Category: ground covers

Mulch Matters: Understanding Materials and Methods

On By durhammastergardenersIn arborist wood chips, Durham NC, fall gardening, garden tasks, gardening, Gardening calendar, Gardening information, ground covers, inorganic mulching material, lasagne mulching, leaf mold for mulch, mulch, mulching and soil chemistry, mulching garden beds, mulching iwth paper, mulching leaves, mulching trees, mulching with arborist wood chips, mulching with cardboard, mulching with paper, organic mulching material, over mulching, plant fungi, plant pathogen, proper mulching techniques, regenerative agriculture, renewable gardening, sheet mulching, soil, soil coverage, soil health, soil microbes, tips for effective mulching, tips for proper mulching, Uncategorized

By Eric Wiebe, NC State Extension Master GardenerSM volunteer of Durham County

(Image credit: Allie Mullin)

As the gardening season is winding down, it’s time to start thinking about your fall and winter tasks. Near the top of that list should be thinking about mulching your plantings. A tour of your yard will probably reveal thin or bare spots where last year’s mulch has either broken down or been washed or blown away (those darn leaf blowers!). You may also want to put a winter cover on areas where you had vegetables growing. Finally, you may also have new areas you are preparing as garden beds where mulch will need to go around the new plantings.

Why Mulch Matters

First, let’s review why you want to be mulching in the first place. The primary reason most people mulch is for a combination of weed suppression and moisture retention. However, there are other just as important reasons to mulch. If you are using organic materials, you have the important benefit of in-place composting and humus generation, improving the biodiversity of the ground ecosystem throughout the mulch and into the surrounding soil and plants. Depending on type and placement, mulch can also help with erosion control and compaction. In addition to absorbing and holding moisture from rainfall, it can also minimize water splash from the underlying soil onto plants–potentially limiting the spread of harmful soil-borne fungi, such as blight. Finally, there may be good aesthetic reasons to mulch to create sharp delineations in color and texture in the landscape.

Types of Mulching Materials

What kinds of materials can you mulch with? Broadly speaking, mulches are grouped into organic and non-organic types. Included in organic materials are natural products, primarily from trees, that have been processed but will readily decompose–this includes cardboard, newspaper, and other paper products. 

Inorganic Mulch

The most common non-organic materials are landscape fabrics made from plastic–either sheet or woven–and various types of stone. These are often used together, as the landscape fabric can be placed under the stone to help suppress weeds and keep the stone from migrating into the soil. Plastic sheet, with or without perforations, is also used in fruit and vegetable production, often applied with specialized machinery. The rest of the blog post will focus on the more popular and beneficial organic mulches used by home gardeners.

Organic Mulch

While shredded hardwood comes to mind when thinking about mulch, other organic materials like leaves are great sources for mulching material. As they decompose, leaves improve soil health by providing nutrients and improving both water retention and percolation. (Left) Newly-raked leaves ready to be spread across the garden. (Right) Leaf mulch after a year of decomposing. (Image credit: Eric Wiebe)

Organic materials can be found from a variety of sources. The best and most convenient materials are those found right in your yard. This includes leaves, new or partially decomposed; grass clippings, compost, and plant and tree trimmings. There is the argument to be made that the best mulch sources are the most local, as they represent the local ecosystem and can best support the macro-/microbiome already in your yard. 

(Left to right) Arborist chip drop ready to be spread and a close-up of this type of mulching material. (Image credit: Eric Wiebe)

If you don’t have enough organic sources of mulch within your own yard, consider arborist wood chips. These are the product of local tree services as they cut down and chip tree limbs and can be acquired for free through a national service, ChipDrop (https://getchipdrop.com/). This product is still relatively local, representing tree species from your area and, for most of the year, contains a mixture of bark, wood, and leaves.

Research has shown that wood chips perform well with regards to moisture retention, temperature moderation, and weed control. While you can buy shredded (often from grinding whole trees), bark, and sawdust mulches from garden centers, many of these products are imported from other areas, bagged in plastic, or dyed. All of these factors both increase your cost and reduce their sustainability. In contrast, the biochemical and physical diversity of arborist chips resists compaction and creates a more diverse habitat for insects and microorganisms. Over time, the bark, wood chips and leaf matter will decompose, adding nutrients to the soil.

