Green Patch: Mulching Tips

Our garden expert explains the benefits of mulching.

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Q. I have a new kitchen garden that will feature vegetables, culinary herbs and edible flowers. Should I mulch after planting?

A. Yes! Mulch is the finishing decorative touch for a new garden, creating a visual contrast between the beds and the rest of the landscape. Mulching is also a conservation technique.

Applying plant-based materials such as straw, pine needles, tree bark and shredded tree leaves around annual flowers, herbs and vegetables does many good things at once.

When using plant-based mulch, be sure to sprinkle a nitrogen source, such as agricultural cornmeal, before mulching to help the plant-based mulch break down. Otherwise, the carbon in this type of mulch (such as sawdust, bark and leaves) will rob nitrogen from the soil and stunt the garden’s plants. For decomposition, the ideal proportion of carbon to nitrogen (called the C/N ratio) is 25 to 30 parts carbon to 1 part nitrogen.

A mixture of mineral-based materials such as coarse sand, greensand, ground granite, lava rock and oyster shells (used only if your soil is not already alkaline) make excellent mulch for Mediterranean natives such as rosemary, sage and thyme.

Q. What will be conserved by applying mulch to my garden?

A. Mulching conserves energy, water and topsoil. Mulch discourages the growth of annual weeds. It shades the soil from sunlight so that instead of evaporating, moisture will remain around plant roots longer, reducing irrigation expense and the labor of dragging hoses. Wind and rain move unprotected topsoil away from the garden. A layer of mulch provides a buffer against these natural forces.

Mulch can also help restore depleted topsoil. All plants contain the elements from the soil in which they grow. As we harvest vegetables and herbs, pull weeds and rake leaves, we remove these elements from the soil cycle. Alternatively, dead plant materials that are returned to the soil are incorporated in the cycle. Decomposers break the tissues down into organic matter, a brown crumbly substance that serves as a reservoir for elements, friendly microorganisms, water and oxygen that feeds existing and new plants. Plant-based mulch replaces the organic matter void left by weeding, leaf raking and the harvest.

Mineral-based mulch contributes potassium, calcium and many other trace elements, depending on the materials you choose. Plants use the chemical elements contained in water, air, and soil and sunlight (in the process of photosynthesis) to grow tissues. Hydrogen, oxygen and carbon are in the water and air. The majority of the rest of the elements comes from the soil. According to Rodale’s All-New Encyclopedia of Organic Gardening (Rodale, 1993), more than 60 elements have been found in plant tissues but only 16 have been proven to be essential to plant growth. The major elements are nitrogen (N), phosphorus (P) and potassium (K). Calcium, magnesium and sulfur are the next most-used elements followed by the trace elements iron, manganese, zinc, copper, boron, molybdenum and chlorine.

Both kinds of mulch will contribute large particles called aggregates to improve drainage and loosen compacted soil. 

Q. Why use mineral-based materials for mulch around the perennial graygreen culinary herbs?

A. Sage, rosemary, thyme and winter savory are native to the Mediterranean region where soils are dry and rocky. The plants grow sparsely on hillsides. Mineral-based mulch will reduce fungal infections by making a barrier between the soil and the lower plant leaves. The light color of the sand and crushed oyster shell reflects sunlight onto the underside of leaves, helping them dry faster. The added light also increases photosynthesis, helping the plants grow. Also, plant-based mulches decompose with the help of fungal organisms. Sometimes these organisms cause rotting diseases in plants, especially those who are native to dry climates.

Q. Should I remove mulch in the spring?

A. Pulling plant-based mulch from the garden in the spring allows the topsoil to warm faster. Mineral-based mulch should be left in place.

Q. When should I add more mulch to the garden?

A. Plant-based mulch decomposes—new layers must be added to maintain its qualities. Replace the mulch in mid-spring after planting and weeding. After a hard freeze in early winter, add more mulch to hold the soil temperature steady. Mulch insulates and moderates the soil temperature, keeping it cool in the summer and helping it resist hard freezing when the temperatures drop. Mineral-based mulches integrate down into the soil and wash away from the surface. Add more after heavy rain and during the winter when roots are exposed by alternate freezing and thawing. When the water in the soil freezes, it expands and heaves. When it thaws, cracks are left in the soil surface, exposing plant roots to the air. The roots then dry out and plants can be killed or stunted

Tina Marie Wilcox has been the head herb gardener at the Ozark Folk Center State Park in Mountain View, Arkansas, since 1984. She also writes Yarb Tales, a weekly column for the Ozark Folk Center ( ).