There are many mulch types available for your organic garden, but which is the best mulch for you?
This article explores some of the most popular types of mulch and ultimately comes to a conclusion with what you should use.
What is mulch? It's really anything that we put on the soil surface to cover the ground.
Landscaping fabric is considered part of our mulch because it is often placed on the soil under various types of mulch in order to help control weeds. The cheap stuff doesn’t work very well, but thicker fabric can work for awhile before weeds start to find their way through the cracks or just start on top of the mulch.
Unfortunately, that thick landscaping fabric can also stop water from getting down to the soil, especially on a slope where the water just slides down the fabric to the bottom. It doesn’t take long for the landscape to show signs of suffering in this case.
But the biggest problem with this fabric is that it doesn’t allow organic matter to recycle into the soil. When you put landscaping fabric on, it means your soil doesn’t get to eat anymore. This is definitely not the best mulch for organic gardening purposes.
Soil needs to be consistently replenished with organic matter, so any of the mulch types we choose have to be composed of organic matter.
Soil is replenished in nature and in our gardens when leaves fall in autumn, and since many of our gardens are low in organic matter anyway, it also happens when we intentionally bring in more leaves, straw, compost and other organic matter to improve the soil.
Putting landscaping fabric in the garden stops all of this and slowly kills the fertility and structure of the soil, and everything living in it. The only potential use is on pathways, since we are compacting them anyway and not trying to increase the organic matter. But we need to look elsewhere for the best mulch.
When it comes to choosing a good mulch, we need to think, what is mulch for?
Weeds. A continuous thick, dense layer of 2”-4” of one of the best mulch types is one of my favorite ways to control weeds because not only does it smother most of them out, it makes the ones that do find their way through so much easier to pull, especially if you have been clever enough to regularly hit the garden (and the mulch) with some water.
We have other organic gardening chores to do, so eliminating most of the weeds is a good goal. It may be necessary to kill some tap-rooted or perennial weeds before placing the mulch on top of them.
In addition, maintaining a dense, multi-layer plant cover on your soil consisting of a groundcover below and flowers, shrubs and trees above will stop most weeds from growing.
But the reason organic matter is that best mulch is that it provides a huge number of benefits to your garden. In fact, t is one of the most important things you can do. If you use an appropriate kind of mulch (we'll get to what is "appropriate" soon), here are the other main benefits:
Soil Health. The best mulch types are persistently and continuously working to improve the health of the soil. They are being broken down by microbes and increasing the organic matter content of the soil.
Organic matter is an incredibly important part of the soil. It improves soil structure, and as it is broken down, its nutrients are releasing into the soil, making it one of the only types of mulch that improves fertility. It prevents compaction from us walking in the garden and erosion from the wind or gravity on steep slopes. It also moderates the soil temperature, which is good for anything (plant, animal and microbe) living there.
Water. Organic matter is the best mulch because it is broken down into humus, which has an incredible ability to hold onto lots of water. But even before it is broken down, mulch holds a lot of water on its own and allows it to more slowly infiltrate into the soil.
It also reduces compaction (and leaching of nutrients) caused by a heavy rain, and erosion that happens when a lot of rain falls and there is runoff. Conversely, when the sun is shining, it also prevents evaporation from the soil surface.
So the best mulch actually improves the biodiversity of your entire soil ecosystem by giving all manner of critters a place to live, food to eat and water to drink. It even looks good in the organic garden if one of the right types of mulch is used.
So what is the best mulch? Well, it has to satisfy all of the conditions in the above article, so we can probably figure it out by a process of elimination. Let's start with mulches that satisfy very few of our conditions and get rid of them right away:
1. Stones or gravel provide some of the benefits in that they protect the soil from erosion and decrease evaporation, but they do not breakdown and so do not do much to improve soil health. They are not one of the best mulch types.
2. Bark mulch and wood chips are no good even though they are some of the most commonly used mulching materials in the garden. You may have seen them in bags or in bulk at the garden center, or you may have used them in your organic gardening before. They do most of the above things well, but unfortunately, they have a couple of big problems making it one of the types of mulch I never use.
The first is that they are very high in carbon and very low in nitrogen. This means that the beneficial microbes can pull all of the available nitrogen from the surrounding area, which often ends up causing a nitrogen deficiency in your plants.
And bark in particular is very low in nutrients (so it doesn't improve soil fertility) and often high in toxins (it’s a tree’s first line of defense against pests), so it causes toxicity problems in the soil. It even contains oils that repel water, rather than more appropriate mulch materials that will hold onto water.
