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Episode 902 - Homegrown: Mulch Madness  


This week, Master Gardener, Fred Hoffman teaches us about different varieties of mulch that you can use in your garden. There are several types of mulch that you can use to protect your plants and keep your garden growing.


What is mulch?
Mulch is any material placed on the soil to cover and protect it. Organic mulches come from plants. They include wood chips, sawdust, leaves, clippings, and bark chips or nuggets. Inorganic mulches include materials such as sand, gravel and pebbles and are generally more expensive and less widely used. See Pests of Landscape Trees and Shrubs for a complete list of organic and inorganic mulches.

Why mulch?
• creates topsoil
• feeds beneficial microbes such as mycorrhizae
• conserves water
• weed control
• cools the soil in summer and warms the soil in winter
• protects plants roots
• builds healthier lawns and gardens
• may help de-toxify pesticides

Adding a four inch layer of mulch now beneath shrubs and trees, out to the drip line, can help stop weeds, keeps the ground more moist and feeds the plant as well. But which mulch is best?

Pros and Cons:
Bark lasts a long time, looks nice, feeds the soil, suppresses weeds.

Beware of finely shredded redwood bark: the hairs can easily ignite if someone tosses in a cigarette butt.

Cocoa shells look nice, last a long time, but may be toxic to dogs.

Chipped/Shredded tree trimmings: inexpensive, feeds the soil, suppresses weeds, but needs to be replaced yearly.

Pine needles: best spread at least three inches deep around acid loving plants.

Compost: Inexpensive, provides soil nutrients and improves soil structure. But weed seeds can germinate in it.

Grass Clippings: inexpensive, but should be applied when thoroughly dried. Also, clippings from weed-like grasses, such as bermuda grass, may get established in other areas.

Weed Cloth: inorganic, suppresses weeds while allowing air and water to pass through. Needs a layer of bark on top to keep it from disintegrating from sunlight. May keep soil too moist.

Newspapers: inexpensive, must be replaced often, needs to be secured in place.

Plastic: suppresses weeds, but does not allow air or water to pass through. In the summer, it may raise soil temperatures too high. Hold in place with bark.

Rock: Looks good, provides no nutrients, may raise soil temperatures too high for the plant.

Proper Use of Mulch
In order for mulch to be effective it must be applied correctly. One of the two most common mistakes people make is applying mulch too deeply or too thinly. A two to four inch layer of mulch keeps down most weeds and retains moisture in the soil. Applying mulch too thinly may not help prevent the suppression of annual weeds.

It is prudent to be cautious of using very thick layers of fine mulch. Too much mulch can reduce the oxygen levels in the soil, which is already a problem in heavy, wet soils. Save the finer, smaller material for use around flowers and vegetables. Excess mulch, particularly if applied right against the stem or trunk of landscape plants leads to constantly wet bark and conditions favorable for root and crown rot diseases. A mulch that is too thick may severely reduce or eliminate drying and lead to water-logged soil, particularly during wet seasons or in heavy clay soil.

Drawbacks to Organic Mulch
There is risk of potential short term nitrogen deficiencies when using green or non-decomposed mulch. Both garden plants and soil microorganisms need carbon and nitrogen to grow. The decomposers (fungi and bacteria) feed on the nitrogen in the organic mulch material. This robs the soil of nitrogen so that the nitrogen in the soil available to plants is reduced. The result is poor plant growth and pale green leaves. Applying organic fertilizer with at least two percent nitrogen such as manure or fish meal should remedy this short time nitrogen depletion. As the decomposers die off nitrogen availability to plants increases.

Organic mulches gradually decompose. Mulch must be periodically added to maintain a sufficiently deep layer to provide good weed control. Furthermore, mulches rarely provide 100 per cent weed control. It may be desirable to apply a pre-emergent herbicide to the soil surface prior to applying the mulch. Supplemental hand-weeding or herbicide applications might also be needed.

Some bark, wood, and foliage contain naturally occurring toxic compounds (allelochemicals) that may damage young plants. Before applying uncomposted mulch around young plants leach the organic mulch with a heavy irrigation. Do not allow the leachate to run off or drain into surface water or storm sewers.

The benefits of mulching outweigh the drawbacks. If you want to keep the weeds down, conserve moisture, use less pesticides, have healthier plants, save money, recycle…MULCH!

Additional Info:
Farmer Fred

Pests of Landscape Trees and Shrubs, California Master Gardener Handbook.
Pests of Landscape Trees and Shrubs, California Master Gardener Handbook

University of California ANR Publication, December, 2002

Cooperative Extension, University of Maryland

Cooperative Extension, Oklahoma

Cooperative Extension, Contra Costa County

Cooperative Extension, Texas

Explore the rest of Episode 902



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