Paper products like newspaper and cardboard have long been used for sheet mulching/lasagne gardening techniques. Here a homeowner is using cardboard shipping boxes stripped of labels and tape as a base layer to be covered by other natural mulching materials. (Image credit: Melinda Heigel)

There are many, many other types of organic mulches that can be used. Pine straw is popular in NC and can be very local if you have a lot of pine trees on your property. While compost can also be used as mulch, its cost typically prohibits its use over a large area. Finally, paper products in the form of cardboard, newspaper, and brown paper have long been used as a form of sheet mulching. Note that you typically have to cover these materials with a somewhat dense mulch (e.g., partially decomposed leaves) in order to hold it in place. While there has been some social media controversy over the use of cardboard, it is still generally believed to be a quality mulch material when applied properly. Note that you should carefully remove non-decomposable elements such as tape and staples, and avoid glossy, non-brown cardboard as they may contain unwanted inks and plastics. While grass clippings can be used as a mulch, it tends to decompose quickly, not drain well when wet or blow away when dry. They may also contain excessive fertilizer or herbicides. Instead, consider mulching it in place in the lawn.

Tips for Effective Mulching

There are best practices to consider when using any mulch, and some considerations for specific types of mulches. First, to the best of your ability, plan how much area you have to cover and what volume of mulch you need. As a rule of thumb, you will need 6 cubic yards of mulch to cover 1,000 sq ft at 2″ deep–you generally will want to keep your mulch 1-3″ thick. However, this thickness will be driven in part by the density of the mulch; how well it retains moisture and allows oxygen to pass through it. Lighter mulch material such as pine straw or newly raked up leaves can often be deeper, while wood chips should be around 2-3.” If mulch is piled too deep, is too fine, or retains too much water, air cannot penetrate the mulch layer and the mulch and underlying soil can be depleted of oxygen. Mulch decomposing in low oxygen (anaerobic) conditions will sour, or ferment, producing methanol and acetic acid, which can be toxic to plants.

(Left to right) A properly-mulched tree has a few inches of mulch that extend to the edge of the drip line (or area that mirrors the tree’s crown). The root flare is visible at the base of the tree, as mulch isn’t in direct contact with the trunk. By contrast, the tree on the right is an example of all-to-familiar over mulching. Known as “volcano mulching,” mounding mulch around the trunk is harmful to your tree, leading to its decline and possible death. (Image credit: Penn State Extension; Eric Wiebe)

For all plants and trees, make sure to apply mulch all the way out to the drip line as you want to provide weed suppression, moisture retention, and soil benefits to all of the roots of the plant. Trees are another case of the dangers of over or improper mulching. Mulch should not come in contact with the bark of the tree as it promotes the kind of dark, moist environments that allow harmful fungi to enter the tree through the bark. Think donuts rather than volcanoes when you mulch your trees.

Impact of Mulch on Soil Chemistry

Using pine straw mulch as an acidifier on blueberries. (Image credit: Eric Wiebe)

Mulches can influence soil chemistry. While most of the time, this is happening in a good way, you need to understand the properties of the mulch you are using. Pine straw and pine bark, for example, has a naturally low pH. While this may be a problem on some plants, you can use it to your advantage with acid-loving plants such as azaleas or blueberries. Organic mulches with a high carbon-to-nitrogen ratio, like sawdust, hardwood bark, or wheat straw, may induce a short-term nitrogen deficiency as it breaks down. If plants in the mulched area become chlorotic and growth slows, consider applying one to two pounds of nitrogen per 1,000 sq ft to compensate. Almost all mulches will have a very localized nitrogen deficiency at the boundary between the soil and mulch. However, this is good as it inhibits weed growth.

Many of the basics and a few of the nuances about mulching have been covered here. As with most aspects of gardening, you can, if you want, continue to go down the rabbit hole of the science of mulching. The resources below will get you started!

