Can too much Mulch Suffocate my Plants? (how can it happen?)


Can mulch suffocate your plants? In certain conditions, yes it can, however, these conditions are created through a lack of education more so than any problem that a mulch will create by itself. These conditions are discussed in depth in this post whose author is an active self-sufficient style gardener who is trained in Permaculture Design.

How mulch works in nature.

It is an educational thing to take the time to look at how the natural systems create mulch without any human intervention.

It is through this lens that we can get some idea on how this natural process works, and then consider how we can mimic the process within our own backyard gardens.

There are multiple factors at work in any given square foot of naturally mulched ground, and if we can get even half of that natural activity in our garden beds we are on the right track.

In the natural setting, mulch is formed from falling leaves, the annual dieback of understory plants, and fallen tree branches that trap blown grasses and leaves. This layer then breaks down over time, sometimes quickly, sometimes slowly.

natural mulch process in action
Spent Turmeric tops create mulch for the dry season.

This is totally dependent on the climatic conditions of the location you are seeing.

Some trees drop a deep layer of leaves, others hardly drop any at all because they are evergreens. The natural systems also create shade in dense thickets where mulch can be concentrated.

When there is dense shade, the mulch is fast to break down into compost because these zones hold moisture for longer and so support soil life. Exposed sites will often dry out and suppress the soil life.

This insight can help with how you might approach the application of mulch to any given area or garden.

The mulch/soil interface is a biological hot zone where surface elements and small life forms interact with subterranean life forms. It is through these interactions that the mulch is converted into soil.

This is the lesson to learn. Life forms break down mulch for us. The more life you have in your garden, the faster the mulch “disappears” and needs replenishing.

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Because there is no natural system in place to self mulch our garden bed, this task falls on us. This is doubly so with raised garden beds.

How much mulch should I use?

For trees, the amount of mulch required to help retain moisture and to limit evaporation is no more that 4 inches in dry climates and just 2-3 inches in the tropics. In raised garden beds the mulch should be no more that 2 inches thick.

This is because the raised beds are more frequently watered and these beds drain water downwards faster than ground level garden beds, generally speaking.

The amount of mulch needed for your garden is directly related to your geographical location. In the wet tropics of Far Northern Queensland, Australia, we have to mulch often because of the soil conditions and also the seasonal changes from wet season to dry season.

In a typical wet season, the mulch we place on the garden beds is converted to soil quite quickly and will only last 2-3 months. In dryer climates, the mulch will last longer because the moisture level is far less.

wood shavings as mulch
Good moisture retention with breathability. Perfect. 4-6 inches deep. Wood shavings from wood turning.

A good rule of thumb is this. The soil life increases exponentially as the moisture levels rise.

It can be understood like this. The sub-tropical and temperate soils and weather conditions create a slow system. The tropical climates create a fast system, in fact it is a system that never sleeps.

Depending on where you live will decide how fast the mulch will break down, and how often you need to top it up.

Why does mulch help in the garden?

Mulch helps in the garden because if provides shelter, food, and protection for millions of microscopic life forms that all play a part in soil creation and nutrient processing. Mulch also helps limit moisture loss, and all life forms require stable moisture availability to prosper.

No matter where you live, the garden will benefit from mulch. All animals need shelter, and mulch provides protection for the life forms that inhabit the soil/surface interface zone.

It is a biological intermixing location that benefits both above ground lifeforms as well as the underground lifeforms.

This zone is potentially the busiest place in your garden, and where there is activity, there is living and dying going on at the microscopic level.

This is one of the mechanisms of creating plant-available nutrients. It can also be a carbon rich area that helps retain moisture.

It is also this interactive zone where soil tunneling life forms like worms are active, and through their movements they create pathways for oxygen to reach deep into the soil structure.

As a relevant side comment, did you know that the continental land forms react to the moons gravitational pull just the same as the oceans of the world? This monthly rise and fall is similar to a long slow breath that the ground takes.

Some people plant by these cycles and this practice is called moon planting.

Mulch also protects against evaporative moisture loss through wind and sun.

We would do well to revisit the natural system that we discussed at the start of this post and recognize that in a healthy ecosystem there is very little bare soil. Any location that is vacant is soon occupied by a new plant.

It could be what some would call a weed, but the gardener with a curious disposition might see it as an opportunist. The process is such that the soil is either covered in plants, mulch, or shade.

What mulch is safe for plants?

Any mulch that is dry and contains low amounts of green matter is safe for most plants.

Trees will benefit from heavier mulches like wood chip, and raised garden beds will do well with straw and/or wood shavings. Re-cycled cardboard is also good, as are natural fiber mats and old carpets.

Mulch to avoid would be green waste from freshly shredded plant material, and heavy layers of manure. Because of the high levels of nitrogen in these materials, it can create conditions that will not be beneficial to your gardens, generally speaking.

The heat generated as this material decays can burn plants. The exception to this rule is in locations where you use the chop and drop method of mulching as your prune and cut back.

This practice is mostly used in well managed permaculture settings that recognize the value of in-situ mulching systems and are often centered around specific trees that are what is termed mineral accumulators.

These plants often are pioneer species and also legumes, however this is not a pre-requisite.

We have an article that goes into some depth on ten different types of mulch and the negatives of them. Some are suited to trees and others are better as food garden covers. The title is “what can be used as garden mulch, safely and cheap” and it details the different conditions that suit different mulches.

The table below describes mulch types that are seasonally appropriate according to expected rainfall, and soil life activity. Tropical soils contain larger proportions of fungal activity than the rest, as this declines more the farther you are from the equator.

