As often as I mention the benefits of mulch, I’ve never devoted an entire episode to the topic – until now. Mulch tends to be an unsung hero in the garden. Yet, mulch and compost are the two most powerful tools that an organic gardener has at his or her disposal. I can’t imagine any garden of mine without both. In fact, I often say that mulch is to the soil above ground what compost is to plants below ground. So this podcast and show notes are dedicated to everything you need to know about mulch.
Mulch is sometimes viewed as an aesthetic element, and it’s very true that a layer of mulch can make your landscape beds look sharp. However, beauty is really just an added bonus. The benefits of mulch run much deeper than appearance.
What is Mulch?
Before I dive into all the benefits it offers, let’s be clear on what mulch is – a layer of material used as a covering over exposed soil. There are many materials you can use for this purpose, but from my perspective, the only types you should ever consider using in your garden are those from natural and biodegradable materials.
Wood chips, pine bark, leaves, straw, grass clippings – these are all examples of beneficial mulch because they decompose and provide many more benefits to your soil. Natural materials which don’t bio-degrade or which take decades to break down – like stone or shells – don’t fall into the category of what I consider a beneficial mulch.
Non-natural materials, like rubber or plastic, are just an all-around bad idea. I’ll get more into that in a minute.
The Benefits of Mulch
Why does it matter if a mulch material is natural and biodegradable? First, let’s take a look at the list of all the benefits of mulch:
Looks Great – I’ve already mentioned this one. It’s the sole reason many gardeners use mulch. Mulch can really set off your lawn and the plants in your garden beds.
Retains Moisture – Mulch acts as an insulating barrier against evaporation from heat and wind. Mulch holds moisture in the soil, allowing you to cut down dramatically on the frequency and duration of watering. This encourages established plants to be more drought tolerant, but it can also make all the difference for the success of new shrubs and trees.
Moderates Soil Temperature – Many of a plant’s feeder roots (which do the heavy lifting when it comes to taking up nutrients for the plant) grow in the first few inches underneath the soil surface. Those roots remain healthier when they aren’t exposed to extreme heat or freezing temperatures. A mulch layer acts as a buffer against temperature shift.
Also, heat from the sun can bake exposed soil and create a crust which inhibits water from infiltrating the surface. When temperatures drop, a blanket of snow can provide protection; but without snow or a mulch layer, an overnight frost or sustained freezing temperatures can kill or damage feeder roots.
Buffers the Soil Surface from Runoff and Erosion – If soil crusts over and becomes impermeable to water, rain and irrigation will flow over the surface, carrying with it particles of topsoil and any chemicals or excess fertilizer. Crusted soil is also more susceptible to erosion from wind.
You may already know that soil compaction can create long-term problems, which is why you should always avoid walking in your garden and landscape beds. Well, did you also know that sustained pounding rain or hail can have the same compacting effect?
Mulch can protect your soil against all of these negative elements.
Even pliable exposed soil is at risk of erosion. During a heavy rain event, water will seek the lowest point in the landscape and can carry away some of your topsoil in the process – unless it’s covered with mulch.
Suppresses Weed Germination and Growth – Weeds are inevitable in the garden, but use mulch properly and you will significantly reduce their numbers. With a layer of mulch covering your garden soil, the weed seeds lurking there won’t receive the light they need to germinate, and mulch can block the seeds which blow in on the wind from reaching soil and taking root.
The weeds which do manage to germinate will be smothered by mulch and less likely to become established.
For all these reasons, mulch is one of the single best things you can do to manage weeds in your garden. I can always spot an area of the GardenFarm where the mulch layer needs replenishment – it’s wherever the weeds are coming up.
Suppresses Soil-borne Diseases – There are many plant diseases which live in the soil. While some diseases infect a plant through the root system, most must make contact with foliage. Splashing water from rain or when you irrigate the bed can carry soil-borne pathogens up from the soil onto the lower foliage of your plants. Disease pathogens making contact with the foliage can infect the plant.
