Mulches are used as a soil covering, for a variety of reasons:
- Water preservation (moisture retention)
- Heat trapping
- Create pathways
- Weed prevention and control
- Protecting roots from fluctuating and extreme temperatures, and…
- To help control soil erosion
Different mulches have different functions and uses. Here we’ll consider the different functions of several mulches:
- Pebble mulch/Gravel
- Rock mulch (medium to large stones)
- Pumice rock mulch
- Straw mulch
- Newspaper and Cardboard
- Grass clippings
- Compost as a mulch
- Landscape fabric
- Wood chips, Shredded bark, and Sawdust
- Cocoa hull mulch
- Pine straw
If you’ve got a bare patch of soil, get some mulch on it. The soil should never be left uncovered. The ground is never left uncovered in nature, and it shouldn’t be left uncovered in a garden or orchard. Why? Read this article here.
For any kind of vegetable garden, we would recommend wood chip mulch far above any other mulch. Just don’t go digging it into your garden. Mulches stay on TOP of the soil, for good reason. (In fact, the definition of a mulch is basically anything that covers the soil.) Well-composted organic matter that is 100% broken down can be dug into your soil, but not mulches. Why? The problem has to do with nitrogen. Nitrogen is necessary for your plants, and it’s also necessary for the organisms that break down the organic matter, whether they’re working on stuff in a pile of compost or mulch that’s sitting on top of your soil. During that breakdown process, the bacteria will sequester nitrogen where they’re working on the organic matter.
So on the surface of your soil, where the mulch is in contact with your soil, there will be almost no nitrogen available to your plants. That’s okay, though, because your plants send their roots several inches into the soil and they’ll find what they need there. But if you’ve tilled the mulch into your soil (and of course we’re talking mulches made out of organic matter here, not rock or pebble mulches) the bacteria and other organisms will sequester nitrogen wherever that mulch is in contact with the soil, even if that’s several inches deep, and your plants’ roots won’t be able to pull any of the nitrogen from the soil, and your plants will become very deficient in nitrogen.
Just remember – if it’s well-composted organic matter, you can work it into your soil. If it’s not, keep it on top for all the amazing benefits mulch brings to the garden.
Okay, now for the mulches. There are lots of types, each with its own functions and uses.
1. Pebble mulch is mostly useful on pathways or driveways. Pebble mulch/ gravel allows water to drain through, which cement and asphalt don’t do. However, the little pebbles/gravel are easily pushed out of their intended area, if they are not confined within a trenched structure. This can cause a big problem over time if you need to mow along the edges of your driveway or pathways. Little pebbles will cause dings and nicks in your mower blade, causing it to become dull much more quickly; and flying pebbels/gravel can cause injury. However, if the gravel is contained (see picture above left), it’s fairly easy to use a wide leaf rake to pull most of the gravel back into its place from time to time. Gravel and pebble mulch also absorb some heat from the sun during the day, and give it off at night, creating a mini micro-climate which can be useful.
2. Rock mulch is sometimes used in perennial flower beds or other perennial plantings. Large rocks absorb even more heat from the sun during the day, creating the potential for larger warm micro-climate areas than small pebbles. It all depends on the size, quantity, and color. Dark stones and rocks will absorb more heat than white or light-colored stones. Larger stones and rocks also cover more area with fewer stones, so if the rock mulch needs to be completely removed, it’s an easier process than with pebble mulch/gravel. A pretty good covering of larger stones will also help prevent soil erosion to some extent.
P.S. Try to get local rock, if at all possible. Moving rocks takes a lot of energy.
