Mulch

Mulching is one of the simplest ways to enhance your garden. It insulates the soil, protects it from erosion, retains moisture, inhibits weed growth, reduces soil compaction, provides a beautiful finishing touch to your garden, and improves soil quality. Mulching, or top dressing, refers to covering the bare ground in your garden by spreading an organic or inorganic material on the soil surface. Many materials have been tried and proven effective as mulches: redwood bark, leaves, woodchips, straw, processed by-products (grape and apple pumice), and cool-season lawn clippings.

For a more detailed list of the pros and cons of different mulches, see the "UC Master Garden Guide on Mulches".

Mulch should not be confused with compost, which is a soil amendment. Mulch is used as a top dressing after plants and irrigation have been installed. Mulch covers drip irrigation, keeps the soil cooler, reduces evaporation, and makes the garden look neat and tidy.

•	Purchase organic mulches that are guaranteed weed-free. Straw, hay, and many types of manure, for instance, may contain weed seeds.

•	Remove weeds before spreading mulch. A pre-emergent herbicide may also be applied after weed removal and prior to spreading mulch.

•	To protect plants from insects and rot, which thrive in moist conditions, keep organic mulches a few inches away from the crowns and trunks of woody plants, like trees, California native plants, and water-wise shrubs.

•	On steep slopes where terracing is not practical, mulch areas with coarse-grade bark, which is heavy enough not to be easily blown by wind or washed off by rain.

•	Under existing oak trees, use a stone mulch to cover the bare soil and do not plant anything, leaving the stone mulch itself as a ground cover.

•	Do not use plastic sheeting as mulch because plastic is an impermeable layer and will prevent water, air, and nutrients from entering the soil. Use landscape fabric (weed block) instead.

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Immediately after the lawn has been cut, spread the lawn clippings on the ground. Do not wait to use the clippings as they will rot and can no longer be used as mulch. Lawn clippings can also be mixed with other garden and vegetable waste to produce compost, which then can be used as a soil amendment.

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Newspaper can be used as mulch. Use only black and white text pages printed in soy ink. Cover the soil with newspapers 2-3 sheets thick and cover with grass clippings, straw, or wood chips to keep the papers from blowing away. This method is especially good for weed control. Newspapers can also be shredded and spread on beds. If you want to plant within the area of the newspaper mulch, cut an “X” through the newspaper, fold the newspaper back making sure the mulch is clear of the plant’s stem to avoid rot, loosen the soil with a trowel, and plant.

Myth No. 1: Mulch provides a permanent barrier to weeds. While mulch reduces the number of weeds, some pesky nuisances may still manage to grow through the mulch or on top of it. Weeds will appear in new landscapes for the first year or two, even with mulch, because of the abundance of weed seeds in the soil. However, once the weeds are brought under control, fewer will follow. Organic mulch needs to be replenished when decomposed to less than 1 inch thick.

Myth No. 2: Mulch placed against your house will attract termites. In reality, it’s moisture, not mulch, which attracts termites. So any landscaping, including shrubs, can lure the unwanted critters. Irrigation systems up against the house also create a termite-conducive environment. Even gravel or rocks around the base of a house can hold in the moisture attractive to termites.

Myth No. 3: There’s no wrong way to mulch. When mulching, keep the bark or rock one to two inches away from stems. Mulch creates a warm and humid environment that is perfect for the establishment of various fungi that can injure or kill the plants. Also, avoid covering the crown of dormant perennials.

•	Determine the square feet of the area you want to cover by multiplying the length of your garden by the width. Mulch is usually sold in cubic feet/cubic yard.

•	Divide this sum by 324. The result is the number of cubic yards of mulch you will need to cover your garden.

"The unmulched garden looks to me like some naked thing which for one reason or another would be better off with a few clothes on.”(Ruth Stout)

Mulch is a layer of material covering the soil to exclude sunlight. It is proven to be one of the most effective and desirable methods of preventing germination and growth of annual weeds. Since most seeds require sunlight to germinate, mulch is effective as a weed control.

Mulches, depending on the material being used, have the added advantage of improving soil, bettering water retention, helping break down clay soil, improving holding capacity of sandy soil and minimizing dust.

The minerals and organic matter gradually released from decaying mulch feeds deficient soils, replacing nutrients taken up by roots as plants grow, often enhancing earthworm populations.

