Using mulch in your garden can help your plants to grow better, and it can make your yard look better. However, is it possible to have too much of a good thing?
So, can too much mulch kill plants? Yes, too much mulch can kill plants, especially if piled up too close to the base of the plants. Mulch that is too thick can suffocate roots, overheat soil in hot sunny weather, and encourage diseases due to excess moisture.
Let’s go into more detail about why too much mulch is bad for plants and how much you should use depending on your soil type. We’ll also answer some common questions about mulch.
Can Too Much Mulch Kill Plants?
Yes, too much mulch can kill plants, especially when the mulch is too close to the base of the plants. (Remember that mulch and soil are not the same thing.)
There are several problems that can occur when mulch is piled too high and too close to plants.
A Thick Layer Of Mulch Becomes Hydrophobic
When a layer of mulch becomes too thick, it becomes hydrophobic, meaning that it repels water. A hydrophobic layer of mulch on top of your soil will not allow water to soak through into the soil where a plant’s roots are.
This means that your plants will slowly die from a lack of water, even if it look like you are providing enough water on the surface.
Too Much Mulch Suffocates Plant Roots
A thick layer of mulch prevents air circulation between the air and the soil. Over time, this results in a lack of oxygen in the soil.
This is a problem because plant roots need oxygen to survive. If the thick layer of mulch persists for too long, then the plant can die due to lack of oxygen for the roots.
Certain soilborne fungi will take advantage of wet soil and a lack of oxygen to cause root rot, which can damage plants, stunt their growth, or kill them.
Too Much Mulch Causes Too Much Heat
A thin layer of mulch can certainly help to insulate plants and regulate soil temperature to prevent fluctuations. This is a good thing. However, too much of a good thing is a bad thing.
When mulch becomes too thick, it traps too much heat in the soil. This is especially likely with dark brown or black mulches that absorb more energy from sunlight.
A thick layer of wet mulch may also start to decompose rapidly in warm weather. This can further raise temperatures (think about how warm a compost pile gets when bacteria are working to decompose organic matter).
These high temperatures will damage your plants, especially if the mulch is piled too close to them.
Too Much Of The Same Mulch Can Change Soil pH
Using too much of the same type of mulch over a long period of time can lead to pH changes in the mulch.
For instance, pine bark mulch has a low pH (it is very acidic). When used repeatedly for many years and broken down over time, it will lower the soil pH.
When soil pH is too low (or too high), nutrients in the soil become unavailable to plant roots. The nutrients are in the soil, but the plant roots simply cannot absorb them.
For more information, check out this article from Research Gate on the effect of soil pH on nutrient availability.
Too Much Mulch Invites The Spread of Disease
Putting down too much mulch can cause too much moisture to be retained, either in the soil or in the mulch. When a thick layer of mulch is piled too close to plants, this will lead to fungal and bacterial diseases.
These fungi and bacteria need moisture to reproduce, and wet mulch provides the perfect breeding ground. It also makes it easier for these diseases to spread between multiple plants in your garden that are in the same mulch bed.
For more information, check out this article about problems from over-mulching trees and shrubs from Rutgers.
Common Questions About Mulch
There are many questions about mulch, from how much to use to whether it can suppress weeds. Let’s start off with the basics and go from there.
How Thick Should Mulch Be Applied?
A layer of mulch that is 2 to 3 inches thick is ideal.
If your mulch is coarse (made up of larger chunks of material), go for a thicker layer (3 to 4 inches).
If your mulch is fine (made up of smaller chunks of material), go for a thinner layer (1 to 2 inches).
Also, you should opt for a thinner layer of mulch to cover soil that does not drain well, such as clay.
For example, to cover clay soil with coarse mulch, use 2 to 3 inches of mulch. To cover clay soil with fine mulch, use only 1 to 2 inches of mulch.
A reasonable layer of mulch has many benefits for plants and trees in your yard and garden.
Mulch helps with heat insulation to protect against rapidly changing temperatures. It also helps with moisture retention to help plants survive drought conditions, or if you are unable to water when on vacation.
