Compost vs. Mulch: What’s the Difference?

by Matt Gibson

Compost is organic matter, everything from leaves to kitchen scraps, that has been decomposed. Mulch is a layer of organic materials that gardeners place over the top layer of their soil as a protective cover. There are many different types of mulch, and compost is in fact, one of the many types of mulch that you can use. Not only does it provide a protective layer and all of the other benefits of any non-organic mulch, but compost also constantly leeches out nutrients into the soil (namely nitrogen and carbon) during irrigation or rain.

In this article we will go into depth about compost and mulch and clear up any confusion you might have. We will define the two useful mediums and clearly explain the difference between them. We will first focus on compost, highlighting what it is, how to make it, its many uses, and when it is best to use it as mulch. Then, we will discuss all of the different types of mulch, and guide you as to when it is best to use each type of mulch, including compost.

What Is Compost?

Compost is organic matter that has been decomposed. To make a compost pile, simply make a pile out of lawn clippings, kitchen scraps, and garden refuse and let it decompose for about a year, stirring the pile occasionally to help quicken the process. Stirring, or turning the compost is one of several ways to speed up the composting process.

One way to keep your compost pile decomposing and breaking down at a high level, keep an eye on the mixture of the formula in the pile. The ideal ratio is two parts brown compost to one part green compost. Brown compost is just what the name implies, any compost material that is brown, such as leaves, dried grass, wood, paper, and straw. Similarly, green compost is any compost material that is green, including fresh grass clippings, freshly picked weeds, plant clippings from the garden and most kitchen scraps.

Aside from keeping the ratio of green to brown compost as close to two parts brown to one part green as possible and occasionally turning the pile, you can also water it from time to time to keep it moist, but never soggy. All of these methods will help to speed up the decomposition process, and keep your compost pile healthy and active. After about a year, your pile should be ready to use in the garden.

Compost that has been fully broken down and decomposed is called humus. Humus is usually dark brown-colored with a mild, pleasant smell. If your compost stinks then it’s not yet finished decomposing. Once it has finished breaking down, the odor should be mild and slightly sweet-smelling.

Uses For Compost

Humus, or finished compost, has a variety of uses. It can be dug into, mixed, and added into garden beds while the soil is being turned over. When planting new plants, gardeners add compost to the hole. Don’t use compost on its own to fill the hole though, as plants tend to get too comfortable when their roots are blanketed in pure compost and don’t spread out into the regular soil, which can stunt their growth. When filling a hole around a new plant, use a mix of 50 percent compost and 50 percent regular soil.

Compost can also be used to side-dress plants that are already in the ground. Side-dress established plants by spreading compost next to them and working it into the soil with a small garden fork. You can also add compost into the mixture you use for starting new plants or seeds. 

If you have a large composting system and have more than enough for amending your soil, you can also use compost for mulching your garden landscape. Using compost for mulch has a lot of benefits. Repurposing yard and kitchen trash is free, so instead of buying bags of wood chips, you can use compost around your garden plants for free.

Using Compost As Mulch

Using compost for garden mulch gives you all of the benefits of standard, non-organic mulches with the added benefit of nutrients being constantly leached into the soil beneath it. As rain water, or irrigation water runs through the layer of compost, micro-doses of nitrogen and carbon are washed downward, improving the soil in the process.

As with other mulch mediums, a thicker layer is a better layer to help shade out sunlight from potential weeds. A two to four inch layer of compost should be added over your soil around all of your perennials, extending out a foot around each plant. As the compost will slowly work its way into the soil below during the growing season, you will want to add additional layers every month or so during the summer and fall.

Using compost as mulch will definitely enrich your soil but keep in mind that whenever any kind of mulch is dry, it’s not a great environment for plant roots. Another way to use compost as mulch is to spread a thin layer around plants and to top it with another mulch, such as chopped leaves, or wood chips. This will keep compost moist and biologically active, providing the best possible benefit for your garden’s plants. 

Benefits of Using Compost

Using compost to fertilize your garden provides more nutrients near the roots of your plants, This makes for easier absorption of nutrients and in turn, more vigorous growth and stronger plants. The nutrients in compost are densely packed, and are released into the soil over a long period of time. Many months, sometimes even years down the road, your plants will still be able to thrive in the nutrient enriched soil.

Regular household items that would otherwise be discarded as trash, can be used as compost and repurposed to create a wonderful soil booster. Items from the kitchen such as used paper towels, egg shells, fruit peels, tea bags, and more can be tossed into the compost heap. Other household items like cotton balls, human and animal hair, and pencil shavings can also be thrown into the compost pile to break down. Repurposing used household items cuts down on your carbon footprint and saves you some wear and tear on your back by reducing the amount of time you spend taking out trash and cutting down the weight of the trash itself.

