of microorganisms that are necessary for roots to uptake nutrients.
6. Provides an environment for beneficial organisms that protect plants from pests.
7. Is absolutely critical for rebuilding depleted or missing topsoil.

As to which mulch is better, that depends upon your tastes. Pine straw is just as good as shredded leaves, shredded wood or almost any other organic material. However, I do NOT recommend the use of cypress or cedar mulches (read this link) as these are harvested from forests that are not being managed in a sustainable manner. Removal of vast areas of cypress trees from our southern coasts has had disastrous consequences...just ask the residents of lower Louisiana where the protective cypress bogs were destroyed thus allowing massive tidal surges during recent hurricanes.

Mulch: True or False?

Fresh mulch from cut trees will release toxins that can kill plants.
False: While fresh mulch can rob soil of Nitrogen if it's mixed in, it's fine to leave on top of the soil, just don't pile it too high or right up against or over plants.

Mulch attracts termites
Barely True: Mulch is mildly attractive to termites. However, termites are everywhere in our environment and they will find your house eventually. It's advisable to keep mulch 12-18 inches away from your foundation and to have a good termite control plan. It's also an urban legend that certain mulches will repel insects.

Mulch will make my soil more acidic.
Barely True: The amount of acid produced by the breakdown of mulch is negligible and poses no problems for most plants. If you are concerned, have a soil test performed and adjust your pH accordingly.

How Much Mulch?

Trees without mulch can suffer from root damage and drought

Depending upon the material, 2-4 inches of settled mulch is sufficient. Don't pile mulch up against or over the trunks, stems or crowns of plants or you may have problems with pests and rotting. Unless you have plant disease problems, don't remove the old mulch. Simply add a new layer over the existing mulch to achieve the necessary depth. The old mulch will naturally decay and become compost. Add mulch at any time to maintain the proper depth.

Hint: Lay new mulch in the fall after the leaves have dropped. This way you won't have to add as much and it will retain a fresh look through the winter.

The table below shows how much mulch I recommend depending upon the material.

Pinestraw:	3 inches (apply 6 inches to allow for settling)
Shredded Hardwood or Pine:	2-3 inches
Nature's Helper®:	1.5-2 inches
Freshly Shredded Leaves:	3 inches
Compost:	1.5-2 inches
Rock:	Not recommended due to heat retention

Mulch versus Compost?

After mulch has been broken down by macro and microorganisms such that it no longer resembles the original material from which it came, it is said to be compost. Compost contains plant nutrients, has the ability to retain moisture and improves soil texture allowing oxygen to penetrate the soil. Compost makes a find soil amendment and can be worked into the soil as needed.

Organic versus Synthetic Mulches

Can you tell the real from the fake?

Organic mulches, as described above, are made up of undecomposed plant material. Synthetic mulches are comprised of man-made materials such as shredded tires (crumb rubber) or recycled plastic made to resemble wood chips or pine straw (pictured above). I am opposed to using synthetic mulches composed of such materials. While these products may help retain soil moisture and control some weeds, they do not provide a required source of organic material necessary to maintain healthy soil and plants and may actually leach harmful substances into the soil. Here are some facts about synthetic mulches:

Plastic and synthetic rubber mulches are recycled petroleum products.
They are not "permanent" and as they break down can release harmful substances including heavy metals into the environment.
Studies have indicated that they are not as efficient at preventing weeds as touted.
Synthetic rubber mulches are flammable and can be hard to extinguish.
Some synthetic mulches may actually increase soil temperature in sunny locations.
Synthetic mulches offer no nutrient value to the soil or plants.
From a practical standpoint, you have to "pick" out any leaves or real pine straw that accumulate in it.
Synthetic mulches could blow or wash into natural areas and streams where their effects on wildlife are not known.

Besides, don't we have enough plastic and synthetic trash already loose in our environment?

Manufacturers of synthetic mulches will proudly tell you that their products won't rot or decompose. To me this indicates a lack of knowledge in soil science and plant nutrition. Mulches are SUPPOSED to decompose. That's how they work to improve your garden.

I could be so cynical as to say that synthetic mulches are garbage and that these industries are using your landscape as a "mini-landfill," but I won't.

References

The Myth of Rubberized Landscapes (pdf)
By: Linda Chalker-Scott, Ph.D., Extension Horticulturist and Associate Professor
Puyallup Research and Extension Center
Washington State University

...Kill Your Garden With Rubber Mulch
Although not written by an academic, it does contain some pretty impressive references at the bottom.

NPR's "Folksy" Commentary on Plastic Pine Straw
A three minute audio commentary from NPR Commentator Diane Roberts dated October 14, 2007.