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Mulch: The Basis of Any Good Garden

Dr. Lakshmi Sridharan, Molecular Biologist

This article is a 2009 AOM winner

A carefully designed eco-friendly landscape with trees, shrubs, vines and herbs adds serenity and beauty to a home and also enhances property value. Water is the lifeline for all creatures, great and small including the plants. Though water occupies three fourths of the Universe, only a small fraction of it is available for supporting and sustaining life. Usable water, therefore, is the most precious natural resource. Hence, gardeners have to use water wisely without wasting a drop of it and conserving it to the maximum. To conserve water, grow just two dozen good roses only (roses are always thirsty for water), include drought tolerant plants in landscape, deliver water through a drip irrigation system, and mulch plants to prevent an excessive loss of water by evaporation from soil surface. Mulching keeps the soil moist, maintains even soil temperature, inhibits weed growth and prevents soil erosion.

Mulching means covering a soil to protect from weed growth. Any covering material can be used for this purpose. However, one has to pay attention to the nutritive or decorative value of the mulching material, the cost and labor involved in mulching, time of application, and methods of application etc. Weigh the pros and cons of using a mulching material prior to its application.

There are two types of mulches: nourishing mulch, and protective decorative mulch. Organic mulches such as wood chips, shredded wood (redwood or cedar), saw dust, newspaper clippings, pine needles, leaf mold, grass clippings or partially decomposed compost are nourishing mulches. Nourishing mulch encourages proliferation of soil microorganisms, the beneficial fungi and bacteria that break down most of these mulches, release the nutrients contained within them, and convert the nutrients into a soluble form for an easy absorption by the roots. During growth, plants absorb soil nutrients depleting the soil of available nutrients. Unless the nutrients are replenished, plants will suffer from nutrient deficiencies. A biodegradable mulch nourishes the soil microorganisms, the garden soil and the plants.

All nourishing mulches modify physical and chemical compositions of a garden soil during microbial degradation. They prevent soil compacting, increase the soil binding capacity to nutrients and water retention. However, nourishing mulches need to be replaced when they have decomposed completely. The rate of decomposition of nourishing mulch varies; ground Hardwood has a moderate rate of composition, while red wood, and cedar barks decompose slowly. Saw dust, wood chips, chipper debris and wood shavings decompose quickly, rob the soil of nitrogen, and require frequent replacements. Raw lawn clippings from treated lawns contain herbicides; hence, they do more harm than good. During decomposition, grass clippings rob the soil of nitrogen and generate lot of heat. During heavy rains, a thick mat of grass clippings traps water, which will cause anaerobic decay, and root rot. It is equally undesirable to use raw compost, as it generates heat during decomposition.

I use Pine needles for mulching the rose beds. As Pine needles prevent the splashing of water, they minimize infections by disease causing fungal spores. Pine needles have been shown to control black spot in roses. The needles decompose quickly, and make the soil acidic. Pine needles are therefore good for acid loving plants such as Rhododendrons, Roses, Azaleas, Hydrangeas, etc. When you mulch roses, make sure to replace the mulch early in spring, as disease causing fungal spores over winter on the mulch. Eucalyptus mulch may offer protection from insects as it contains volatile oil.

Another commonly used mulch is mushroom compost. Mushroom compost can supply nutrients and increase the water-holding capacity of the soil. However, one should exercise caution in using mushroom compost as mulch. The soluble salts and other nutrients in fresh, undiluted mushroom compost are too concentrated for germinating seeds, young plants and other salt-sensitive plants including members of the heath family such as rhododendrons, blueberries and azaleas. Cure the mushroom compost in winter before using it as mulch. Order a supply of mushroom compost in the fall and let it sit uncovered, to "cure" over the winter.

Straw or salt marsh hay is an excellent mulching material. They do not pack down the soil. However, they can be a fire hazard. Be careful of smoking materials when working near them.

