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how much mulch is too much

 
gardener
Posts: 6686
Location: Arkansas - Zone 7B/8A stoney, sandy loam soil pH 6.5
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I started this topic simply because there is a true threshold for the depth of mulch for crop production. This threshold varies by crop type and once you have breached it, production goes down.

While the benefits of mulch are many, it is possible to have to much of a good thing at which point it can become detrimental.

Several studies have been done for different crops and in different climates, most show that Maize is tolerant of mulch layers between 2" and 4" thick, more than that and you will begin to have problems with stalk rot.
The makeup of the mulch used is also a factor. Wood chips, Straw, Compost, and cardboard/ newspaper are all used as mulch layers but each has differing properties that should be taken into account when deciding which one or combination of them is best for the situation.

Climates with high humidity are best approached with a mulch layer that has the properties of breathability, in this type of climate soil respiration is a factor in preventing mold growth from overtaking the base of the plantings, it also will prevent rampant black spot from gaining the advantage.
In this situation wood chips, straw and gravels are the best choices for mulching, they also have the advantage of being able to take a topping of compost for infiltration to the soil layer through watering or rains.
There is also the option of pulling back the mulch for compost or other fertilizer application then the mulch would be put back into place.

In partially arid climates, where rainfall occurs for only a specific period of the year and with low humidity, there is the issue of evaporation being the biggest factor in water retention.
In these areas a thicker much may seem to be the best choice but once again there are limits as to how thick a layer the plants will be able to tolerate and still produce maximum crops.
Again you have the same choices of materials to use but you might find that something like straw is the best choice for maximum water retention, or a lasagna style could be found to be the best choice.

In areas of rainforest type climate such as is found along the upper West Coast, it might be best to have only a thin layer of mulch covering a topping of compost or other fertilizing medium.
Wood chips in this climate would most likely break down at a rate where within two or three years they could be planted into directly with no need to pull the mulch back for planting.
Straw in this climate would also break down quickly so it might be a good practice to use it along with woodchips and compost in the lasagna methodology. Mulching in this manner would not need to be very thick at all, perhaps a maximum of 4".

As with all things in the growing world, experimentation is the best method to discover what works best for your particular situation. Best results can be found quickly by using smaller plots for different content and thickness of mulches.
By following these scientific methods you can even determine which thickness is the threshold for each crop you grow and thus be able to make adjustments for each situation for maximum productivity without suffering huge losses along the way to determining what works best for which crop.

Example one: Maize (corn), In the high humidity, high heat of Arkansas, USA Maize responds best with a three inch thick mulch of loose straw or wood bark chips.
This type of mulch retains water in the soil but also allows enough air to get to the soil so that the fungi and molds that most affect the ears and stalk can not get a strangle hold on the plants.
When the mulch layer is put down at a thickness of four inches, problems begin to arise from molds having the correct environment to thrive and infect the stalks with rot. This causes stalk fall and crop loss.

In this same environment wheat benefits from a straw mulch that is no thicker than three inches, thicker causes stems to soften and rot can set in allowing the wheat to be blown over by light winds resulting in crop loss similar to storm damage.

After seven years of trials in Arkansas I determined that for Pole and Bush beans, Corn, Squash, and Cucumbers a lasagna mulch of 1" compost covered with 2-3 inches of either straw or wood chips or a blend of the two results in good water retention and reduces the mold/fungus problems created by thicker mulch layers of any material. If I need to break down a grassy/weedy area for a new bed, putting down two layers of cardboard then covering with a minimum of 5" of straw, wood chips, or a blend of these two will effectively smother the undesired plants and prevent them from coming back if left on for a 12 month period. Morning glory and the other vining plants require a triple layer of cardboard instead of two and the mulch needs to be a minimum of 6" thick when packed down.
Strawberries are best mulched with straw and it should be kept away from the root crown to prevent diseases from becoming established and killing off the plants.

When you are prepping your garden areas and mulching at the right coverage for your particular area, you are minimizing the amount of water needed as well as providing nutrients and preventing invasive plants from gaining the advantage over your plantings.
This leads to better yields with less inputs of fertilizer and less sweat equity.
 
steward
Posts: 5419
Location: Cache Valley, zone 4b, Irrigated, 9" rain in badlands.
2051
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I have pretty much determined that for my lifestyle and world-view, any amount of mulch is too much... Because mulch requires money and/or labor to acquire and to install into the garden. Even free mulch isn't free if I have to buy gasoline to haul it, or provide labor to spread it around. This year I'm focusing on nurturing a sick family member so I cut way back on gardening. I'm only planting about 80,000 square feet this summer. Nevertheless, even with the reduced garden size, installing mulch on that much ground would cost more in labor and materials than I am able to provide.

I recently tilled a garden for someone. It was approximately terracotta bedrock type clay. He's planting corn. We intend to till that into the bedrock in the fall. That's as best as I'm able to do regarding mulch.
 
Bryant RedHawk
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Location: Arkansas - Zone 7B/8A stoney, sandy loam soil pH 6.5
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I am very sorry to hear you have a sick family member Joseph, I hope they get better and I will offer up prayers for them.

Mulch is indeed not for everyone or even every situation. I know a little about your setup from photos you have posted and I agree with your assessment of your situation.
 