3. Straw and Hay are not the most aesthetically pleasing, but they are fairly good types of mulch. They are used in organic gardening, but the main issue for most people will be that it is not always easy to find and it breaks down so quickly that it has to be applied multiple times a year.
You may not want to use straw or hay from ryegrass as it has toxins in it, and definitely not from grass that has been sprayed with pesticides such as Roundup, which is common in many countries. The difference between straw and hay is that hay has seeds, so it will often actually produce weeds.
4. Grass clippings are not the best mulch to use in organic gardening because they get so tightly packed together that they inhibit air circulation. Besides, they are far too important for the soil of your lawn to bring into the garden. They do not contribute to thatch or any other lawn problems, but they provide many benefits so they must be left there.
5. We're getting closer to our my favorite of all mulch types, but not yet. With all this talk about organic matter, why not just use compost? A little bit of thought tells us why. It does a lot of things right, but fails to stop the weeds! The same goes with manure, and manure needs to be composted before applied to soil anyway. We should use compost and manure, but they are not really the best mulch. I cover compost in detail in the Smiling Gardener Academy along with cover crops because they are both excellent ways to increase the organic matter content of the soil, but here, we're looking for the best mulch.
What is mulch, I mean the best mulch - what is it? It's organic matter and it's provided by nature. In a well-designed organic garden, this is one of the only types of mulch that magically appears in our beds in autumn, protecting the soil over winter, and breaking down throughout the following spring and summer until a new batch magically appears in autumn.
Of all the mulch types, by far the best for organic gardening is: leaves! They do absolutely everything right. That's why when we're designing our gardens we want to make sure to use plants that make a lot of leaves - not just evergreens - and we want to design the beds to catch all of these leaves.
Those that fall on the lawn and non-garden surfaces can be raked into the gardens or mulched right on the lawn.
If you don't have enough leaves, your neighbors will usually be happy to give you theirs, since they would otherwise have to rake them up and dispose of them. In many cities, you can rake your leaves to the curb and a big truck will come by to pick them up. But why would you want to give the best mulch ever away unless you have too many?
Ironically, some organic gardeners do this and then buy the leaves back as leaf mould in the spring. Leaf mould is just leaves that have been slightly decomposed. Leaf mould is one of the best mulch types, too, but in most cases, the gardener would have done much better to save the money and keep the leaves in the garden over winter where they can protect the soil.
If you have a thick enough layer of leaves in your garden (2" to 4" is nice), many weeds will be smothered. You will still get some weeds, but they will be so easy to pull that it won't matter. You can just drop them back on top of the leaves to become part of the best mulch ever.
Some people think leaves are not one of the most attractive types of mulch for the garden, but is a forest floor unattractive? Or is the forest floor covered in 2 inches of bark?
We’ve been conditioned to think that bark mulch or bare soil is the most aesthetically pleasing, but if you covered your organic garden in a rainbow of autumn leaves, I think you’ll see it differently, especially now that you know all the benefits they provide.
When we remove the leaves, we are breaking nature's cycle and creating more work for ourselves.
So leaves are the number one best mulch.
There is one other organic gardening material that doesn't take the place of leaves as the ultimate of all the types of mulch, but it is beneficial to have as well. It's called a living mulch, i.e. plants. A living mulch is a dense plant cover on the soil, especially low growing "cover" crops.
These can be annual crops (also known as green manures) that we plant in our vegetable gardens to protect the soil during certain times of year and to provide organic matter to the soil, or perennial plants (also known as groundcovers) that live permanently in our ornamental gardens underneath our flowers, shrubs and trees.
A good goal is to make sure all of your soil is consistently covered with plants, and cover crops help achieve this goal. Plants send out hormones in the vicinity of their roots that tell weed seeds not to grow.
As long as your organic garden is dense with plants and leaves (the best mulch ever), and your lawn is dense with healthy grass, those weed seeds will mostly stay dormant and you have a whole list of other benefits to which you can look forward.
Actually, when I was a kid, I recall my dad would pile a bunch of leaves in the back of the pickup truck (we lived in the country), head down the street to where there were no houses, drop the tailgate, and hit the gas. It was so much fun watching the leaves get caught by the wind and cover the sky like a thousand red and yellow butterflies. In hindsight, I have no idea why we did this, but it was fun at the time.
I know someone reading this is wondering about oak leaves. I've never had a problem with the fact that oak leaves don't break down quickly. I've always enjoyed that about them because it just means my mulch stays around longer. And nope, they don't acidify the soil. But again, if you have too many, don't force it.
Feel free to ask a question or comment below.