This fungal activity is why we have included small logs as part of the mulch we use here where we live. We know it works. There is relevant information about using logs or more specifically wood shavings from particular logs as a mulch in this post titled “Growing lots of ginger, is this the best mulch?” and we do recommend the read. It really surprised us with the results of this mulch.

ClimateMulch for
Mulch for
Food gardens
TropicalSmall logs/wood chipBark chipHay/bagasse
Sub-tropicalWood chipStraw/bark chipHay/bagasse
TemperateBark chipsBark chipsHay/straw
MediterraneanBark chipsBark chipsHay/straw
It is logical to use appropriate mulch for your climate.

Can mulch harbor disease?

In certain conditions mulch can harbor disease. Unless the mulch was placed already holding the disease within it, the disease is most likely a soil borne condition that benefits from the life rich area of the soil/mulch interface zone.

There can be times when mulch can be the starting point for a disease outbreak, but this would occur after a localized event most likely.

What can cause something of this nature is the heavy application of a mulch that packs down tightly and severely limits air movement through the mulch. This can potentially create anaerobic conditions and this is a primary vector for disease.

It can also be the result of the mulch being too green and creating problems. This article “will too much mulch burn my plant?” looks at this angle.

Mulch is most effective and beneficial when air can pass through the layer relatively freely. It is desirable as a gardener to avoid creating any anaerobic conditions if at all possible. If it does occur, the possible solution is below.

Look at your soil as a matrix of favorable locations for bacteria and fungi to inhabit.

The same goes for mulch, and compost. If a disease has control of a location, that spot is taken and will not be available for a beneficial microbe to inhabit.

This process can potentially be reset by creating a good compost that is ready for the garden, and getting a garden fork or similar, and turn the soil over. Expose the soil to the sun for a day or so at a time, and repeat this process several times.

After a few turns, add a generous amount of fresh compost to the garden.

What you are trying to do is create a bacterial kill-off via tilling and sun exposure, and in the process of adding the compost you allow the beneficial bacteria and fungi within the compost to lay claim to those locations that the disease inhabited.

This is something that you can try before reaching for that chemical bottle.

Does mulch encourage worms?

Yes, if mulch is kept moist and is thick enough to provide shelter from the heat of the day, worms will be active beneath the mulch. Many small life forms will be active, and this is what helps drives healthy plant growth.

Where we live, during our wet season there is a defined acceleration in activity beneath the mulch. The change from the dry to the wet and the amount of visible soil life activity that comes with the wet is astonishing.

It is this time of the year that we let the chickens out of the pen every few days for a hour or so at a time. They get pretty excited at the life under the mulch as they go about their scratching.

Do worms help aerate soil under mulch?

Yes, worms will aerate soil beneath moist mulch, if earthworms are local to your area. It is the movement of worms through your soil that creates voids that help oxygenate the soil.

Most soils have some worms hanging around. It is amazing when mulch is placed in an area where there was no garden before. It will not be long before worms arrive. How they know when to come is beyond me.

Healthy soil is active soil. Earthworms are often the measure of a healthy garden, but these animals cannot exist alone. They require a support network of symbiotic life forms to help build a community that can then in turn support your plants.

This entire process can only take place when conditions are suitable (read moist), and this is where mulch fits in. In your garden you become the orchestras conductor.

What kind of mulch should you avoid?

Mulches that have water rejection behavior should be avoided. This behavior is termed hydrophobic and it applies to mulch like lawn cuttings or clippings.

Good mulch provides the necessary environment for soil borne life forms to interact with healthy plants. It is a life regulating material in a way.

This entire process can be stopped quickly when a type of mulch is mistakenly applied with good intentions. One particular type is lawn clippings and what can happen if grass clippings are applied as a blanket mulch is the layer becomes hydrophobic.

Hydrophobic activity, or rejection of water, is often the result of fungal activity that colonizes the mulch surface and can cause the inability of water to filter through the mulch. It basically becomes water proof.

Any water runs off the mulch and cannot soak through, and this can create a double whammy effect against the soil moisture levels.

This double whammy happens because the underside of the mulch will absorb some moisture from the soil below, taking it away from the plants roots. It also severely limits any water entry to the soil surface through hydrophobic action.

We have personal experience with this through a neighbors garden where, in the middle of our wet season, he was watering vegetables with a hose and had to blast holes in the mulch to get moisture into the soil. The plants looked like they had not had water for many days and were in bad shape. We learned all about this process that day.

Why does mulch kill trees?

This is a curious topic. Mulch does not kill the tree but careless placement of mulch can create conditions that will kill a tree.

To simplify this subject and to avoid any issues, we again look to the natural systems where trees naturally grow. They do not have mulch against the trunk, and they have grown this way for centuries in many cases. We would be wise to mimic these teachers.

There is a crazy notion that appears on many websites called volcano mulching. This is the ultimate in stupidity; both the idea that it might work, and all commentators giving oxygen to such an absurd idea.

If you have read this far you will have understood that healthy soil conditions require aerated mulch and this does not mean shallow mulch per-se, but means well ventilated mulch at whatever depth it will breathe.

The material structure will inform you on this. Coconut husks can be placed a foot deep and still be a good mulch. A foot of wood chip will smother everything below it.

Structure matters.


Mulch can be the greatest benefit to a gardens production, be it food growing like in our case, or simply growing flowering plants. The same rules apply across the board.

Over time you too will learn the simple lessons that the natural systems can teach us if you take the time to stop and notice.

Article by Tim Blanch for He is a qualified Permaculture designer.