In addition, mulch slowly adds organic matter to soil as it decomposes. Finally, mulch helps to suppress weeds, which have trouble growing through a layer of mulch.
What Can I Use As Mulch?
You can use wood chips, hay, straw, grass clippings, compost, chopped leaves, or even cardboard as mulch. For the sake of making your yard look nice, mulch will usually consist of wood chips.
However, if all you need to do is protect plants in your garden against temperature spikes, drought, and weeds, then any of the other materials listed above will do.
How Often Should You Replace Mulch?
A layer of mulch should be replaced when it becomes thin due to decomposition. How often this happens depends on what you use for mulch and how quickly it breaks down.
Clemson University suggests replacing organic mulches (such as wood chips) every 2 to 3 years. Of course, if you want to keep landscaping mulch looking fresh, then you may want to scrape away a thin layer and replace it every year.
When Should You Put Mulch Down?
If you are mulching to protect plants in your garden, then do it right after planting. This will help to insulate the plants against extreme temperatures, retain soil moisture, and prevent weeds from competing with your crops.
For more information, check out this article on mulch from the Virginia Cooperative Extension.
On the other hand, you should apply mulch in the fall if you want to prevent winter soil erosion and infestations of weeds in the spring.
For more information, check out this article on mulch from the Oregon State University Extension.
How Close Should Mulch Be To Plants?
For plants, keep the mulch 3 to 4 inches away from the base of the plants, to prevent the problems mentioned above.
For trees, keep the mulch 12 to 18 inches away from the base of the tree. For more information, check out this article from the University of Florida.
Also, avoid “volcano mulching”, which means building a hill of mulch at the base of a tree, with the tree “erupting” from the top of the hill.
Volcano mulching suffocates roots, and encourages pests such as voles. A steep slope also sheds water, keeping the soil underneath too dry, especially if the layer of mulch is thick.
In addition, volcano mulching causes decay on the lower trunk of a tree. At the same time, the excess mulch will hide the decay from your view until it is too lat.
This leaves you spending time, money, or both to get rid of or replace the tree.
Can Plants Grow Through Mulch?
Yes, plants can grow through mulch. However, a layer of mulch will slow down the growth of plants such as weeds. A thick layer of mulch will also decrease germination rates for the seeds of weeds.
Can I Just Put Mulch Over Weeds?
You can put mulch over weeds to kill them. However, the mulch needs to be thick enough so that the resulting heat can kill them, or so that the lack of air will suffocate them.
However, be careful about mulching over weeds that have already produced seeds. You could be in for an unpleasant surprise next year if the seeds survive the winter and germinate the next spring!
Do You Have To Remove Old Mulch Before Applying New Mulch?
It depends on how thick the existing layer of mulch is.
If you already have 3 to 4 inches of mulch on top of the soil, then it is not a good idea to continue adding more (see the guidelines above, depending on how fine the mulch is and how well your soil drains).
If the existing mulch is already too thick, you have two options.
Your first option is to stir up the mulch you already have. Even if the top layer is faded, the mulch underneath may still look good. This could give you an extra year between mulch replacements.
Your second option is to rake up some of the old mulch and remove it before adding new mulch.
Can You Till Mulch Into Soil?
No, you should not till mulch into soil. It is a poor soil amendment, since it has not completely decomposed (as opposed to compost, which has had a chance to decompose and break down).
However, you can take excess mulch and put it into a compost pile to give it time to decompose further. Then, you can use the compost as a soil amendment for your garden.
For more information, check out my article on how to make compost.
Do You Throw Away Old Mulch?
No, you should not throw away old mulch. Instead of sending it to a landfill, you can compost it instead. See the link above to find my article about how to make compost!
By now, you know that too much mulch can kill plants – and you know exactly why that happens. You also have some guidelines for applying and replacing mulch.
I hope you found this article helpful – if so, please share it with someone who can use the information. If you have any questions about mulch, please leave a comment below.