When To Use Compost As Mulch

Should compost be used as mulch year round? Having their roots covered throughout the winter months won’t hurt your plants. In fact, it may help to insulate and protect young plants from the most severe times of ice and snow. Once springtime comes, remove the compost from around the plants in order to allow more sunlight to reach the soil to help warm and thaw the ground. 

What Is Mulch?

Mulch is simply the layer of organic materials placed on top of the soil as a protective covering. Mulch works to suppress weeds, keep valuable moisture locked in, insulate the soil during cold weather periods, and reduce erosion from wind and rain. Mulch also helps improve soil quality by contributing nutrients to the soil by gradually breaking down over time.

Throughout the gardening season, mulch should be applied heavily to garden beds, using grass clippings, straw, and shredded leaves. Though a lawn mower can handle the task, we recommend an electric shredder to make the job quicker and easier.

On perennial beds, cedar chips are a great mulch choice. Aside from their pleasant smell and appearance, cedar chips break down slowly, making it a great mulch choice to lay down and keep in place for several seasons. Cedar also discourages several types of garden pest insects.

What Is The Difference Between Compost And Mulch?

The main difference between compost and mulch is the location in which they benefit the soil. Compost goes to work within the soil, while mulch goes to work on top of the soil, sort of like a blanket.

Compost is generally tilled into the soil as a nutrient enhancing additive. Plants benefit from the nutrient-rich food source, and soil-dwelling organisms feed off of it as well. Earthworms, for example, feast on the nutrient-heavy compost while boring holes into the soil, keeping it loose and less compacted, which makes it easier for roots to spread out extend into the soil.

Mulch is applied directly on top of the soil, keeping moisture locked in to the soil below. Straw, leaves, and wood chips all serve this purpose, as well as hindering the growth of weeds from popping up in between plants in your garden bed. Like compost, mulch does leach some nutrients into the soil below but not at the same high levels that compost can produce. Mulch is subject to decay and needs to be reapplied periodically depending on the material. 

Different Types of Mulch

There are two types of mulch, organic and inorganic. Organic mulches include formally-living materials like: chopped leaves, straw, paper, wood chips, compost, shredded bark, grass clippings, sawdust, and pine needles. Inorganic mulches include black plastic and landscape fabrics, such as geotextiles.

Both types fight against weeds but organic mulch will also improve the soil as it decomposes, whereas inorganic soil doesn’t break down. Inorganic mulches have their advantages sometimes however, like black plastic, which absorbs heat during the day and stays warm and cozy at night, which is perfect for cherry tomatoes and eggplants, both of which thrive with a bit of nighttime warmth. Each type of mulch has its own function.

Wood Chips and Shredded Leaves – There is no shortage of fallen leaves on any property and they can be shredded with a lawn mower or electric trimmer. Some lawn mowers even bag up the leaves and grass clippings for you. Wood chips and shredded leaf mulch look great in garden pathways, shrub borders, and flower beds. Wood chips are not a great idea for spreading in your vegetable garden or annual flower beds though, as they need to be dug up and tilled each year, and wood chips can get in the way.

Grass Clippings – The nitrogen-rich grass clippings are a great choice of mulch for vegetable gardens, as veggies need lots of nitrogen to thrive. However, you don’t want to strip away too much of your grass clippings from the lawn itself, as they serve as a wonderful natural fertilizer to help keep your lawn healthy, fresh, and green.

Compost – If you have enough compost to spare, it makes a lovely nutrient-rich mulch, but you may want to use a light layer of compost and cover it with chopped leaves, or hay to help insulate it, keeping it moist and biologically active, as dried out compost will not provide a great environment for plant roots.

Straw and Hay – Straw and hay mulches look good and come with many of the benefits of other mulches, just make sure that any hay that you might use is weed and seed free or you’ll just be making a mess. Also, don’t pile up straw or hay mulch past the stems of vegetable plants or the trunks of fruit trees, or you may be inviting rodents and slugs to your garden unknowingly.

Plastic Mulch – Using a black plastic film in place of mulch in a vegetable garden can work wonders. Plastic sheeting will increase the heat of the soil and help keep the surface warm and dry, which will help fight rotting issues. Plastic sheeting will also prevent weeds and retain moisture. Do not use plastic mulch under trees or shrubs.

Landscape Fabrics – Also known as geotextiles, landscape fabrics let in water and air but keep weeds from growing up but they will degrade over time and they are not very attractive, so it’s best to cover them with a second type of mulch. Another drawback of landscape fabrics is that shrub roots tend to grow into the fabric, causing real problems when you eventually want to remove it.