Mulch acts like a shield to prevent the disease movement through the water, so it significantly reduces your risk of plant diseases.
Adds Nutrients and Organic Materials – Healthy soil should be made up of around 30% organic matter (by volume). Native soils – clay and sand, for example – are often lacking in organic material. Compost is a great source of organic material to improve soil health, but so is mulch. Over time, natural mulch material breaks down. It feeds the soil food web which, in turn, feeds your plants. Organic matter also improves soil drainage over time.
This is a significant benefit of mulch, and one that non-natural mulches or non-biodegradable mulches don’t offer. This is why I only recommend natural and biodegradable mulch materials.
Reduces Landfill Waste – Many mulches are materials which would otherwise end up in the landfill. Wood products, leaves, pine bark – these are all too often considered trash. Instead, think of them as nature’s gifts to gardeners.
By using wood mulch products, we’re really cutting down on landfill waste. In fact, 60% of all material going into a landfill can – and should – be composted, used as mulch or recycled.
What makes a good mulch?
Some of the greatest benefits of mulch come as a result of decomposition. So, it only makes sense that a good mulch is one that will break down over time and which won’t introduce any harmful elements in the process. The material should also come from a sustainable source.
The best mulch for your application depends on your region and what you are mulching.
The smaller the materials are, the faster they decompose. That’s true in a compost heap and in a layer of mulch.
In my garden beds – especially my raised vegetable beds – I want a material which will break down quickly. Shredded leaves and finely ground bark will protect my soil through the growing season; but by the time the season is waning and it’s time to amend with compost, most of those small mulch fragments have broken down into a soil-like material. So, I can amend the bed with compost without having to pull back the mulch layer.
In walking paths, I prefer to use larger hardwood mulch. I’m not looking for significant soil improvement in these areas – instead, my goal is to protect against erosion and weed intrusion. Hardwood is slower to break down than softer woods, like pine, and the larger pieces will also need to be replenished less often.
When you purchase mulch, know what you are buying. It’s not uncommon for mulch to be made out of contaminated wood products. Dyed mulch is a great example. It’s not the dye that is the cause for concern. These products typically use food grade coloring. The issue is that those colored mulches are often made of chipped shipping pallets.
Many shipping pallets are treated with a toxic insecticide, methyl bromide, to kill insects which might be hitching a ride into the U.S. from other countries. Pallets which reach the end of their use are ground up, dyed to look nice, and sold on the market as mulch. You don’t want methyl bromide around your plants – especially in and around edibles. So although they may look pretty, avoid dyed mulches.
I also recommend against cypress mulch products. These tend to be popular because cypress is long-lasting. More often than not, it’s not sustainably harvested.
These products usually come from wild cypress forests. Too often, these trees are cut down specifically for use as mulch, and it takes decades for trees to mature. Louisiana and Florida cypress forests are in serious states of decline, yet they continue to be harvested as a mulch source. There are just so many more environmentally-responsible options to choose from.
If you need more than 27 bags or so to mulch your landscape, consider buying in bulk. It will be less expensive, and many bulk mulch suppliers will deliver to your property. Another benefit to buying in bulk is that the material tends to be locally sourced, so it has a lower carbon footprint.
If you do buy in bulk, a little homework on the source is still a good idea. Ask the supplier where they get their material.
I was once all set to buy from a local supplier when a casual conversation revealed that their heaps of wood mulch came not only from area tree service companies (a good resource) but also from area work crews demolishing old play sets, decks and other wood structures. It was the thought of all that construction material that stopped my purchase cold.
Those structures are typically built with treated wood, and one of the primary chemicals in older treated wood is arsenic. The chemicals are then released through all the exposed areas of the wood chips. That’s just not something I was comfortable spreading around the landscape of my home. I found a different supplier.
