There is a common misconception about mulch and its potential to get hot and harm plants. This post hopefully presents the reasons why mulch can get hot and what you as a gardener can do to stop it from happening, and this all starts with why it happens in the first place.
Too much mulch does not burn plants if the mulch is dry and stable. There is more on what this means below.
Using mulch appropriately.
Mulch comes in many different shapes and sizes, however, there are some types of mulch that will out-perform others in a given situation. It seems common sense, right?
If a mulch works great in one setting and a different mulch performs poorly, you want to know about it. How do you know what goes where?
| Mulch type | Heat potential | Longevity |
|---|---|---|
| Small logs and Branches | low | long |
| Wood chip | low | medium |
| Straw | low | low |
| Shredded Tree waste | high | medium |
A good rule of thumb is to see the mulch as a blanket that keeps moisture in the soil but lets air move through it at the same time. If either of these two requirements are not met then you don’t have mulch. If the “mulch” is too thin and sparse you have litter.
If it is too thick and fails to let air flow you have a compost pile. This is not to say that heavy layers of mulch equals compost, because that is not entirely accurate. It is a form of compost, but with an understanding that there are good and bad composts.
We will go into that in another post.
To use mulch appropriately, you should consider what plant type you are working with, the climate that you are in, and the mulch material you have available.
What makes good mulch?
A good mulch is one that will maximize soil moisture retention by minimizing evaporation, while simultaneously providing habitat for micro-organisms that convert plant-based materials in nutrients and minerals.
This is a process that you can see happening if you lift some mulch that has been down for a few months and the season has not been dry. A few days after rain is a good time to have a look. We have a post on an accidental event here at home where we grew a lot of ginger with mulch that was unique to us. It is titled “Growing lots of ginger, is this the best mulch?“
What kinds of mulch get hot?
Mulch that gets hot is potentially full of nitrogen rich materials, like shredded branches with lots of green leaves. Piles of fresh-pulled weeds will also get hot if it is a big enough pile.
The reason these mulches get hot is from the accelerated breaking down of the material into lower form components, and in bad cases can actually turn anaerobic on you and become a danger to your plants.
It is the decomposition of this green matter that is related to the process within a compost pile.
How do you fix hot mulch?
To fix hot mulch you need to add dry materials like dead leaves or similar.
As we noted above, the heat is a fast process of plant decay, and is not conducive for mulch use. It is however a great start for a compost pile. Think of the hot mulch as being being too wet, and you need to balance it out with a dry substance.
If it is in a large pile waiting to be spread and is showing signs of overheating, spread the pile if possible and let it get some air. This type of scenario has potential to turn bad on you because of the lack of oxygen. Turn it often and let it dry out. This could take a few days.
If your mulch is dry and there is no green waste in it, then there should be no reason to stop you from applying it to your gardens.
You should be able to be generous with the thickness without fear of burning your plants because the nitrogen component is just a small percentage. If there is green waste, the answer is above for you.
Is mulch the same as compost?
While mulch and compost share some common ingredients this is where the similarities end.
Mulch is not the same as compost because mulch is a dry mixture that is designed to break down slowly if possible and to serve the purpose of moisture retention at the soil surface and below.
Compost is the process of biologically breaking down natural materials into plant available nutrients and minerals through careful manipulation the ratio of carbon-based vs nitrogen-based materials.
Through design we can manipulate the biological activity to turn mulch like components into soil ready compost inside of a month and the compost will not resemble anything that was originally part of the starting pile.
Is using too much mulch the same as hot mulch?
No, hot mulch is where the wrong type of material is being used as a mulch and too much mulch is simply a layer of good mulch that is too heavy and thick.
Too much mulch can be quantified by the kind of material being used. To much green material placed 3 inches deep over a garden is too much vs 3 inches of a dry mulch is a good layer that will serve the intended purpose.
It comes down to the material you have available and what are the attributes of that material. Thickness of mulch plays only a part of the equation, and does not create issues in some cases.
Thin mulch has it’s own set of questions also. “How often should you replace mulch?” is one such question. When you are looking at depth of fresh mulch and potential heat, pay attention to the old mulch before you discard it.
I mentioned coconut husks as an example of a mulch in a separate article where we asked the question “can too much mulch suffocate my plants?” and it mentions that you can place these husks a foot deep and still get airflow at the soil surface.
A foot of smaller particle mulch will likely smother. If there is green materials in the mulch, it has the potential to get hot.
Conclusions.
As I write this article it has helped me understand the process of mulch far more intricately than before.
We are primarily self-sufficient in most fruit and vegetables here, and having the ability to put posts like this together to help others is reminding me of how intricate and intertwined the natural systems that we rely on really are.
If you have other questions on mulch, we have several articles on site and you can find them all in the one place by using the search icon at the top of the page and entering “mulch.”
Article by Tim Blanch for TheTropicalHomestead.com. He is a qualified Permaculture designer.