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Aug 24, 2023

Ask the Master Gardener: Best mulches decompose and improve the soil beneath them

Dear Master Gardener: I’m on the landscape committee of our homeowner’s association and we received a bid for cleaning up a weedy area and doing some planting. On the bid is the cost of adding

Dear Master Gardener: I’m on the landscape committee of our homeowner’s association and we received a bid for cleaning up a weedy area and doing some planting. On the bid is the cost of adding landscape fabric after killing all the weeds, then using rock as mulch. Aren’t landscape fabric and rock mulch out of favor?

Answer: It used to be popular to use landscape fabric with rock over it, especially in commercial landscaping, in order to suppress weeds and reduce maintenance. It does help suppress weeds for a while, but over time it ends up doing more harm than good. The big selling point for landscape fabric is its permeability so water will still reach the plants’ roots. Unfortunately, in a downpour the water tends to run off instead of soaking through the fabric. In addition, over time the fabric pores get clogged with soil and sediment and the fabric becomes less permeable or not permeable at all. Water and oxygen can no longer reach the roots of the trees, shrubs, perennials, or annuals and they dry out. The other selling point is weed prevention. Weeds may not come up through the fabric (although some eventually do) but dirt blows around and settles on top of it and so do weed seeds. The weeds plant themselves on top of the fabric instead.

Rock mulches do suppress weeds and help retain moisture, but they tend to absorb and retain heat, increasing soil temperatures during hot summer days. Over time leaf litter and other organic matter decompose among the rock particles and build up a soil base that invites weeds. The biggest objection many — if not most — rock mulch users have is when they want to change or remove it. The stones work their way down into the soil below them, even with a layer of landscape fabric beneath them, often as much as a foot deep. That makes their removal a messy, dirty and heavy task. Another product that is not recommended is black plastic. It is effective in suppressing weeds, but it prevents water and air from reaching plant roots. Organic products are best. As they decompose, they improve the soil beneath them. Some, like shredded leaves, quickly decompose and add nutrients to the soil, but they must be replaced often, sometimes several times a season. Straw is an excellent insulator but is messy and tends to contain many weed seeds. Cocoa bean hulls have a pretty color and smell wonderfully of chocolate but are quite expensive and have a tendency to crust. Bark nuggets, a recycled byproduct of the forest industry, are slow to decompose, and come in various sizes. Wood chips also come in many sizes. They can be found readily and sometimes are even delivered by tree services with chippers. Shredded rice hulls and pine needles are excellent, inexpensive, usually free mulches. They are light in weight, tend to stay in place and are easy to handle. Contrary to popular belief, pine needles do not acidify soil. So, what is the recommendation? Definitely an organic mulch: wood chips, rice hulls or pine needles. All mulches should be placed 3 to 4 inches deep to be effective.

Dear Master Gardener: Can I get rid of some old dry beans (navy beans, etc.) by adding them to a compost pile?

Answer: Yes, you can compost old dry beans. Composting is a very important organic practice. It is a process that allows naturally occurring microbes to convert yard waste into a useful soil amendment. Most of the soil in this area tends to be sandy, so adding compost to sandy soil will aid in moisture retention and nutrition. There are two things to keep in mind when starting your compost. First, the microorganisms needed for decomposition require oxygen, water, and nitrogen. Second, the smaller the plant pieces, the more quickly they will break down.

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According to Carl Rosen, a soil scientist at the University of Minnesota, many organic materials can be composted besides grass and leaves: non-woody shrub trimmings or twigs less than one-fourth inch in diameter, faded flowers, weeds without seeds, healthy leftover plants at the end of the gardening season, lake plants, straw, coffee grounds, eggshells, fruit and vegetable scraps, shredded newspaper (black and white print), small amounts of wood ash, and sawdust. Sawdust requires the addition of extra nitrogen and wood ash raises compost alkalinity and may result in nitrogen loss from the pile.

Ideally, grass clippings should be kept on your lawn, but if you decide to add them to your compost, mix them with other yard waste as they tend to pack down and restrict airflow, limiting the oxygen needed for decomposition.

Never compost pet feces because they can transmit disease. Do not compost meat, bones, grease, whole eggs and dairy products because they attract rodents and other animals. Keep out any badly diseased plants.

Dear Master Gardener: How do I take care of the yellow spots in my yard caused by dog urine?

Answer: Dogs often cause brown spots of dead grass in the lawn. Their nitrogen-rich urine acts just like fertilizer burn. The treatment is the same. Thoroughly water the area to dilute the urine and wash it through the soil. It may not be practical, but it is effective. The surrounding grass will eventually fill in these brown areas. Nature and time do heal many landscape problems. There are mixed reports on some of the urine-neutralizing products. Several people have reported great success while others find watering works just as well. Some gardeners train their dogs to go in a specific area.

Dear Master Gardener: There is a shady area in my lawn where grass won’t grow and I would like to replace it with a ground cover of some sort. On a list of ground covers I saw wintergreen. Would it be a good choice for this area?

Answer: The short answer is “probably.” Hardiness is not the issue because wintergreen (Gaultheria procumbens) is hardy in USDA zone 3. It does, however, require part to deep shade and acidic soil that is moist and rich in organic matter. It has glossy, evergreen leaves that release the wintergreen scent when crushed; small, egg-shaped, white flowers in summer; and striking, edible, red berries in the fall. It grows from rhizomes that creep along underground, sending up short 6-inch stems at intervals, slowly expanding its territory. Birds like it and it has a lovely burgundy fall color. It may not be readily available in local nurseries but it can be purchased from catalogs or — if you have it growing wild in your woods — it can be propagated from cuttings in the summer or from rooted suckers in the spring. People who like wintergreen-flavored chewing gum, toothpaste and candy are often unaware that their fresh, minty taste comes from oils from this hardy, beautiful, low-growing shrub. Native Americans used wintergreen medicinally because it has some mild analgesic and fever-reducing qualities, but most of us just enjoy its flavor.

You may get your garden questions answered by calling the new Master Gardener Help Line at 218-824-1068 and leaving a message. A Master Gardener will return your call. Or, emailing me at [email protected] and I will answer you in the column if space allows.

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University of Minnesota Extension Master Gardeners are trained and certified volunteers for the University of Minnesota Extension. Information given in this column is based on university research.

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