Speaking of ground up building materials, dyed mulch products are also known to include demolition byproducts and treated wood. The vegetable-based dyes are added to make the wood from all those various sources look consistent, but there’s no way for you to tell what chemicals may be lurking in those beautifully-colored wood chips.
Now that we’ve explored what constitutes a good mulch, let’s take a look at the benefits and potential drawbacks of the most commonly available types.
Leaves – Let’s start with my favorite. Sometimes, I want to shout about my love of leaves from the rooftops. I collect them every fall – not only from my yard, but from friends and neighbors too. I even offer to pick up bagged leaves destined for the landfill with posts on local social media sites. Let’s face it, in fall I become leaf-obsessed.
Not only do I feel great about reducing landfill waste, but I’m gathering one of nature’s greatest gifts for my soil. Once I get them home, I spread them out and run over them with a mulching mower. Mine has a bagger attachment, so I can dump the ground leaves directly into a garden bed or a storage container. Before I had a bagger attachment, I would rake up and bag the ground leaves the old fashioned way. It took a little longer, but it’s great exercise and so worthwhile.
I collect and grind so many leaves, that I created a wire “corral” to contain them over the winter.
Shredded leaves are great to work with. They look nice, and they hit my decomposition “sweet spot,” protecting my garden soil all through the growing season and reaching a soil-like consistency just in time for my fall compost amendment. I add compost directly over the decomposing leaves, which are rich in calcium and micronutrients, to create amazing soil that keeps getting better year after year.
Wood Mulch – There are plenty of options for wood mulch, some of which I’ve already touched on. Chip products, finely-ground options, hardwood, pine and more – how do you know which is best? Well, aside from the products and risk I’ve warned about, what you should use comes down to personal preference.
For example, how long would you prefer the mulch to last? As I mentioned, I use longer-lasting hardwood chip mulch in my GardenFarm paths but softer, smaller wood products anywhere I want something which will break down quickly to improve the soil.
I use arborist wood chips regularly in my landscape beds. I love that you can find this material for free from local tree services. Try contacting your area services to let them know that you are interested in taking their load of chipped wood. Oftentimes, you are doing the company a favor, because they won’t have to haul the load as far or pay a landfill fee.
GetChipDrop.com is a good online resource for connecting gardeners and companies with chips to unload.
Some gardeners worry about using arborist wood chips and ask if chips which come from a diseased tree will spread disease in their landscape. Most studies indicate that diseased mulch does not spread disease to surrounding plants, trees and shrubs; so the risk would seem to be low.
Here’s something else you don’t need to worry about: That pine bark will lower the pH of your soil – or that wood chips will rob your soil of nitrogen. Those are both myths.
When pine or other wood products are used as a mulch above the soil, there isn’t enough contact with the soil to affect pH or cause nitrogen depletion. It’s only when the wood material is mixed into the soil – when soil makes contact with all the small surface areas – that the pH or nutrient levels can be negatively affected.
Most of the wood chips break down within a year or two and begin to look like a blended soil mix. At that point, the material can be incorporated into your soil, if need be, without any negative impact.
Be cautious when it comes to any material from black walnut trees. There is an allelopathic chemical, known as juglone, in black walnut trees which can inhibit the growth of sensitive plants in your landscape. This chemical is most present in the tree roots, but it is found in all parts of black walnut – including the leaves.
If you have leaves or chips from black walnut or think there may be black walnut in material you’ve received, allow the material to age before you use it as mulch. Juglone will begin to dissipate after about six months, but the longer you can allow it to age, the better.
Hay or Straw – These materials are readily available from home improvement stores, garden centers and farm suppliers. Both options are easy to spread, and they look nice in the landscape. There are key differences between these two materials.
Straw comes from grain crops and is just the stalk of the crop. For that reason, straw typically doesn’t contain seed heads. It’s easier to work with but slower to break down than hay.