Resources and Additional Information

General guidelines on mulching trees and shrubs: https://extension.umd.edu/resource/mulching-trees-and-shrubs/

Mulching garden beds:

https://carteret.ces.ncsu.edu/2021/08/mulching-your-garden-beds

https://yardandgarden.extension.iastate.edu/how-to/using-mulch-garden

Benefits of using arborist wood chips: https://pubs.extension.wsu.edu/using-arborist-wood-chips-as-a-landscape-mulch-home-garden-series

Tips for mulching in the spring: https://ext.vt.edu/lawngarden/turfandgardentips/tips/Springtime_mulch.html

Comprehensive guide to mulching materials: https://extension.unh.edu/sites/default/files/migrated_unmanaged_files/Resource007193_Rep10385.pdf

Sheet mulching/lasagne gardening gardening with paper products: https://extension.oregonstate.edu/gardening/soil-compost/sheet-mulching-aka-lasagna-composting-builds-soil-saves-time

https://extensiongardener.ces.ncsu.edu/2020/01/sheet-mulching-2020-winter-piedmont-news/

Problems with over mulching: https://pender.ces.ncsu.edu/2020/10/can-too-much-mulch-killl-plants/

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Bringing Plants of the Piedmont back to Downtown Durham

On By durhammastergardenersIn bees, beneficial insects, birds, cover crops, CPTED, crime prevention through environmental design, Downtown Durham, NC, drought tolerant plants, Durham NC, garden design, gardening, Gardening calendar, Gardening information, gardening NC, green mulch, ground covers, ground covers for living mulch, how to improve soil, insects, landscaping for wildlife, living mulch, native plant list, native plant list for Piedmont NC, native plants, Natives, NC Native Plants, Perennial, phenology calendar, Piedmont Prairie Garden at Rotary Plaza, pollinator gardening, polyculture, renewable gardening, soil, soil health, sustainability, sustainable garden design, sustainable watering, sustanability, Uncategorized, urban challenges in landscaping, urban garden, urban gardening, urban heat islands, urban planning, urban prairie garden, urban wildlife, uuban service benefits in landscapes, water wise plants, wildlife

By Eric Wiebe, NC State Extension Master GardenerSM volunteer of Durham County

A once neglected site, Rotary Plaza in downtown Durham comes alive in July 2021 with sunflowers that produce seeds for wildlife. Millet, a grain used here in the design and novel to many urban dwellers, also serves as a great food source for gold finches and other birds. (Image credit: Ben Bergmann)

Have you noticed changes to the plantings in downtown Durham over the past few years? If not, you need to head on down, walk around, and appreciate the transformation of the many of the city-owned green spaces. This transformation, and the strategies behind it, point to a new way of thinking about our relationship to the landscape, both in urban settings like downtown, but also in our own yards. 

I had the pleasure of both walking around downtown Durham with and then sitting down and talking to Ben Bergmann, the architect of this transformation. Ben was hired by City of Durham in September 2020 and charged with re-envisioning and elevating horticulture in downtown Durham, including creating a Downtown Horticulture Crew within Landscape Services. Ben brought a widely varied background to this position, trained with BS, MS, and PhD degrees in plant sciences, and working as everything from small business owner and farmer to tropical agroecologist and research scientist. One thing all this work has in common is plants and his great love of them. Ben feels this broad range of experiences has helped prepare him for a job that has him applying both the art and science horticulture to a municipal government position with its many constraints and opportunities and multitude of stakeholders. 

“When I started, I realized I needed a mission statement to clearly and concisely communicate to my supervisors and the public what I and my unit in government is doing.” He stated that his landscape design and management are guided by the goal of enhancing

  • Environmental sustainability
  • Programmatic stability
  • Aesthetic appeal
  • Service benefits

He notes that it is important to recognize that these categories are not mutually exclusive, but each has a role to play in his horticultural work supporting the living landscape in downtown Durham. 

Designing Sustainably

Ben explains that a crucial re-orientation is to start planning a landscape by thinking about how it can best align with the ecosystem, and that means starting with native plants that can be used. “For urban contexts some people would say that there is nothing native to these downtown spaces, and they will use this as an ‘out’ to plant whatever they want. I don’t embrace this–something was native, originally, in these spots. Many times these native plants can and will grow and thrive in these spaces.”

When Ben thinks about which native species to use, he first looks for plants native to the North Carolina Piedmont and secondarily to the larger Piedmont of eastern U.S. He notes, however, that it is very important to select plants with a long-range vision. That is, when accounting for global climate change, plants that are going to be here in 20 years may very well be in an environment that looks more like the southern Piedmont, our coastal plains, or South Carolina.