3. Pumice rock is a very lightweight, porous rock that comes from volcanic eruptions. It is often used as a mulch in flower beds and other perennial garden beds. It has the ability to trap and retain moisture, because it’s so porous, which none of the other rock mulches are able to do. However, if it is sitting entirely on the top of the soil level, it takes a significantly longer amount of time for water to penetrate down into the soil (which means, for example, that you’ll have to set your sprinkler to water for an hour instead of 30 minutes when you first use pumice rock as a mulch). Once the pumice rock has settled into the soil and ‘combined’ with the soil somewhat, the rate of water penetration takes less time (water gets down into the soil much more quickly), and the pumice maintains the advantage of retaining water. Since pumice is quite porous, it will absorb some heat from the sun, but not as much as the other rock mulches. (The red rock in the above picture is pumice. Click on the picture for a closer view.)
4. Straw mulch happens to be one of my favorites… in memory at least, because I remember spreading straw mulch on pathways in our family garden when I was growing up. However, in my opinion, there are several problems with straw mulch that don’t make it a very desirable option. Straw has a high carbon to nitrogen ratio, and it doesn’t break down very quickly (see this article to understand more about the carbon to nitrogen ration, and why it matters in composting/mulching). That wouldn’t matter so much, because it’s being used on top of the soil, rather than in the soil, except that if you spread straw as a mulch in your garden in the summer, it will make the perfect home for all sorts of mice, voles, and other critters for at least a year or two, right through the winter and into your next growing season. Also, straw is light colored and unless you spread a very thick layer, it doesn’t block out the light very well, and the weed seed will have plenty of light to germinate and grow right through it. Also, straw can sometimes contain weed seed, which may only add a problem to your garden. All in all, I don’t recommend using straw mulch.
If you already have straw mulch in your garden and especially if you’re experiencing problems with rodents, it might be a good idea to get a cat or two. Rodents can cause big problems, and hugely decrease yields in your garden. It’s worth it to get a cat and get on top of the problem as soon as you can, because they’ll breed, well, like rodents!
5. Newspapers and cardboard can be very useful in the garden. They’re very similar in how they’re used, and their function is mainly to suppress weeds and eventually decompose and add a little bit of organic matter. Laying down a mat of newspaper or cardboard is guaranteed to block out all light from the soil beneath. However, newspaper is more effective because it forms to the contours of the land, making it a more effective weed control. Once you’ve laid down a cover of newspaper, then you just make a hole in it (cut an X with a knife, or just punch right through it with your hand or a trowel), and plant your plants exactly where you want them. Or spread the newspaper right around established plants. Newspaper is a bit unsightly, though, so I would recommend laying another type of mulch, perhaps compost or wood chips, on top of them. Besides, that’s added organic matter, and that is definitely a good thing.
One interesting thing that I’ve observed has to do with worms. After spreading newspaper/cardboard on the ground and covering it with a layer of compost (in my veg garden), whether I’ve done it in the spring, summer, or fall… the next time I’ve cultivated the ground, when I shift about the layers of partly decomposed paper stuff, there always seems to be huge gatherings of worms under and around them. I don’t know the reason for it, but I’ve come up with a few theories:
- Perhaps the worms like sliding around right underneath the newspaper because it’s like having soil above them, as they usually do when they dig their tunnels, but it’s not quite so much work to wriggle around right underneath a layer of newspaper.
- It gets pretty moist under newspaper/cardboard, especially after it rains, and as usual, the worms try to come up for air as their tunnels are flooded, but they make it as far as the newspaper/cardboard, but they’re not able to get through that layer (which doesn’t seem to matter too much, because the worms are always alive beneath the newspaper/cardboard, that I’ve seen). This might actually be an advantage, because when worms come up for air after a big rainstorm, it makes them easy pickings for birds. But if they stay under the layer of newspaper/cardboard, they’re safe from birds, while still avoiding being drowned.
- Maybe it’s simply because it’s a little warmer there in the winter and cooler in the summer… who knows?
One issue I want you to be aware of, in regards to newspapers, is that colored ink in newspaper can be hazardous. The jury is still out on this one, as far as public opinion goes, and it’s a much debated topic in gardening forums that I’m part of. So I’ll just give you my two cents worth, and you can decide for yourself.