Hay comes from grass crops, like fescue and bluegrass, so it will likely contain seed heads. The bigger concern, however, is the possibility that the grass crop was treated with a persistent herbicide (used to kill broadleaf weeds but just as effectively lethal on many plants), which will take years to dissipate and can ruin your soil in the process.
My garden was hard hit by horse manure from hay which contained persistent herbicide. You can read all about this killer compost experience and, hopefully, learn from my mistake.
When I have used hay as mulch, I made sure to get it from a reliable source, and I pulled it up at the end of the season, so any persistent herbicide lurking in the material wouldn’t have the opportunity to work its way into the soil.
Pine Straw – I really love the look of pine straw (pine needles). This is a common mulch here in the southeastern U.S. Pine straw is slow to break down, and although it’s acidic, it won’t have much impact on your soil pH. This material is more difficult to spread, but once it’s down, it holds together nicely and can even be shaped.
I recommend wearing long sleeves and gloves to protect your forearms from the scrapes and sticky residue that’s a part of the bargain when it comes to applying pine straw.
Grass Clippings – I love free materials, and grass clippings are no exception. If you don’t mulch when you mow (also known as grasscycling), dry your clippings to use as mulch. If you can spread them on a driveway or other hard surface and place them between two layers of clear plastic, it will only take a few hours on a hot sunny day before they dry to a lightweight, crispy material which is easy to work with.
I recommend against mulching with fresh, wet clippings; because they can generate heat and mold which can create problems.
Plant Hulls – Materials like rice, peanut or buckwheat hulls are becoming more commonly available. However, since these materials come from a food crop, there is a greater likelihood the origin was treated with a pesticide or herbicide, which can remain in the hulls and transfer to your garden.
Cocoa Mulch – Cocoa shell and cocoa bean mulches may also have been treated with a chemical which will transfer to your garden. These materials also present another risk – they contain Theobromine and caffeine which are toxic to dogs. Even if you don’t own a dog, consider other pets in your neighborhood. Dogs are drawn to the sweet smell of cocoa materials, but they can become sick or die from eating them.
Shredded Paper – It’s biodegradable and plentiful, so feel free to use shredded paper as mulch. Inks these days are typically soy-based and, so, not a health concern.
It’s not very aesthetic and will bind together once it’s wet, but shredded paper could be a great option for mulching plants in a hoop house or a low tunnel. I prefer to add it to my compost heap, but if you would like to use it as a mulch, go for it. I’m a fan of whatever keeps this material out of the landfill.
Living Mulch – What is a living mulch?. It could be a ground-hugging plant, like creeping thyme or phlox, or a cover crop, like vetch or clover. Living mulch – particularly cover crops – offer a host of benefits to soil health. These options protect the soil surface, and they can outcompete and shade out weeds too.
Dyed Mulch – By now, you know the risks presented by dyed mulch. This just isn’t a good option in my opinion.
Plastic Sheeting – While there are some agricultural applications where plastic sheet material as mulch makes sense, it’s just not a good idea in the home garden. It offers no benefits to your soil. It can inhibit water infiltration and increase soil temperatures. In fact, there are more potential drawbacks to using plastic sheeting than there are benefits. Plus – may I add that we should all be looking for ways to use less plastic in our lives, not more?
Plastic or Rubber – Chips of these industrial materials aren’t uncommon as a mulch option. There was a time when I said that rubber mulch might be appropriate for pathways or playgrounds, but not anymore. These petroleum-based products just don’t belong anywhere they make contact with our soil. They degrade in the heat of the sun over time and leach chemicals into the surrounding area in the process.
These materials aren’t good for birds or other beneficial creatures or for any aspect of our environment. You could argue that these mulches are worthwhile because they are a recycling byproduct. I’m all for recycling, however, I just can’t feel good about recommending these products in our garden beds.
Once you’ve decided on the type of mulch, how much do you need and how should it be used? There are a lot of misconceptions out there – even in the landscaping industry.
