Mulch is great for keeping weeds at bay and retaining moisture in soil. The decision to use mulch in a raised bed really depends on what you are growing.
So, should you mulch a raised garden bed? You should mulch a raised garden bed if you need to suppress weeds, retain moisture, or regulate temperature in the soil. However, plastic mulches may make the soil too warm for cool-weather crops such as lettuce or spinach. A layer of mulch that is too thick or too close to plants will hurt or kill them.
Of course, retaining soil moisture in a raised bed is important, since they tend to drain faster than the surrounding soil.
In this article, we’ll talk about why you might want to mulch a raised garden bed. We’ll also take a look at the different types of mulch you can choose from and when to use each one.
Let’s get started.
Should You Mulch A Raised Garden Bed?
In most cases, it is beneficial to mulch a raised garden bed. However, you should consider the pros and cons, as well as the type of material you use as mulch.
Advantages Of Mulching A Raised Garden Bed
Adding a layer of mulch over a raised garden bed has several advantages, including:
- Fewer Weeds – mulch prevents the seeds of weeds from sprouting. It can also suppress existing weeds if you cover them.
- Even Soil Temperature – a layer of mulch insulates the soil below. This prevents rapid changes in temperature so plant roots will not experience such extreme heat or cold.
- Moisture Retention – mulch retains water in the soil beneath it. With a layer of mulch, water evaporates from soil much more slowly. This is important in a raised bed, where water drains faster than on level ground.
- Restores Nutrients – this only applies to organic mulches that decompose. For example, grass clippings and leaves will restore nutrients and organic material to the soil as they decompose.
Disadvantages Of Mulching A Raised Garden Bed
There are a few drawbacks to putting mulch over a raised bed. For example:
- Decomposition – some mulch (such as leaves) will decompose during the growing season. This will result in an unpleasant smell in your garden. Even worse, the decomposition can deplete nitrogen from soil in the short term (especially with carbon-rich material, such as sawdust).
- Rotting Stems – if you place mulch too close to the base of a plant, its stem will rot (in part due to the increased moisture retained by the mulch).
- Danger to Plants – too much mulch can kill plants. A thick layer of mulch prevents soil aeration, which will suffocate plant roots.
- Potential For Mold – mulch can sometimes host mold growth. This is more likely under ideal conditions: organic material (such as wood), humid air, moist soil, and warm temperatures (77 to 86 degrees Fahrenheit).
Of course, if you choose the right mulch, apply the right amount, and keep it away from plant stems, you can avoid most of the problems.
How Do You Mulch A Raised Garden Bed?
On average, a layer of mulch for a raised bed should be 2 to 3 inches thick. However, the thickness varies depending on the type of mulch:
- Coarse Mulch – use a thicker layer (3 to 4 inches deep). Coarse mulch is made up of larger pieces of material. For example: large wood chunks or whole leaves.
- Fine Mulch – use a thinner layer (1 to 2 inches deep). Fine mulch is made up of smaller pieces of material, and has often been processed. For example: sawdust, grass clippings, or chopped leaves.
For soil that does not drain well (clay), use a thinner layer of mulch. Apply mulch in mid to late spring.
No matter which mulch you choose, be sure to make your raised garden bed deep enough to hold the mulch.
Mulching too early in the season will keep the soil cool. This will delay transplant of warm-weather crops such as tomatoes and peppers.
Mulching in the fall will give organic mulches time to decompose over the winter. In some cases, the mulch will decompose to the point where it won’t do much to stop weed growth in the spring.
You will need to replace organic mulches often, since they will decompose over time. Inorganic mulches may not need to be replaced for many years.
When applying mulch to a raised garden bed, don’t put it too close to the plants. Otherwise, the stems will rot due to contact with the mulch, especially when it stays wet.
A good rule is to keep mulch 3 to 4 inches away from the base of plants. For trees, keep mulch 12 to 18 inches away from the trunk.
For more information, check out this article from the University of Florida.
What Is The Best Mulch For Raised Beds?
The type of mulch you use for raised beds will depend on a few factors:
- Crop – for example, warm weather crops (such as tomatoes and peppers) may benefit from plastic mulch to keep them warm in spring. Cool weather crops (such as lettuce and spinach) may not need cold protection.
- Soil – as mentioned earlier, clay soil (which drains slowly) may not need a layer of mulch to retain moisture. Sandy soil (which drains quickly) is more likely to need a layer of mulch to retain water.
- Weather – if the weather is rainy and humid, you might want to avoid organic mulches that will decompose and attract mold.
There are two basic types of mulch you can use for raised beds: organic and inorganic. We’ll start by taking a closer look at types of organic mulch.