(Left to right) By selecting plants that can cope with a warming environment, Bergmann installed dwarf palmetto (Sabal minor), a plant typically associated with climates in the native ranges east and south of Durham, NC, and shown in green on the USDA’s zone map. (Image credit: Ben Bergmann; https://plants.usda.gov)

While native species take the lead when planning bed renovations, Ben states that non-native plants can still play a role. Returning to his guiding principles, non-natives can help fill functional gaps that might be hard to resolve in the short run with native species. Similarly, there are times that a spot planting of a non-native will help achieve a particular aesthetic goal by providing highlight color, texture, or architecture.

(Left to right) Before, a non-imaginative flag pole bed at Durham Parks and Recreation building included a highly invasive and exotic species: Nandina domestica. After a re-imagining, the same bed demonstrates there is a place for non-native plants alongside natives, especially during transition phases. (Image credit: Ben Bergmann1

Because the legacy plantings downtown were almost 100% non-native, it means that during a bed’s transition period, some non-natives may be left to help visually and functionally stabilize the bed until new, native plants can get established and then take over. Among other things, the non-native plants retained just for the transition can provide shelter from the sun and wind during establishment.

(Left) The process of establishing an urban prairie at the Rotary Plaza began in September 2020 with beds filled with weeds and remnants of long past garden attempts. (Middle) In March 2020, the the beds are full of cover crops: crimson clover, a nitrogen fixer that is great for pollinators in early spring, and winter rye, which has high biomass production and deeply penetrating roots. The intricate planting pattern reflects intentionality and provides visual interest. (Right) By July 2022 the Piedmont Prairie Garden at Rotary Plaza is realized and chock-full of sustainable native plants. (Image credit: Ben Bergmann)2

Even given how much he has achieved is such a short time, Ben still takes the long view when planning and planting. For example, cover crops have been used in very purposeful ways in areas such as the Rotary Garden. During the initial transition of this space, cover crops were planted and maintained in ways that not only helped build the soil, but created visual interest. Over time, the cover crops matured and eventually replaced with native perennials, creating an evolving horticultural narrative in this space. As Ben said, “Some [landscape] designers shy away from transitions–they want to ‘put a tarp over it’ until it has reached its final phase. I don’t shy away from transitions; they are an important part of my plans.”

Ben notes that using select non-native plants, cover crops, and other annuals allows him to more easily use seeds, plugs, and small plants in the beds. In the case of herbaceous perennials and woody plants, besides being less expensive, younger, smaller plants will typically develop more robust root systems and quickly outgrow their bigger counterparts.

Addressing Urban Challenges

Ben was quick to remind me of the many contextual limitations and challenges the Downtown Horticulture Crew faces daily that home suburban gardeners rarely have to contend with. For example, CPTED (Crime Prevention Through Environmental Design), is an important set of guiding heuristics that often preempts other design considerations. In addition to making sure plantings do not screen illicit activities, it also disallows designs that interfere with security cameras or impair pedestrian safety, including driver site lines at street crossings. 

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The center of the Chapel Hill Street parking lot is a perfect example of an urban heat island with even hotter microclimates where landscapes are needed. (1) In January of 2022, the lot was cleared of weeds and other unhealthy, dated, and non-native landscape plants. (2-5) The view in 2024, after Bergmann and his team revitalized the area with an assortment of heat-tolerant natives. (Image credit: Ben Bergmann)3

Most of the beds Ben and his crew manage are surrounded by heat absorbing asphalt and concrete and buildings with glass that intensify solar radiation by reflecting light onto them. This creates heat islands much hotter than in surrounding communities. In addition, tall buildings funnel wind through these spaces and block rain, further desiccating plants. 

Humans also are unhelpful, by walking on, driving over, or otherwise physically challenging plantings. Some soils are more like landfill or construction sites, and many have the negative impact of overuse by dogs on their walks. Even with these challenges, Ben has identified quite a few native species that are up for environmental extremes while still fulfilling functional and aesthetic goals. Not surprisingly, he is currently exploring the possibility of a demonstration xeriscape-inspired bed. This one may very well include some non-native plants more typically found in desert environments.

