What To Do With Old Mulch (4 Options To Recycle Old Mulch)


Mulching your yard and garden is a common annual task, since wood chunks often decompose quite a bit over the course of a year. However, you might be wondering what is the best way to deal with existing mulch.

So, what should you do with old mulch?  If the mulch has not decomposed, you can loosen it and turn it over to reuse it or add a thin layer to the top of it to refresh it. You can also move it to a new location. If the mulch has begun to decompose, you can mix it into the soil or add it to your compost pile to speed up decomposition.

Of course, when certain mulches (like sawdust) are mixed into soil, they tie up nitrogen as they decompose.  This temporarily prevents plant growth, so be careful when mixing mulch into soil where you want plants to grow.

In this article, we’ll talk about what to do with old mulch.  We’ll also answer some common questions about mixing mulch into soil and turning it into compost.

Let’s get started.

What To Do With Old Mulch

Having old mulch in your yard isn’t necessarily a problem.  You have a few options when deciding what to do with it, including:

  • Loosen or turn over the old mulch – this is probably the easiest method.  It might be necessary if the old layer of mulch is “crusted”.  Be careful not to put too much new mulch on top of the old layer!
  • Mix old mulch into soil – this method takes a little more work than loosening.  It is a good method if the mulch will compost in place without tying up too much nitrogen in the soil.
  • Add old mulch to compost – this method takes the most work, since you must rake up all the mulch, shovel it into a wheelbarrow, and move it to your compost pile.
  • Reuse old mulch elsewhere – this method also takes a lot of work, but if the mulch has not decomposed, you can move it to use elsewhere.
red cedar mulch
You have several options for old mulch, including turning it over, mixing it into the soil, adding it to a compost pile, or reusing it elsewhere.

Loosen Or Turn Over Old Mulch

Loosening old mulch by turning it over or fluffing it up is one of the easiest ways to reuse mulch.  When you turn old much over, the entire layer loosens.

This prevents the mulch from condensing into a hard layer, which can prevent water from getting into the soil below.  A thick layer of mulch is more likely to become hydrophobic (meaning it repels water).

mulch
A layer of mulch may condense and become hydrophobic, repelling water. Loosening the mulch with a rake will prevent this (or fix it if it happens).

After loosening old mulch, you can add another new layer of mulch on top of it.  Just remember that the total amount of mulch only needs to be 1 to 4 inches, depending on soil type and the size of the mulch chunks (see the table below for more detail).

Mulch Size
& Soil Type
ClaySand
Small1-22-3
Large2-33-4
This table summarizes mulch layer
thickness (in inches) for various
mulch chunk sizes and soil types.

Keep in mind that too much mulch can kill plants if piled up too close to the base of the plant or layered too thickly.

You can learn about what to avoid with mulch in this article.

If most of the old mulch has decomposed, you can sometimes mix it into the soil without harming plants (more on this later).

How Do I Turn Over Old Mulch?

To turn over old mulch, you can use a rock rake to loosen the pieces and move them around a bit.  This will prevent the mulch from forming a hydrophobic layer.

rake
Use a rock rake to loosen and move pieces of mulch. Go easy to avoid damaging shallow roots that may be present.

Raking will also prepare for another thin layer of new mulch to refresh the old one.  Be sure to rake lightly to avoid damaging any shallow plant roots that may be present in the mulch or in the soil just below the mulch.

For the same reason, avoid digging with a shovel, trowel, or other sharp tool.  These can sever plant roots and damage any landscape fabric material that you have underneath the mulch.

Mix Old Mulch Into Soil

If the old mulch is partially decomposed or made of fine particles, you can just mix it into the soil.  This will speed up decomposition of the mulch, adding both organic material and nutrients to the soil.

mulch
If some of the mulch is decomposed, you might opt to mix it into the soil instead.

However, there are some cautions when it comes to mixing mulch into soil.

Is It OK To Mix Mulch Into Soil?

For most mulches, mixing them into the soil will speed up decomposition.  This will add organic material (from rotted wood, etc.) and nutrients to the soil.

However, certain mulch types will temporarily tie up nitrogen in the soil as they decompose.  One of the biggest culprits is sawdust.

sawdust
When sawdust decomposes, it temporarily ties up nitrogen in the soil, denying plants this vital nutrient.

Sawdust has lots of carbon, as does any type of wood mulch.  However, the small particle size of sawdust means that it will decompose quickly.

As sawdust decomposes, the carbon in the wood ties up nitrogen in soil as it decomposes.  This makes nitrogen unavailable for plants, stunting their growth or killing them.

