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.
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).
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).
& Soil Type
thickness (in inches) for various
mulch chunk sizes and soil types.
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.
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.
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 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.)
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)
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).
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).
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.
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.
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.
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).
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:
- Helps the mulch to settle – that way, you can see if there are any thin spots and you can thicken them up a bit with more mulch.
- Prevents hydrophobic mulch – watering helps to prevent a hydrophobic layer of mulch that repels water (a hydrophobic layer of mulch prevents water from getting to the soil and plant roots below).
Now you know what to do with old mulch and how to repurpose it for other garden uses.
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