Mulch Matters: Understanding Materials and Methods

On November 13, 2024November 13, 2024 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 Gardener^SM 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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Plant Detective: Identifying Black Knot Disease

On March 13, 2024March 11, 2024 By durhammastergardenersIn ornamental trees, plant disease, plant fungi, plant pathogen, trees

By Melinda Heigel, NC State Extension Master Gardener^SM volunteer of Durham County

This is the first in an ongoing series called Plant Detective, where we’ll uncover science-based information on strange and mysterious sightings in the plant world.

I often learn the most about the wondrous world of plant life through observation. Taking our dogs for daily walks in the neighborhood, around local parks, and on greenways provides great opportunities to study trees, flowers, plants, weeds, and sometimes what are strange unidentified objects. I jokingly call these “case files,” and they often turn into fun research projects that enrich my understanding our environment. I ran into one such puzzler recently.

(Left to right, top to bottom) Glorious late-winter color of the Japanese flowering apricot (Prunus mume); unsightly black wart-like structure on a blooming branch; closeup of a gall encircling a branch; and proliferation of this material on the blooming tree. (Image credit: Form, in bloom jr CC BY-NC 2.0; Melinda Heigel)

Who doesn’t love the sight of blooming trees in mid-February like this Japanese flowering apricot (Prunus mume)? As my eyes were drawn to the stunning pink flowers against the bright February blue sky, I noticed something a lot less attractive: black wart-like clumps of material on the limbs of the tree. What was this unsightly vision? Turns out it is a pathogen that affects numerous species of the Prunus genus. While I won’t disclose the initial terms I typed into the search engine (hint: it’s often referred to as cat poop on a stick), I quickly discovered this growth was the fungal disease known as black knot (Apiosporina morbosa).

The fungal pathogen Apiosporina morbosa occurs on wild, native, and cultivated plums, prunes and cherries and, according to Cornell University, it’s been identified on 24 species of trees. As in our “investigation,” it can sometimes also target others in the Prunus genus including chokecherries and apricots. While initially discovered in the US Northeast in the early 19th century and thought to be a native pathogen, black knot can now be found throughout all of North America.

Evidence of Black Knot Disease

(Left to right) At an earlier stage in the infection, springtime swellings of fungal tissue often have a velvet-like appearance and are olive green in color. By fall, these structures rupture, harden and turn dark brown to black. As the disease progresses, knots continue to spread girdling twigs, limbs, and sometimes the trunk. If left unchecked, black knot weakens the tree, can make it more susceptible to insects and other pathogens, and can lead to its death. (Image credit: Melinda Heigel)

Black knot is a slowly developing disease, and the first visual evidence is subtle. Generally after a tree has been infected for a year, small light-brown swellings or nodes will appear on woody new growth, often at the base of leaves. The spring and early summer’s warm, moist weather provides perfect conditions for this pathogen to flourish, and the seasonal flush of new leaves makes detection tough. Fungal spores that cause the infection are carried by wind and water.

This fungus overwinters on the tree, and these swellings continue to grow and take on a distinctive spongey, velvety texture and green hue in the spring. As the growing season progresses, the knots elongate over time, some encircling branches and twigs. The galls eventually rupture, harden and turn dark and brittle in the fall. By the second year, more and more galls become evident, making detection easy work, especially in winter once leaves have fallen. In the spring, the fugal life cycle begins again.

In addition to the galls, changes in leaves and bark can also signal black knot disease. While some cultivars resistant to the pathogen can sport healthy green leaves in the presence of galls, leaves on more susceptible Prunus species may appear wilted, brown, or die back as the infection progresses. Changes in trunk appearance like black, bumpy and swollen bark and oozing from cracks in the bark are also potential indicators.

*Fall-time view of Prunus sp. with with established black knot infection. The black warty galls are made up of both fungal and plant tissues.* (Image credit: Melinda Heigel)

Management of Black Knot Disease

How do homeowners, home fruit growers, or ornamental tree lovers attempt to prevent or mitigate black knot infections in their beloved Prunus?

