Mulches and Mulching

 Applying a mulch around our plants can be one of the most effective ways to improve their health and success, especially during hot and dry conditions like we had in 2012. Mulches have many benefits, but it is important to know how to use them to avoid problems.

 Mulch conserves water by reducing evaporation 10%-50%. Usually a 2”-4” deep layer is best, and the material needs to be open enough to admit rain and irrigation and dense enough to resist evaporation. It is a good idea to apply mulch after the soil has been deeply watered or soaked with a good rain. Then the mulch will hold the moisture. Beware of materials like unshredded leaves which can act like shingles, and dry compost or sawdust which are hydrophobic, meaning they are difficult to wet. These problems are worse on a slope where water can run off instead of penetrating.

Years ago I was mulching my roses in the fall with 4” of good compost and was puzzled to see winter dieback on roses that are bone hardy. When I dug through the compost I discovered that moisture was only penetrating a couple inches and realized that my roses were dying of drought. Mixing the compost with some sand, or switching to Soil Pep, a partially composted fine bark allowed water to penetrate. Where water is limited, a mulch can make the difference between life and death to a plant, a good yield or dropped fruit, and stunted or vigorous growth.

Mulches are also valuable to suppress weeds which compete for water, light and nutrients. Lee Reich, expert gardener and garden writer goes into detail in his book Weedless Gardening. It is necessary to use at least a 2” deep layer and 4” is better around shrubs and in vegetable gardens. To be most effective, weed before mulching, or cut the vegetation to the ground and put overlapping pieces of cardboard or thick layers of newspaper down, followed by a 2”-4” layer of compost with sand or mulch. Of course it helps if your mulch is weed-free. The few weeds that grow up through the mulch are easier to pull or dig out.

Another benefit to mulches is their ability to moderate the soil temperature. This is a great advantage in hot summer conditions when uncovered soils can heat to over 100 degrees F. Most plants cannot metabolize properly over 85 degrees which means growth, flowering and fruiting decreases in high heat. The insulating effect of a mulch can reduce this problem. Earthworms dive deeper when the soil is hot, and organic matter in the soil is rapidly oxidized when overheated, losing carbon value and releasing carbon dioxide into the atmosphere. The insulating effect can also be valuable to protect from the cold. Plants can continue to flower, fruit and make roots longer in the season when mulched. And fruit trees and roses can be kept dormant longer in the spring, protecting them from late freezes.

One of the most important values of mulch that has been long appreciated in Nature, but until recently, little known by most humans, is its value in building a nutritious and healthy topsoil. The famous biodynamic soil scientist E.E. Pfeiffer said that mulches “…foster the development of a very fine humus and crumbly soil structure underneath.” This outcome occurs through several mechanisms. Under mulch, the soil surface is much less compacted by rain and footsteps which can close off the pores that let in water and air. And worms and other organisms move through the moister and more nutritious layer under the mulch, making holes and paths that admit water to deeper levels.

We are also becoming more aware, with recent revelations in soil science, of the nutritional advantages that mulching provides by supporting a diverse population of microorganisms. These beneficial bacteria and fungi “digest” raw and complex nutrients into simpler forms that can be used by plants. This is Nature’s approach to fertility which is soil and soil life based as opposed to 20th Century top-down chemical fertility. Mulches also provide both the materials and the environment for microorganisms to improve soil structure which means more spaces and more oxygen in the soil. They accomplish this by secreting sticky substances that bind small particles together into aggregates that aerate the soil and make it easier for root hairs and trowels to dig.

Jane Shellenberger, in her book Organic Gardener’s Companion,

discusses how to use sheet mulching to create soil fertility. “During one particularly warm, dry, windy winter, I was able to create loamy chocolate-cake soil loaded with worms in three large dried out and neglected raised beds simply by spreading a thick (6”) layer of chopped leaves on top, watering well, and covering with flakes of hay.” She describes this method in some detail, as does Lee Reich, who says this method replaces the need to plow, rototill or double-dig.