For the colored inks, however, a variety of other elements/metals are used (think sulfur, for example… it creates a great yellow color). For the colored inks, there is such a huge variety of color needed and printed every day, that you can never be sure what they’ve used. Last I read, about a year and a half ago, lead was still used in some inks in those shiny weekly inserts produced by some stores (I won’t name any here, because I don’t want to get sued, if it happens to have changed since 18 months ago).
Given that information, I would recommend only using black-inked, gray-papered newspaper, and definitely avoid the shiny, white-paged, heavily colored newspapers.
Now, for the other side of cardboard: Cardboard generally consists of one or two layers of a ‘sandwich’ of paper: bottom is flat, middle is wavy, top is flat… that equals one layer. To hold the layers together, usually a ‘glue’ of cornstarch is used. At the open corner, however, where two sides are glued together to make the box shape complete, a different glue is used. I don’t know what that glue is made of… there seem to be no sources online that will give me the details. And it’s likely there are several different types of glue used, depending on who makes the box and what it’s intended use is. This being the case, I would recommend that you cut out that corner of ‘mystery glue’, and use the rest of the box. BUT, there’s one more thing to be aware of…
Some boxes are treated with water and/or flame retardant chemicals. How do you know if the cardboard you’ve got has been treated or not? Well, you won’t always know. Sometimes a glossy sheen might tell you, but that could also just be wax (which you’ll often see on boxes that ship produce). Sometimes there will be no indication whatsoever that it’s been treated. So… I know it sounds a little tough, but it’s better to be safe than sorry, and I don’t use cardboard in my garden unless I’m sure it hasn’t been treated or contaminated in any way (which, sadly, isn’t often). Remember, if you put it on or in your soil, it’s easily eaten by bacteria, and those chemicals move on up the food chain to worms and birds. Also, your plants could easily take up some of those chemicals, and you eat your plants! When I think of it like that, I’d rather pass on the glue and chemicals.
6. Grass clippings can be a great addition to your garden, and depending on the stage they’re in, they can be added on top of your soil as a mulch, or worked into your soil for some added organic matter. There are two stages of grass clippings… when they’re still green, and when they’ve turned brown. If they’re still green, as you see in the picture to the left, grass clippings are full of nitrogen (that’s what gives them the green color) and they’ll break down really well in the soil, releasing all those nutrients, including nitrogen, into the soil. But if you’ve left them out for a bit and they’ve turned brown (it doesn’t take long), all that nitrogen has been lost (see this article to understand more about the carbon to nitrogen ration, and why it matters in composting/mulching). Nitrogen is quite volatile and escape back to the atmosphere very quickly. That’s one reason why grass clippings turn brown quite quickly after being cut or pulled from the soil. Once they’ve turned brown, just use the grass clippings on top, as a mulch, or add them to a compost pile and make sure they’re well-composted before they’re added to garden soil.
One other thing to keep in mind is whether or not the grass has been treated with any herbicides, fungicides, pesticides, etc. If any chemicals have been used on the lawn where the grass clippings came from, it’s best to chuck them. All the more reason to avoid using chemicals on your lawn… so you don’t have to throw away all that awesome organic matter that could otherwise be benefiting your garden!
7. Compost is great to use as a mulch. As long as it’s well-decomposed, it can go on top of or in your soil without any concern whatsoever. It is best, if you’re making compost yourself, to make sure the pile is turned regularly so it gets hot enough to kill any weed seed that may have made it into the compost pile.
Compost also tends to be slightly acidic, so it’s an especially great addition to a garden with alkaline soil (for more on that topic, read this article to understand Soil pH and the effect it has on nutrient uptake). Other than that, I have no recommendations, except to make as much compost as you can, and enjoy using it on your garden!