Common Questions and Answers About Compost Versus Mulch

Can I spray paint my mulch?

When you have seen people spraying something like a mulch colorant, paint, or dye onto their mulch, they’re likely using sprayable mulch dye instead of standard spray paint. These methods of coloring mulch last for several months and are available at garden centers and other places gardeners shop. You can use a tarp over your plants, garden sculptures, and other fixtures to avoid dyeing them when you apply mulch colorant. However, the optimal replacement timeline for mulch is twice a year, which will refresh the color as well, so with proper application, this product may not be necessary. A standard spray paint contains chemicals which aren’t the best addition to your garden.

Can I use compost as a mulch?

Compost can be used as an organic mulch that prevents weeds while also adding nutrition to soil. One of the best ways to use compost as mulch is to add compost in a thin layer, then top with another type of mulch, such as shredded leaves. This technique works well because it prevents the compost from drying out, and mulch is most effective when it remains moist.

Can too much compost hurt plants?

When garden soil contains more than about five percent compost, the level of compost is too high in the soil, and it can be harmful to plants. Too much compost can leave an excess of certain nutrients in the soil, such as nitrogen or phosphorous. Problems resulting from too much compost can include plants that grow too fast or increased pests and disease as plants fail to produce natural pesticides. About an inch or two of compost per year in flower beds and for landscaping is plenty; in vegetable gardens, you can use up to three inches per year without worrying that you’ve used too much.

Can you put mulch over weeds?

Using a mulch layer directly over weeds that are growing in your garden will have some effect and kill a portion of the weeds, but there are better options to kill the weeds than adding mulch directly on top of them. Some gardeners use solarization to kill weeds before they mulch, spreading clear plastic over an area and securing the plastic with stakes or weights in the corners. The plastic raises the temperature of soil underneath, killing weeds from sprouting seedlings to established plants. A layer of landscaping fabric can be used prior to a mulching layer, but some plants will sneak through the weave of the fabric. Gardeners can also use a layer of newspapers or cardboard underneath their mulch to prevent the growth of weeds and kill existing invasive plants. If you choose to use mulch alone, fine-textured mulch should be spread at least two or three inches thick for optimal weed prevention, while coarser mulches need a deeper layer of four to six inches to be effective against weeds.

Do you have to remove old mulch before applying new mulch?

If you don’t remove the last batch of mulch before applying a new layer, you risk preventing the mulch in the old layer from decomposing. The result of failing to take out old mulch before you refresh it can be a permanent barrier to plants or plant death due to lack of nutrients and rot. Never allow your mulch to get deeper than three inches. If you spread all three inches at once, you’ll need to remove the old layer before putting down anything else. If you only use one inch of mulch at a time and mulch twice a year, you can wait a year and a half before you need to remove any mulch from your garden.

Do you need to pull weeds before mulching?

Mulch blocks sunlight from reaching the soil when it’s spread thickly enough. However, while mulch will help prevent the growth of weeds, it isn’t foolproof, so some gardeners choose to spread a layer of newspaper or cardboard under their mulch to help kill weeds. Solarization is another popular approach, in which clear plastic is spread over the garden prior to mulching, which heats up the soil and kills weeds as well as preventing diseases.

Do you throw away old mulch?

Mulch should be cleared away before you add more if it threatens to get more than three inches deep. Simply use a rake to pick up any mulch you need to throw out before you refresh your mulch, which should happen twice a year or so.

Do you water mulch after putting it down?

Watering your mulch right after you’ve spread it in the garden helps to moisturize it and also encourages the individual pieces in the mulch to settle, helping the layer of mulch to stay in place and maximizing weed prevention. You should water the area before applying mulch as well to get the soil as moist as possible.

Does mulch turn into soil?

Mulch will eventually decompose and become part of your garden’s soil. In fact, that’s why it’s recommended to refresh your mulch twice a year. You can keep mulch from decomposing as quickly by using treated instead of untreated mulches in your garden. After one year, mulch has begun its process of breaking down into soil, and it will be complete after three years.

Does the color of mulch matter?

Choosing which color of mulch to use in your garden is about more than just aesthetics. Black mulch provides lots of contrast when used with vivid greens, like sweet potato vines. It also looks especially nice with gray homes or those with contemporary building styles. Brown mulch is recommended to match red brick homes, and red mulch pairs well with gardens that employ lots of terra cotta as well as pueblo-style homes.

How do you maintain mulch?