Nourishing mulches such as grass clippings, pine needles, and leaves are recyclable materials available in ones own garden. Homeowners want to have a lawn that is the envy of their neighborhood. To keep a lawn beautiful, one has to water it deeply at least twice a week. In California, when there is acute water shortage, the city does not want the homeowners to irrigate the lawn. Be water-wise and mulch the lawn. A lush lawn requires mowing more than once. Use the grass clippings for mulching the lawn then and there itself.

Instead of bagging the clippings and throwing it in containers for the city to recycle or use them for landfill, recycle the grass in the lawn while mowing. If performed correctly, returning grass clippings to the lawn should not mar the appearance of the lawn or cause an accumulation of thatch. For clippings to break down rapidly mow the lawn frequently enough so that large amounts of leaf residue do not remain on the surface of the turf. A mulching mower should be part of your landscape machinery. The rotary mowers are designed to keep the clippings circulating under the mower deck and chop the grass blades into finer pieces. If you want to use the clipping for mulching other plants, dry the grass clippings before using them as mulch.

Leaves are good for mulching trees. Fall is the best time to bag the raked leaves for mulching. If you do not have deciduous trees in your garden, collect them from curbsides of homeowners before the green waste trucks collect them. When I come across heaps of pine needles, I bring them home for mulching the roses in my garden.

Redwood or cedar wood barks cost money, but economical as you do not have to replace them quickly; also they give a neater appearance to a landscape. Mulch after watering the plants as a thick mat of mulch may slow down water penetration. Three to six inches of mulch offer the desirable results such as maintaining moisture in the soil, even soil temperatures and inhibiting weed growth. Do not compact mulch closer to the tree trunks or the bases of plants. Waterlogging near the bases of plants will result in trunk rot. Do not bury young seedlings especially in summer, as the heat will cook the seedlings. When using mushroom compost as mulch, till about 3 inches of the compost into the top six inches of fairly dry garden soil for flowerbeds and vegetable gardens. For container grown plants, use fresh mushroom compost about one-quarter of the volume of soil in the container.

Winter mulching is important when you garden in cold climates. Plant roots have to be protected from freezing temperatures. Apply mulches for winter protection after the soil has begun to harden. Too early applications encourage small animals to seek boarding and lodging under a thick cover of mulch, where they make their nests in the fall. Protected by the mulch and a snow cover the hungry beasts feed on bark and roots of thin barked ornamentals, and fruit trees during the food-scarce weeks of winter.

Pull away winter mulches from garden plants in early spring to promote earlier warming of the soil by the sun. Apply new mulches for the summer after the soil has warmed and fertilizer has been applied.

One may also use living mulches such as low growing alfalfa or clover. While these plants may compete with the main crop for water and nutrients, they keep the soil moist from scorching heat and breeze because of their low growing habit. In addition, the legumes fix nitrogen, which is made available to the main crop by the beneficial fungi, the mycorrhizae, which colonize the root system and the root zone.

Decorative rocks (pea gravel, lava or dolomite) that come in different shapes, colors and sizes are used as protective mulch. Though expensive and labor intensive, they need no replacements. They add color and a finished neat appearance to a landscape. Microorganisms do not degrade the hard rocks. They have no nutritive value. They do not hold water but retain soil moisture.

Black plastic is often used for mulching. Do not use plastic sheets, as they will trap heat, which will harm the root system of a plant. Make slits in the plastic or use shredded plastic. Woven fabric may also be used for covering a soil to prevent weed growth. Organic mulch is preferable to plastic. Microorganisms do not decompose plastic or woven fabric.

Mulch helps in the control of weeds by cutting off sunlight for weed growth and choking the weeds. However, care should be taken in the choice and application of mulch. Thick mulch may reduce air circulation, result in waterlogging especially during rainy seasons, and provide shelter to hibernating insects during winter.

Living with Nature, not against Nature should be the goal of every homeowner. One can easily have a beautiful landscape by conserving water and using recyclable material from ones own yard as mulch.

All photos by Rita Perwich


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