All About Mulch

Discover these useful mulching techniques to nourish soil and make your gardening work easier.

By Margaret A. Haapoja


November/December 2016

Mulching

Compost is among the most beneficial mulches, helping to reduce weeds and nourish soil.

Photo by iStock

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Often called “the mother of mulch,” Ruth Stout, garden expert and author of The Ruth Stout No-Work Garden Book, never watered or weeded and seldom fertilized her garden — all thanks to her use of mulches. Mulch is a covering spread around plants to control weeds and hold moisture in the soil, creating an insulating layer between the soil and the sun or cold wind. Mulch may be composed of organic materials such as compost or decayed leaves, or it might be synthetic, such as black plastic or fabric. Using mulch well can make your garden easier to maintain, help protect crops and extend the growing season. But sometimes mulch is not advised. Read on to learn how to use mulch to make your garden healthier and easier to grow. 

The Many Benefits of Mulch

The practice serves many purposes and saves time; in addition to preserving soil moisture and preventing weed seeds from germinating by blocking sunlight, mulching usually improves soil’s texture and can even reduce disease. Some gardeners find that mulching can double the length of time between waterings. 

Mulch also acts as insulation, keeping soil cooler in summer and warmer in winter. For example, clematis thrives when its roots are kept cool in summer with mulch. Alternatively, mulch can keep soil a few degrees warmer and prevent plants that are not quite winter-hardy from dying in cold weather. Mulch attracts beneficial earthworms; prevents the loss of topsoil to erosion; improves soil structure and fertility; keeps plants and their fruits clean; provides a place to walk; and makes garden beds more attractive.

Which Mulch to Choose

There are many types of mulch — both organic and synthetic. Organic mulches include autumn leaves (best piled in fall and applied to the garden next spring, after they have begun to decompose); grass clippings (don’t use clippings from chemically treated lawns); hay; sawdust (if not applied too densely); yard trimmings; pulled weeds; pine needles; cardboard; and newspaper. Compost makes a wonderful mulch if you have a large supply. It not only improves soil structure, but provides an excellent source of plant nutrients. Pine needles increase the acidity of the soil, so they work best around acid-loving plants such as rhododendrons and blueberries. When using newspaper, only use text pages (black ink), as color dyes may be harmful to soil life. 

Randel Agrella, seed production manager at Baker Creek Heirloom Seed Company, defines mulch as anything that covers the soil, restricting evaporation and inhibiting weed growth. He recommends using organic mulches such as sawdust, wood chips, hay, leaves and grass clippings. In his own garden, Agrella mulches with pulled weeds, laid on the soil with roots exposed to prevent re-rooting. 

But sometimes he uses non-organic mulches, such as plastic weed barrier or landscape fabric, which can be useful if you’re aiming to warm up the soil faster than organic mulches can. “Applying black plastic can warm up the soil faster than organic mulch on bare soil. It is often used to get an early crop going, or in cases where the soil needs to be warmer than it would naturally get, as for growing melons or sweet potatoes in cool-season conditions.”

Betsy Danielson, garden designer and owner of Dazzle Gardens in Sandstone, Minnesota, says she believes the only good mulches are materials that decompose. Her favorite options are finished compost, well-aged manure, chopped leaves, grass clippings and pine needles. “Because I’m a huge proponent of no-till, don’t dig or till in the material,” she says. “Just let it decompose in place.” 

To use mulch to protect trees, add a layer of mulch two to three inches deep, being careful to keep it at least six inches away from trunks. “For trees, you don’t want volcanoes around the trunk,” says Mike Zins, associate professor emeritus at the University of Minnesota. “Think bagel-shaped, not volcano. If the mulch is piled against the trunk, it can soften the bark and make it easier for insects, rodents and other pathogens to get through the protective area over the cambium layer.” 

When to Mulch

Zins says one should differentiate between winter and summer mulch. Winter mulch is primarily for the protection of plants and summer mulch is primarily for weed control and conserving moisture. He offers these tips.

Mulch in cold seasons: In winter, mulching shields plant roots from extreme cold, helps plants ride 
out sudden temperature changes, and helps prevent the cycle of freezing and thawing that kills many plants.Mulching strawberries with straw helps protect the perennial plants during winter, even down to 40 degrees below zero, and keeps the fruits cleaner in summer. Plastic mulch will generate lots of heat when spring rolls around, while organic mulches keep soil cooler in spring but insulate roots better. Mulching can also prevent cold-weather damage to some early-blooming plants in spring.  

Mulch in warm seasons: Mulching insulates soil, keeping it cool during hot times and preventing crusting, when the top few inches of soil dry out when in direct contact with the air. The less organic matter, the worse the crusting will be in summer. “Summer mulch is used to control weeds and keep soil moist and cool,” says northern Minnesota extension professor Terry Nennich. Some gardeners also use mulch between rows of potatoes to reduce sunburn on exposed tubers. For tomatoes, market farmer Kent Lorentzen uses clear plastic mulch. “Over the years,” he says, “I’ve done some side-by-side trials of different mulches for tomatoes: grass clippings, sawdust, paper, black plastic, IR [colored] plastic, clear plastic and bare ground. I’ve found any mulch is better than no mulch with not too much difference between them.”

When Not to Mulch

In some cases mulching is not beneficial. In certain circumstances, mulch can keep soil too cool or moist, which can cause plants to rot. Where the season is very short or cool, don’t use mulches except in pathways. In wet gardens, mulches may worsen slug and disease problems. 

Organic mulch should not be used on newly seeded areas, because most seeds need warm soil temperatures to germinate. Plastic mulch, on the other hand, will help soil warm faster.

Keep in mind that mulch can sometimes deplete soil of nitrogen, because the breakdown of organic matter uses the nitrogen crops might otherwise use. Nennich says people call him up and say, “Gee, my plants are yellow and I put this wonderful mulch on them.” He tells them the mulch is sucking nitrogen out of the soil and recommends adding finished compost or a nitrogen-based organic fertilizer. Usually one application is enough. “One problem with organic mulch is that the soil will try to break it down,” he says. “The organisms that break it down have the ability to use the nitrogen before the plants do. After the compost is broken down, the nitrogen is then available to the plants.”  

Specific mulches can also cause potential problems: lawn clippings can become smelly if they get too hot; thick sawdust can form a crust that inhibits moisture from moving; and piles of herbaceous material can sometimes attract rodents. “I don’t use mulch between rows of beets, carrots or other root crops,” Lorentzen says. “Rodents seem to find it a good home right next to a good food supply.”