Check the depth of your mulch occasionally to ensure it’s between two and four inches deep. Mulch will eventually decompose and become a part of your garden’s soil. Sometimes, gardeners choose to remove old mulch before they add more to prevent a buildup of mulch past two to four inches. Too much mulch can stop the decomposition process, causing a layer that will prevent plants from growing or kill existing plants due to malnutrition and rot. Many gardeners find it works best to mulch twice a year, whether that means completely replacing a rotted layer of mulch that you remove first or simply topping off your existing mulch with a layer of an inch to refresh the color. A few times each season, you should mix your mulch up a bit with a rake or turn it over with your hands, breaking up any large clumps. Be sure to keep mulch from touching plants and trees directly.

Is black mulch toxic to dogs?

Mulch that is made from cocoa shells is poisonous to dogs if ingested because of the compound that makes chocolate toxic to them, theobromine. Ingesting large amounts of cocoa shell mulch can be fatal for dogs. Safer alternatives for pet owners include shredded leaves, pine needles, rubber mulch, cedar (which will also deflect insects), and natural untreated woods. If your dog has ingested cocoa shell mulch and is showing any symptoms or you are concerned they may become ill, you should visit your veterinarian, who can administer treatments to induce vomiting and prevent poisoning.

Is it cheaper to buy mulch in bulk or bags?

It is often cheaper to buy mulch in bulk or as an amount suitable for the yard instead of by the bag, although delivery fees from some installers can make this guideline untrue. The very least expensive option is often to buy mulch yourself and pick it up yourself, if you own a truck or can rent one for the project.

Should landscape fabric go under mulch?

Using landscape fabric under mulch is especially recommended if you’re using rocks or gravel, as it will make cleanup much easier in the long run. Landscaping fabric will also help keep weeds at minimum, but some are bound to grow through its weave, and they’re difficult to remove once they’ve done so. A layer of landscaping fabric also makes weeding a difficult process in general. Landscape fabric should never be used under organic mulches, when the goal is for the mulch to become one with the soil as it breaks down over time.

Should mulch touch plants?

Be careful not to let your mulch touch plants or trees directly. The reason separation is needed is that mulch can leach nutrition from the soil as well as keeping water sealed away from the roots of plants and rotting the bark where the mulch touches trees.

Should you replace mulch every year?

Because climate, mulch material, and other factors all influence how long it takes a layer of mulch to decompose, you’ll want to keep an eye on your mulch to see how deep the layer is by checking a few times a year. A good time to do this is when you’re turning the mulch over with your hands or a rake and breaking up any unusually large pieces. Mulch should stay between two and four inches thick; once it begins to decompose and gets thinner than that, it should be replaced.

What are the advantages of using mulch?

Mulch is more than just a way to add visual contrast to your garden. It also helps keep soil cool and seal in moisture in areas where a hot, dry climate makes it hard to keep plants hydrated. Organic mulches add nutrition to the garden soil as they decompose and become a part of it. A layer of mulch also helps keep weeds at bay and reduce the damage cold weather can do to your plants in the winter. Certain mulch materials, such as cedar, cypress, and pine, also repel insects from the garden.

What color mulch fades the least?

Color doesn’t have a lot of variation in fading time when it comes to mulch. However, mulches that are dyed tend to hold their color much longer than mulches that are not dyed and keep their natural color.

What is the safest mulch for dogs?

Many mulches are appropriate for use around dogs, with the only mulch that’s toxic to them being cocoa bean shell mulch. Pine needle mulch can also be problematic if ingestion occurs, as they can puncture your pet’s stomach lining. However, some pet owners may experience problems with rock mulches or wood chip mulches if their individual pets are attracted to eating those materials. Alternatives include shredded leaves and rubber mulch. Problems can occur with any mulch your pet ingests, so supervise your pet around a new type of mulch.

What is a good price for mulch?

Mulch prices vary widely depending on the material, from $16-19 per yard for natural mulches to $60 to $75 per yard for hardwoods. Bulk orders tend to be cheaper than purchasing mulch by the bag, especially if you have a truck (or can rent one) and will pick the mulch up yourself. Delivery fees may increase the cost of bulk orders when they apply, so always ask about these charges.

What is the best time to put down mulch?

The best time to put down mulch is mid to late spring, once soil has warmed somewhat for the season. Mulching your garden before the soil has had a chance to warm itself can keep it from soaking up the sun and slow the warming process down. Mulch might need to be reapplied in the summer or winter as it breaks down and decomposes. A few times per year, turn your mulch over with a rake or use your hands, also breaking up any large chunks. Take this opportunity to check the depth of your mulch later, adding new mulch as needed to keep it between two and four inches.

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