Vigilant Inspection: Continually monitor tree health throughout the year and look for signs and symptoms year round. While it’s a challenge to initially identify, early detection can provide the opportunity to prolong and potentially save the tree if infected. Look for any swelling, discoloration, or galls especially around newly emerging leaves (where first signs appear). Inspection also comes into play when selecting and purchasing a tree at the nursery. Like with all plants purchases, you want to buy a healthy specimen that has no visible signs of this disease or any other disease.

Location Awareness: When locating a Prunus in your landscape, make sure you know if there are other wild or cultivated cherry or plum trees close by which might harbor the fungus either now or later. Since the fungal spores are dispersed by wind and water, plant any new nursery specimen upwind at the very least. Avoid planting new trees around any site where a known problem exists with black knot among wild or cultivated trees. As an example, a local pocket park in my neighborhood has trees with black knot problems and a homeowner adjacent to the park has just identified this disease on her Japanese flowering apricot trees.

Pruning and Proper Disposal: One of the best strategies for controlling black knot is to remove infected plant material, which helps control future spore dispersal and disease progression. Most experts recommend thoroughly pruning all shoots and branches with knots in the fall and winter. This timing is important because the diseased plant parts are most evident and it’s before springtime spore dispersal, which happens around the time of bud break. Some recommend a second inspection and pruning just prior to bud break.

Make cuts on infected limbs 4 – 8 inches past a gall to ensure removal of affected material. If a large branch has multiple galls or is engulfed with knots, consider removing the entire branch. Sanitation is paramount, so disinfect your pruners or loppers between every pruning cut you make.¹

Proper handling and disposal of affected plant material are critical. Take care not to drop any diseased or pruned material, as spores could be transmitted to the nearby ground where they can survive. Likewise, don’t compost or place the pruned material at the curb for yard waste pickup. It’s best to destroy the diseased clippings or bag them and put them in the trash. You should likely consider removing seriously infected trees.

Chemical Controls: While most experts agree that certain fungicides can be an important tool in controlling this problem, chemical controls alone cannot stem disease progression without the pruning and sanitation measures mentioned above. If homeowners decide to explore the use of appropriate fungicides in an integrated pest management approach, note that it can be complex: timing based on weather conditions, temperatures, and bloom stage is critical. Copper sprays applied during dormancy and Neem oil have been shown to help inhibit fungal spore production but not eradicate the disease. If you decide to include the use of any chemical controls, make sure you have properly identified the pathogen, read and follow all label directions for timing, application, warnings, and use of personal protective equipment. Some experts recommend that homeowners with ornamental and fruit-bearing trees affected by black knot leave the spraying to commercial growers and stick to to cultural and sanitation controls only. (See resources below)

Disease-resistant Varieties and Alternatives: When choosing Prunus cultivars to plant in your home landscape or for fruit production, look for disease resistant (not disease proof) options. Talk with your local nursery about what types do best in your area, especially if you live in a hot and humid environment like central North Carolina. If you are looking specifically for spectacular spring-blooming trees for ornamental purposes, a native tree like the redbud (Cercis canadensis) or the serviceberry (Amelanchier genus) can certainly fit the bill.

Keep an eye out next time you are walking. Who knows what intriguing things you might see! As spring and summer are on the horizon, what plant mystery will we solve next?

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Notes

1–Rubbing alcohol (70% isopropyl alcohol) or a 10 percent solution of household bleach are great options for sanitizing garden tools. See Iowa State University’s Extension and Outreach site for more details. https://hortnews.extension.iastate.edu/faq/how-do-i-sanitize-my-pruners-prevent-spread-disease

Resources and Additional Information

To learn more about growing fruit from the Prunus genus like plums in the home landscape, including great varieties suited to central North Carolina, consult the online version of the North Carolina Extension Gardener Handbook.

https://content.ces.ncsu.edu/extension-gardener-handbook/15-tree-fruit-and-nuts#section_heading_9399

For more detailed information on black knot (Apiosporina morbosa), check out the following online factsheets from West Virginia Extension and Cornell University.

https://extension.wvu.edu/lawn-gardening-pests/plant-disease/tree-fruit-disease/an-introduction-to-black-knot

http://plantclinic.cornell.edu/factsheets/blackknot.pdf

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