8. Landscape fabric is a black fabric, generally made of woven polypropylene, which means it’s a form of plastic. Being made into a woven, fabric-like material makes it breathable. It’s supposed to let the water in but keep sunlight out. And it does generally work, as far as I’ve seen. But it’s still a plastic material.
It does have one great benefit, especially for gardeners who live further north and typically have a shorter growing season. Melons, particularly watermelons, need plenty of heat and sunlight to grow a healthy, strong plant, and to produce delicious, large, sweet watermelons. Using a black, landscape fabric catches the heat of the sun, warming up the soil beneath it sooner than usual, and keeping the soil a little warmer at night, making it possible to plant out watermelons and other heat-loving crops a little sooner than you otherwise would be able to.
There are two problems with landscape fabric. First, although it’s supposed to let water in and keep light out, I have found that it takes a bit of doing to get the water to actually go through the cloth, especially when it’s brand new. That means instead of setting your sprinklers to water for 30 minutes, you might need to water for 60 minutes, to make sure enough water is making it through the landscape fabric.
It becomes more penetrable as it grows older, though… both from above and below. That means that while the water can get in more easily as the fabric ages, the weeds can more easily catch a bit of sunlight, germinate, and push its way up through the landscape fabric, which I’ve seen happen on countless occasions. When that happens, if you want to maintain a weed-free garden, you’ll have to replace the landscape fabric or use another method. This brings us to our next problem.
If you’ve laid down a huge landscape fabric, made some cuts in it, and planted perennials in the openings, when it comes time to replace the fabric, you might have a bit of a job on your hands. Depending how large the area is, if you don’t want to dig up all your plants, you’ll have to rip holes in the fabric big enough to pull it up and away from your plant. And then imagine trying to get a new one settled back over your plants. Time to find a different kind of weed-suppresser… how about newspapers covered with wood chip mulch?
The last thing I’ll say about it is that the only reason I would personally use landscape fabric is to grow melons better and earlier. But with a bit of thought, there are alternatives that can be used. Like using big rocks (especially if they’re dark) to create a warmer microclimate area for a melon patch. Or building raised beds sided by dark-colored bricks. That would be a beautiful display anyway, with the melon vines falling over the sides! So, I’ll leave the ultimate opinion on landscape fabric to you, but I think I’ll use something else.
9. Wood chips and shredded bark have similar functions, so I’m grouping them together, but in reality, shredded bark doesn’t even compare to wood chips. I don’t know why. You’d think they were similar, but wood chips out-perform shredded bark by a long shot. They’re a high carbon-to-nitrogen material, so they’re great if you use them on top of the soil as a mulch, but DON’T dig them into your soil. They cover and insulate plants’ roots quite well, which helps protect against fluctuating and extreme temperatures. They hold moisture fairly well, which will help keep the ground around your plants moist in between waterings. And they’ll slowly break down, adding organic matter to the soil. Wood chips seem to be a “miracle mulch” in my opinion.
One thing to consider in regards to wood chip mulches has to do with the subject of allelopathy. In the dictionary, allelopathy is described as the phenomenon when the growth of one plant is suppressed by chemicals produced by another plant growing nearby. The negative side of allelopathy always seems to get the focus, but allelopathy isn’t always a negative thing. It can also refer to a positive situation, where one plant produces chemicals that enhances the health and growth of other plants.
As far as wood mulch/shavings go, however, we do need to consider the negative effects of allelopathy. One of the best examples of this is the black walnut. Everyone seems to know that black walnut isn’t a very good tree to have growing near other plants. The reason is that the Black Walnut tree exudes a compound called juglone from its roots, nuts, and leaves, which suppresses nearby plant growth. You probably won’t find any black walnut mulch being sold anywhere, but just in case you have a big black walnut tree on your property that you’re going to chop down and have it chopped up into a mulch… well, now you know that it wouldn’t be the best idea to use that mulch in your garden. Below is a list of common allopathic trees/bushes in the United States.