According to the Texas A&M University Extension, this lack of nitrogen can last anywhere from several months to several years.  The time frame depends on the amount of sawdust added to the soil and the soil conditions (including moisture and aeration.)

sawdust
Consider using sawdust for animal bedding or to suppress weeds, rather than mixing it into soil.

A better option is to use sawdust for:

  • animal bedding (when mixed with the high nitrogen content in manure, it will decompose faster, and the mixture will make a good compost)
  • covering ground between rows in a garden (this will smother weeds and prevent new ones from growing)
  • compost (add plenty of grass clippings or other nitrogen-rich materials and water the pile to make it decompose faster)

There are lots of other alternatives to mulch and sawdust, including:

  • Grass clippings
  • Pine needles (pine straw)

You can learn more about other alternatives to wood mulch here.

cutting grass
Grass clippings make a good mulch that will not tie up nitrogen in the soil (in fact, they are rich in nitrogen and will provide more!)

Does Mulch Turn Into Topsoil?

Mulch does not turn into topsoil.  Instead, mulch turns into compost as it decomposes over time (this compost, in turn, feeds the topsoil).

soil
Mulch can decay into compost over time, but it will not contain the minerals, clay, and sand from rocks that topsoil has.

Remember that topsoil contains some compost (organic material and nutrients), but it also contains minerals, clay, and sand (from rocks affected by weather over a long time period).

You can learn about the difference between topsoil and mulch here.

Add Old Mulch To Compost

As mentioned earlier, you can turn old mulch into compost by adding it to your pile.  This takes some work, since you must scrape up the mulch with a rake, shovel it into a wheelbarrow, and move it to the compost pile.

compost bin
You can add mulch to your compost pile to speed up its decomposition.

However, this lets you add a completely new layer of fresh mulch to your landscape.  Before you add old mulch to compost, here is a quick checklist to look over:

  • Is the mulch biodegradable?  Gravel and rubber mulch take a long time to decompose, so they should not be added to a compost pile (you are better off reusing them for another garden project).
  • Is the mulch clean?  Some mulch is treated with dyes to give it a different color.  Other mulch comes from trees that were treated with pesticides or herbicides that can hurt plants or bees.  Either way, consider the source before you put wood mulch in a compost pile.
  • Can you balance the compost pile?  Adding too much carbon-rich material (like sawdust) without a balance of nitrogen-rich material (like grass clippings) will slow down or stop a compost pile from decomposing.
coarse wood mulch
You might not want to use dyed mulch or material that was treated with pesticides or herbicides in your compost pile.

Should You Remove Old Mulch?

You do not need to remove old mulch before adding a new layer of mulch.  As mentioned earlier, you can loosen it up with a rake to refresh it or mix it into the soil before adding a new layer.

However, if the existing layer of mulch is too thick and you want to add new mulch, you should remove some or all of the old layer.

Also, if your mulch is moldy, you may want to remove the old stuff before adding new clean mulch.

yellow slime mold
You might want to remove old mulch if it is moldy.

Does Old Mulch Make Good Compost?

Old mulch can make good compost, subject to the checklist mentioned above (mulch should be biodegradable, free from pesticides or herbicides, and balanced by nitrogen-rich materials in the compost pile).

Reuse Old Mulch Elsewhere

The last option is to reuse old mulch in another part of your yard.  If wood mulch is not decomposed (or if you used rubber, gravel, etc.), you can recycle it.

Rubber or gravel mulch is your best bet in a location where you don’t want plants to grow (since these will not decompose to add nutrients and organic material to the soil below).

gravel small stones
You can reuse gravel elsewhere, but it will be tough to move. It will not decompose to add nutrients and organic material to the soil below.

Wood mulch is your best bet in a location where you have some plants growing (since it will decompose to slowly feed the soil over time).

Should I Water Mulch After Putting It Down?

You should water mulch after putting it down.  This serves two purposes:

hose spray nozzle
Water mulch after you put it down. This will help it to settle and prevent it from forming a hydrophobic layer that repels water.

Conclusion

Now you know what to do with old mulch and how to repurpose it for other garden uses.

I hope you found this article helpful.  If so, please share it with someone who can use the information.

You can learn about how to prevent mulch from washing away here.

You can learn about how cedar mulch compares to other types of wood mulch here.

If you want to read some of my most popular posts, check out the “Best of GreenUpSide” page here.  Enjoy!

~Jonathon

jonathon.david.madore

Hi, I'm Jon. Let's solve your gardening problems, spend more time growing, and get the best harvest every year!

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