Educating the Community

Ben understands that an important part of his job is educating not only the workers and residents of downtown Durham but all city residents and visitors as to a new way of envisioning landscapes. Landscapes that, while they exist in a human-built environment, much more clearly reflect real ecosystems of the NC Piedmont. This means not only planting predominantly native species but planting them in soil nourished by natural mulches created by the plants themselves rather than imported, dyed wood chips. He also allows the plants to go through full cycles, creating visual landscapes that sometimes predominate in browns, grays, and blacks, as much as greens, yellows, and reds. 

(Left to right) The Piedmont Prairie Garden at Rotary Plaza in early January with the last vibrant colors and in late February, when the remaining plants still create an almost black-and-white picture. (Image credit: Ben Bergmann)4 (Below) By designing a phenology calendar for his plantings, Bergmann pays careful attention to nature’s timing. This tool helps gardeners know when to plant, specific bloom times, and when colors, textures, and other elements are present throughout the seasons. For a closer view, click on the link for an electronic version of the calendar in our Resources and Additional Information section below.

He is heartened that there is a growing critical mass of professionals helping him in this educational work. The City of Raleigh and Wake County has been hard at work creating plantings that reflect this same vision. He also has strong allies at Duke Gardens, where Annabel Renwick, has provided both inspiration and plants for some of the bed designs. Ben recognizes that it will take a while both for his beds to reach the final stages of their design vision and for Durham residents to embrace this new look, but he has already received lots of positive feedback for his renovated beds. It will also be up to Durham Master Gardeners, and committed gardeners everywhere, to help continue to educate the public and support this work.

Plant lists for featured Downtown Durham Sites

1–The planting design at the Parks and Recreation administration site include a mix of natives (*) and non-native plants: threadleaf coreopsis* (Coreopsis verticillata), creeping phlox* (Phlox subulata), eastern ninebark* (Physocarpus opulifolius), panicle hydrangea (Hydrangea paniculata), lantana (Lantana camara), coleus (Solenostemon scutellarioides).

2–Plants in the Piedmont Prairie Garden at Rotary Plaza include swamp milkweed (Asclepias incarnata), blue mistflower (Conoclinium coelestinum), purple coneflower (Echinacea purpurea), Joe-Pye weed (Eutrochium fistulosum), Spotted Joe-Pye weed (Eutrochium maculatum), common sunflower (Helianthus annuus), scarlet rosemallow (Hibiscus coccineus), larger blue flag iris (Iris versicolor), cardinal flower (Lobelia cardinalis), switch grass (Panicum virgatum), garden phlox (Phlox paniculata), hoary mountain mint (Pycnanthemum incanum), brown-eyed Susan (Rudbeckia triloba).

3–Center of Chapel Hill Street parking lot showcases the following native plants: sedge (Carex bicknellii), purple love grass (Eragrostis spectabilis), Elliott’s lovegrass (Eragrostis elliottii), scarlet rosemallow (Hibiscus coccineus), swamp rosemallow (Hibiscus moscheutos), Virginia sweetspire (Itea virginica), tulip poplar (Liriodendron tulipifera), black-eyed Susan (Rudbeckia hirta).

4–The Piedmont Prairie Garden at Rotary Plaza in late February still offers color, texture, and winter interest. They include climbing aster (Ampleaster carolinianus), scarlet rosemallow (Hibiscus coccineus), switch grass (Panicum virgatum), hoary mountain mint (Pycnanthemum incanum), white goldenrod (Solidago bicolor), showy goldenrod (Solidago speciosa)–with touches of hopeful green, beardtounge (Penstemon digitalis), eastern beardtounge (Penstemon laevigatus), stoke’s aster (Stokesia laevis), adam’s needle yucca (Yucca filamentosa).

5– See below for Bergmann’s list of natives that are tried-and-true performers for Durham and central North Carolina.

Resources and Additional Information

Piedmont Prairie Garden at Rotary Plaza Bloom Time / Color Calendar (electronic version)

https://go.ncsu.edu/piedmontprairiecalendar

NC State Extension on landscaping for wildlife with native plants in urban environments

https://content.ces.ncsu.edu/landscaping-for-wildlife-with-native-plants

North Carolina Native Plant Society’s list of recommended native species

Recommended Native Species

NC State Extension on all things native plants

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