Organic mulch comes from living things: most often, some type of plant or tree. There are lots of options for organic mulch in a raised garden bed.
Large wood chips take longer to break down than finer pieces (such as sawdust). This means that a layer of wood chip mulch will last longer, so you won’t need to replace it so often.
Cedar mulch will last even longer than other types of wood chips. Cedar has natural oils that help it to resist decay.
However, wood chips might not be a great choice if you want to replace nutrients and organic material in your raised beds every year. In many cases, wood chips are used for decorative purposes.
Of course, you could let wood chips rot and use what is left as compost in your garden. You could also choose one of the many alternatives to wood mulch.
Sawdust is a cheap source of mulch. Just make sure to avoid sawdust from wood that is treated with chemicals (for example, some pallets or railroad ties).
Sawdust particles are small and fine, so this mulch provides good insulation for soil.
Remember that if you mix sawdust into the soil, the carbon will “tie up” nitrogen as the sawdust decomposes. This is good for suppressing weeds, but bad for growing crops in your garden.
Grass clippings make a cheap source of mulch for a raised garden bed. However, make sure to avoid grass clippings from lawns that were treated with herbicides (which can kill your plants) or pesticides (which can kill or repel bees).
Grass clippings decompose fast, so you will probably need to replace them every year. However, the decomposition process will restore nutrients and organic material to the soil.
Leaves are another cheap source of mulch for a raised garden bed. Smaller pieces will break down faster, so use a lawnmower to chop up leaves to speed up decomposition.
Leaves decompose fairly quickly. After this happens, they will provide the perfect place for weeds to grow.
For this reason, you will need to replace leaves often to keep weeds at bay.
Straw is made from grain crops by removing the grain and chaff. Straw will suppress weeds while it lasts, but it will decompose quickly.
Dry straw has lots of space between the pieces, so it won’t insulate soil as well as other finer materials.
Straw probably does not contain the seeds of weeds. However, it may contain pesticides, so know your source before adding it to your garden.
Hay may contain the seeds of weeds, so opt for straw if possible. You can learn more about the difference between straw and hay in my article here.
Pine Needles (Pine Straw)
Pine needles have a pleasant smell, and they can provide decent insulation for soil in a raised garden bed. You might be able to find pine needles nearby for free from your yard or a neighbor’s yard.
Pine needles decompose, so they will need to be replaced often. However, they will add nutrients and organic material back to your soil.
Compost that is completely broken down will do a good job of insulating soil and retaining moisture. However, it might be a good place for weeds to grow.
You can make compost from all of the organic materials listed here – to learn more, check out my article on how to make compost.
Newspaper is cheap and plentiful, and it is easy to add multiple layers to make it as thick as you want.
Newspaper might help to suppress weeds for a while. However, it breaks down quickly, especially when wet.
As a result, you might need to replace newspaper multiple times during one growing season. You can also use newspaper as a first layer of mulch, with something piled on top for a second layer.
Cardboard varies in its size and thickness, but it will work better than newspaper to suppress weeds. Cardboard gets heavy when it is wet, and decomposes fairly quickly.
You can sometimes find empty cardboard boxes for free at grocery or retail stores. Sometimes all you need to do is ask!
Inorganic mulches do not break down over time. As such, they are a good choice if you do not want to constantly replace mulch.
Inorganic mulches are not a good choice if you want mulch that decomposes to restore nutrients and organic material to the soil.
Gravel is made from rocks of various sizes. Gravel has a nice appearance for landscaping, but is not well-suited for a vegetable garden in a raised bed.
Gravel is also expensive, and heavy to work with. You would be better served by almost any other type of mulch for a raised bed.
Rubber mulch is made from recycled chunks of tires. It might be suitable for landscaping and weed suppression, but it is not a great choice for a raised bed vegetable garden.
Landscape fabric is good for suppressing weeds and preventing them from growing. Use organic mulch over a piece of landscape fabric to provide additional insulation and water retention.
Another benefit is that you can cut holes in a piece of landscape fabric. This will let you plant crops with the correct spacing while still preventing weeds from growing.
A plastic tarp will do a good job of smothering existing weeds and preventing new weeds from growing. A tarp is thicker than landscape fabric and will last much longer.
Just remember that plastic is not permeable, so the soil underneath it might become dry if you don’t water carefully. However, it might be a good option for plants that can tolerate drought.
As with landscape fabric, you can use organic mulch over a piece of plastic tarp to provide additional insulation and water retention.
Now you know how to decide whether to mulch a raised garden bed. You also know what to consider when choosing the type of mulch to use.
I hope you found this article helpful. If so, please share it with someone who can use the information.