         So what’s not to like about mulch? Well, mulches can have faults when conditions are too wet and the soil or the mulch goes anaerobic (doesn’t get enough oxygen). Then plants can’t breath and decline or die, and the mulch can produce toxic substances like methanol and acetic acid. Sometimes there appears a disgusting, bright yellow spongy fungal mass called Witches Butter, but even that is not harmful. And slugs and snails like mulch that is wet all the time. Also it is said that mulching too thickly can reduce oxygen entering the soil. Lee Reich says apply no more than 2”-4” deep, but Dr. Bob Kremer, a USDA soil microbiologist, whom I questioned suggested 6” of mulch in hot, dry conditions. He said it is important to know that “…all soil microorganisms are essentially aquatic…they need some amount of water to function.”

What materials make good mulches and which have problems? Shredded leaves make a great mulch, and straw free from seeds and herbicides is great in a veggie garden. Some prefer hay in a veggie garden, claiming they have no problem with seeds if they use thick “flakes”. Woodchips are an excellent local resource that looks better when big chunks and stringy pieces are screened out. I prefer the fine grade of wood chips both because they mat tighter and don’t blow as easily, and because they break down faster and feed the soil faster. I also believe that large bark chunks, and cedar and redwood chips and bark are poor choices for a mulch around plants , because they contain natural chemicals that repel microorganisms and therefore are slow to provide nutrients and other benefits. They are fine for mulching paths. And please don’t pile mulches against tree trunks; they can rot the tree’s bark and damage or kill the tree.

Some inorganic mulches are good. Fine gravel like pea gravel or squeegee are preferred for mulching alpines and xeric plants like penstemons and cacti that will rot with an organic mulch. I have seen a pile of rocks nurse a young tree into strong growth, and I have heard that young trees really benefit from a leaf mulch that is covered with large flat rocks. Even old boards and used carpet have been used effectively to make paths in raspberry patches.

I don’t recommend black plastic and geotextiles, known as weedbarriers. They sound like they will free us from maintenance, giving us a “permanent” solution, but don’t believe it. Black plastic kills by drought, suffocation and overheating. Weedbarriers are fine for paths or temporary barriers for poison ivy or thistles, but fabric pores fill up in a few years, preventing air and water penetration. In addition, we can’t amend the soil or feed through them except with liquids fertilizers. Worst of all they don’t allow worms to come to the surface to aerate and mix in organic matter, and they suppress other soil life. In addition, they must have mulch covering the fabric so the sunlight doesn’t make them break down and mulch can’t stick to fabric, like it does in soil, so it invariably blows or washes off leaving ugly bald patches.

Mulching can also reduce erosion by wind and water, reduce the need to cover every bare spot with a plant, improve the appearance of our gardens, and, according to a recent study at Morton Arboretum, rejuvenate soil that has been compacted during housing construction.

I believe that mulching is one of the most important gardening and farming arts to be practiced and developed in the 21st Century. Mulching helps to build nutritious topsoil that can lead to nutrient-dense food which is the best medicine. It can be a very significant method for recycling our waste materials, and through this, a very important means of holding (sequestering) carbon in the soil and soil life so that it does not become carbon dioxide, a greenhouse gas.

Plants are excellent solar collectors and when we employ them in growing our own food and creating our landscapes, we are adding to our personal economies. When we apply mulch to soil, we are partnering with Nature’s own biology and technology for building fertility and health from the soil up, and helping to heal our ailing earth and atmosphere.

Resources for more info on mulches and mulching:

The Mulch Book by Stu Campbell

Organic Gardener’s Companion by Jane Shellenberger

Weedless Gardening by Lee Reich

Lasagna Gardening by Patricia Lanza

Building Soils Naturally by Phil Nauta

Growing Green by Jenny Hall and Iain Tolhurst

The No-Work Garden Book by Ruth Stout