Common allelopathic trees/bushes
- Sage brush
- Tree of Heaven (sometimes called ‘ghetto palm’ or ‘stink tree’)
- Several pine, juniper, and cedar trees
You’ll notice that pine, juniper, and cedar are on the list, even though wood chip mulches are often made from these woods. Their effect isn’t nearly as strong as the black walnut, and in truth, very little would be contained in or exuded by the wood chips.
10. The use of Cocoa hull mulch is a much-debated topic. There are some people who love it, some who are afraid of it, and some who have never used it. Cocoa hull mulch (or just cocoa mulch) is just a woody type mulch, made from the shell of the cocoa bean. It is light weight, and a bit hard, like a nut shell. It has it’s ebbs and flows of popularity and rejection, mostly because now and then people become wary and afraid of it because of its possibility of toxicity to animals.
Everyone knows that chocolate is toxic to dogs and cats, because of its content of theobromine and caffeine. Cocoa hull mulch contains both compounds as well. However, the content of theobromine in cocoa hulls may be nearly non-existent, or it may be nearly twice as much as chocolate. It depends on how it’s treated. Sometimes the cocoa hull mulch is heat-treated to remove as much of the oil or fats within the cocoa hull as possible, which will remove some of the scent and theobromine (since the scent compounds and theobrimines are associated with the oil/fat), decreasing the sweet smell that attracts animals as well as the harmful theobrimines. Sometimes the mulch isn’t treated at all, and retains both its scent and its theobromine content. So the amount of theobromine in cocoa hulls is hugely varied. Considering that cocoa hull mulch can contain from 300-1200 mg/ounce to begin with (that’s a huge range – a four fold difference!), and also considering that some cocoa hull mulch is treated and some isn’t, there’s a lot of variability in the possible dangers of cocoa mulch.
Considering all that, if you have animals, it may be taking a bit of a gamble on your dog or cat getting sick to use cocoa mulch on your property. It’s not often that an animal is killed from eating cocoa mulch, but it does happen. And you don’t want your neighbor’s dog to die, either. That could lead to all sorts of issues. So… just be sure to make a wise decision, depending on your circumstances, on whether or not you use cocoa hull mulch.
11. Pine straw is a great possibility for a mulch in the right area. Since pine needles are thin and wispy, like straw, you’ll need a fairly thick layer of them to be sure to suppress weeds. But they last for forever, and if you can find a free source, it’s a very economical option as well.
Here are a couple of things to consider. First, as a benefit, pine needles are quite acidic, which can be a really good thing for alkaline soils. Optimally, soil should be slightly acidic. Using pine needles as a mulch will slowly affect the pH of the soil, making it more alkaline. But again, don’t dig the pine needles into your soil… unless they’ve been thoroughly composted first. Just use them on top.
Second, pine needles may also have an allopathic effect. Fresh pine needles contain a great deal of allelopathic compounds, but the amount decreases as the pine needles age and decompose. Different allopathic compounds work in different ways to affect the growth of other plants. Some compounds prevent other plants from being able to respirate, some prevent the germination of seeds, etc. The compounds in pine affect seed germination, so fresh pine needles spread around existing, established plants, in a perennial bed with bushes or other perennial plants, would be an excellent way to suppress the germination of weed seeds. As long as it’s not suppressing any seeds you want to germinate, pine needles would be very useful around perennial plants.
And since the allelopathic effect of the chemicals decreases as the pine needles age and decompose, a compost of pine needles would probably be a great option for a garden because of its acidifying effect on the soil. If you want to be safe, you could work in the composted pine needles in areas where you won’t be directly planting seeds in the ground, but where you’d be setting out transplants instead.
Well, I hope you enjoyed that thorough analysis in the different mulches, and I certainly hope you find it useful! Please let us know of any comments or questions you may have. We always love hearing from our readers!
Photo credits: cardboard picture from http://www.ikatbag.com; grass clippings from http://www.clearchoicescleanwater.org; deb dramby;