For starters, don’t spread mulch too thinly, or it won’t be sufficient to provide the benefits you would expect. A thin layer won’t be as effective at suppressing weeds or buffering the soil against heat, erosion, etc.
That said, spread mulch too thickly, and you create new problems. When mulch is too thick it can actually prevent water from reaching the soil. It can also smother your plants.
The sweet spot is a 2” to 4” layer. Aim to spread and maintain your mulch at a depth of no more than 4” and no less than 2” and you will see a notable difference in the health of your plants and soil. Plus, you’ll see a drop in supplemental water needs.
This leads me to one of my ultimate gardening pet peeves – mulch volcanoes. No doubt you’ve seen these somewhere. In spite of how much harm they can cause, some “professional” landscape companies across North America still implement them as part of their service.
A mulch volcano is the result of an enormous heap of mulch being piled around the trunk of a tree – oftentimes, over a foot deep – and tapered out to the drip line or lawn border.
Not only is this an expensive waste of a resource, but it’s also harmful to the health of the tree.
Mulch under plants and trees is a very good thing, but it shouldn’t make contact with the trunk of trees or shrubs. Why? Mulch right up against a trunk creates a pathway for pests and diseases and can promote rot.
Always keep the mulch layer pulled back about 2” from the trunk base. Remember the 2-inch rule: 2” deep and 2” away. You can go up to 4” in either case, but stick with that range for both depth and distance.
A mulch volcano also covers the root flare of the tree. Trees are healthiest when their root flares are exposed, so for the sake of the longevity of your expensive trees, never cover the root flare with mulch or soil.
Many trees have been killed thanks to mulch volcanoes. So, just don’t do it, and don’t let a landscape company do this on your property either.
Mulch beneath trees and shrubs should be spread at least as far as the drip line (the furthest outer reach of the foliage). The root systems of these plants spread out to the drip line too (oftentimes beyond), so that’s a practical guide for laying mulch to protect roots.
I often hear questions when it comes to mulching over newly-sown areas. Seeds need to be able to germinate and reach the light without additional impediment, so this is one time when a 2-4” layer isn’t a good idea. Most seeds germinate so quickly that it’s just not necessary to mulch for the brief period between sowing and when the plants reach a few inches in height.
If you do feel inclined to mulch over seeded areas, a very light sprinkling of hay, straw or (especially) dried grass clippings is just the ticket. Just don’t use so much that the germinating seeds might be encumbered by the weight or depth of the material, once they breach the soil.
When I sow seeds, I tend to pull the mulch layer back and leave the row exposed once I’ve sown. As the plants mature to a height of a few inches, I gently push the mulch back around the area for the rest of the season.
I’m also asked how to amend the soil with compost or fertilizer when mulch is in place. If you want to make it easy on yourself, add compost or fertilizer directly over the mulch layer. Over time, water from irrigation and rain will wash the nutrients through the mulch and down to the soil. There is nothing wrong with this approach.
I prefer to pull the mulch layer back and out of the way (unless the mulch is mostly decomposed). Once I’ve applied compost, I push the mulch back into place to protect the soil – and the compost. Direct contact with the soil allows the compost to provide results more quickly to the overall health of the bed.
What about using compost as mulch? There’s nothing wrong with that. However, compost is so valuable and can be more expensive or less readily-available than mulch.
Compost is lightweight enough that it can be lost to wind or rain erosion, and it’s just too precious to me to want to risk losing any.
The best use of compost is to add it as an amendment and, then, to cover and protect it with a layer of another natural mulch material. That way, you’ll get more mileage from the compost – and you’ll also reap the benefits that a layer of natural mulch provides to soil health. They each have their place, and they really do go hand in hand for a successful garden. I can’t imagine ever going without either one.
If you haven’t already listened in to my discussion on mulch, click the Play icon in the green bar under the title at the top of this page. Then, I’d love to hear which type of mulch you prefer and why. Share your thoughts and feedback in the Comments section here.
Links & Resources
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