Joseph Lofthouse
steward
Posts: 5419
Location: Cache Valley, zone 4b, Irrigated, 9" rain in badlands.
2051
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Thanks Bryant: We are getting along in years. Recovery seems beyond normal expectations of the human condition. And how would I even approach changing the less than optimal lifestyle choices of others when I can barely address my own, and then only occasionally? The best we hope for is to muddle through in relative peace until we don't.

I loved mulching my garden when I lived back East, and the rain fell abundantly and generated lots and lots of organic matter which was viewed as a nuisance and people would practically pay you to haul it away for them. I didn't like the damping off and mold problems, but the sandy soil I was working with really performed well with added mulch.
 
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I rarely use a mulch that is less than 6 inches thick. Usually Ii like 8" of mulch for annuals and up to a foot on perennials. Ii should qualify this by saying that live in a hot dry Mediterranean climate. The issue with fungus should not be if the soil and plants are healthy. Consider spraying with compost tea or horsetail spray.
 
pollinator
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Someone in my neighborhood says he got rot problems from mulching just with the leaves off his tree.  He chopped them up with a mulching blade. (We are about 40" yearly in even distribution).  He also said it started after the drought a few years back.

I can't imagine that little mulch is the problem.  More likely that he waters with garden hose.

I'm confused since he says he watered yet the drought started this problem.

But can a mulch be a carrier for a fungus? Isn't it more an environment for the spores that were already everywhere anyway? (Pasteur at the end of his life: "le terrain, cest tous" the environment is everything. )

Any thoughts?

By the way, even USA Today(!) has said mow and drop, don't rake, your leaves (!!) The conversation started bc the weather was rough and people posted that the Town wasnt picking up their leaves.  I said "USA Today has printed an article saying it's better to mow your leaves and drop them than rake and bag them." But then he had actually tried years ago and had this problem.

 
pollinator
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Example of too much mulch ->

https://www.thetreecareguide.com/volcano-mulching-can-kill-your-tree/





I killed a citrus tree by having too much mulch at the base of the tree I put 10 inches all the way to the base of the tree...

But if we look at nature I wonder in a forest how deep does the leaf layer get in the forest each year?     I know if you have certain species of worms they convert the leaves into compost very quickly....

But I learn how much is too much by killing the plants myself...     then finding the right amount as time progresses.

That said I have beans that come up thru 1 foot of wood chips,     which really shocked me.      
 
master steward
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Location: Pacific Northwest
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When I go out into my forest and look at the soil under a tree in the fall after the leaves have fallen, there's about 3 inches of leaves in the deepest spots under the Big Leaf Maple. Usually it's more like 1-2 inches. Once it starts raining, that'll flatten down easily to an inch or less. Under that layer, it's about 2 inches of decomposing leaves and earth. The earthworms make pretty quick work of the previous years leaves. And, below that is nice soil.

The forest microbiome decomposes the mulch rather quickly, and the earthworms, etc mix it up with the lower layers.

If I want to mulch a 4x8 foot garden bed with 3+ inches of wet leaves, I have to steal pretty much all the leaves from 1 full grown maple tree. It makes sense to me that more than 3 inches of mulch is usually too much for most plants, as it's really not something you'll find too easily in nature.
 
Mart Hale
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Nicole Alderman wrote:When I go out into my forest and look at the soil under a tree in the fall after the leaves have fallen, there's about 3 inches of leaves in the deepest spots under the Big Leaf Maple. Usually it's more like 1-2 inches. Once it starts raining, that'll flatten down easily to an inch or less. Under that layer, it's about 2 inches of decomposing leaves and earth. The earthworms make pretty quick work of the previous years leaves. And, below that is nice soil.

The forest microbiome decomposes the mulch rather quickly, and the earthworms, etc mix it up with the lower layers.

If I want to mulch a 4x8 foot garden bed with 3+ inches of wet leaves, I have to steal pretty much all the leaves from 1 full grown maple tree. It makes sense to me that more than 3 inches of mulch is usually too much for most plants, as it's really not something you'll find too easily in nature.



The best mulch I had was when I buried 20 inches of leaves   with 10 inches of wood chips on top.       The worms broke down all of the leaves and the soil left behind grew some of the biggest pumpkins I had.     So getting the leaves broken down by the worms to make like worm castings is the way I like to prepare an area to plant.
 
gardener
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I think it not only matters where you are, but what you are mulching, and what your goal is.  I put weed cloth down and covered it about 6" of wood chips In my garden paths.  I planted two grapes by the back garden fence.  The first year they didn't do well, and I thought it was because I planted them too late, but the next year when they didn't die, but didn't grow I knew something was up.  I did some research and one site said grapes didn't like there roots too shaded, not to mulch it.  I moved the mulch back from the grapes and they have been growing and doing well ever since.  An other place I'm trying to keep the weeds from growing.  I put an 8" layer of wood chips down and the Johnson grass still grows through it.  Its less than without the mulch, but still a lot grows through.  I guess I will dump a few more inches on and see if that does the trick.  This year I will mulch my veggie garden for the first time, in the past I didn't do it because I was afraid the mulch would smother the veggie seeds, but it seems if its not to thick it wont.  What is a good place to start?  2" maybe and adjust as needed?  I live in N. California zone 9b, and I look forward to trying this, I think it will help a lot.  
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