Although mulching can require an initial investment, either in time, money or both, the long term benefits reduce ongoing weed management costs. If you have prepared your site, killed or removed weeds, follow immediately with an application of mulch. Regular hand pulling of weed seedlings may be all that is needed for weed management. Although mulch is effective against newly emerging weeds, it often fails to control established perennial weeds such as johnsongrass, bermudagrass, and field bindweed. Remove these species manually as soon as they appear or by spraying glyphosate (Roundup).

3. Early vegetables may, in fact, need bare soil. Exposed soil warms faster in the spring than mulched topsoil.

5. Mulching eggplant or bell peppers and possibly tomatoes too early can depress growth.

6. Organic mulching materials that are not partially decomposed can rob soil of nitrogen.

7. Rather than mulching poorly drained soil, leave it bare so that evaporation can occur. This may prevent root rot.

Organic materials (general): Can conserve moisture, prevent surface crusting, improve water penetration and harbor insect pests.

Bark chips and ground bark: Can harbor insects and termites, often placed over plastic as decorative material.

Compost: Excellent source of organic matter; may harbor certain weed seeds or plant pathogens if not properly prepared.

Grass clippings: Readily available, can reapply over time, may contain weed seeds or bermudagras rhizomes, may mat and reduce water penetration if not dried first.

Hay and straw: Allows good water penetration, may contain grain seed. Can mold and mat in wet weather.

Leaf mold: Can add needed acidity to alkaline soils; must be carefully prepared or purchased. Whole leaves can be effectively used, but appears unkept or unattractive.

Newspapers (shredded): Readily available, low cost, no weed seeds, attracts earwigs, sowbugs, not stable in windy locations.

Newspapers (folded) Use 6 sheets thick, attracts earwigs, beetles, sowbugs, not stable in windy locations. Requires compost or other weighted materials to hold down.

Raisin paper (with resin) Is not considered organic because the water resistant resin is made with chemicals. Virgin paper is available but it breaks down easier than wet strength (raisin paper with resin) but not as fast as newspaper. It is effective as a mulch using only a single thickness, and with aged manure or compost added on top to hold down the paper.

Peat moss: Increases water holding capacity but resistant to wetting when completely dry. Adds acidity to alkaline soils, expensive.

Sawdust: A fine, short-term soil amendment; will mat and inhibit water penetration; robs soil of nitrogen but composting will eliminate some problems; need to add additional nitrogen.

Wood chips: Robs soil of nitrogen; less depletion if rotted prior to application; need to add additional nitrogen.

Nonorganic mulches: Do not benefit the soil by breaking down into organic matter that feeds the soil. To some, it is considered unattractive. It requires special irrigation procedures; must later be removed from field or garden but conserves moisture.

Black plastic. Use in the winter to store heat. Effective; need to add drip irrigation or furrows; warms up soil in spring, considered unattractive.

Clear plastic: Same as black except does not control weeds beneath unless solarization procedures follows.

Aluminum foil: Disorients aphids (and white flies) within 12 inches, expensive, may reflect too much heat in summer.

Nonwoven polypropylene fabric: Allows air and water penetration. It is usually held in place by wood chips or stone.

Photodegradable plastic film: May not need to be removed from garden or field; degrades during the life of the crop, although degradation may not be complete with some products.

Stone: Keeps its color and is longlasting. Tends to heat up, but it retains the heat within the stones.

(The mulching material list has been modified from Pests of the Garden: A growers guide to using less pesticide by Mary Louse Flint, University of California Press)

Ramona Frances of Madera is a University of California Master Gardener. This column is provided by the University Cooperative Extension Master Gardener Program in Madera County. The Master Gardener program extends research based information in home horticulture and pest management, verified by University of California experts to the citizens of our state.

If you have gardening questions, you can call the Master Gardeners at the following numbers:

•	Fresno office 559-456-7563 or stop by the office at 1720 S. Maple Avenue Mon-Fri from 9-12 to speak to a Master Gardener in person. See http://www.mgfresno.ucdavis.edu/.

•	Madera office 559-675-7879 Ext.204 to leave a message or stop by the office at 328 S. Madera Avenue on Mondays from 1:00 - 3:00 p.m. to speak to a Master Gardener in person. See http://www.cemadera.ucdavis.edu.