Every location in your garden will have different conditions acting on it, like sun, shade, moist and dry areas. These all play a part in how long any type of mulch will last for, as does the material you are using as the mulch.
So, to answer the question on how often to replace it, the logical reply is when the mulch has decayed to one inch thick it should be topped up with fresh mulch.
We suggest leaving it no longer due to the biological harm that can happen if the mulch is left to totally disappear. More on this below.
Why mulch disappears over time.
When an appropriate mulch is place around your plants, it serves some very important uses.
It protects the soil from moisture evaporation and can help against erosion. It also provides wind protection and overheating of the soil.
The mulch surface often looks dry and lifeless, and this is natural. It should create a protective layer that will allow moisture to pass through from above, and this water movement will act on the mulch.
The moisture will expand the mulch, stretching it and creating small cracks and openings in it. This is called weathering and it is a very natural process that cannot be stopped. Everything weathers, even rocks.
That is just on the top side of the mulch.
The underside of the mulch is where rapid decay can occur, and depending on what kind of garden it is, this can benefit or be a pain. The food gardener will be ok with the mulch decay because it is through this process that the soil benefits.
The landscape gardener will look for a mulch that can resist the decay for as long as possible, and that makes sense.
Why mulch decays and disappears is as follows. The contact point between the soil and the mulch is a location where surface biology meets underground biology. If this zone is kept moist the activity is accelerated and it is a major part of the natural process of converting plant based material (mulch) into soil. If the zone is dry, the activity slows down dramatically.
Does old mulch need to be removed?
Old mulch can be home to many beneficial life forms that are suited to your climate and soil conditions so if old mulch is removed you are potentially removing a population of plant supporting organisms unwittingly.
The entire point to gardening is to have healthy plants that can either feed us, or give aesthetic value to a landscape. Removing mulch just to replace it can handicap a garden for a short while, but is rarely a case of killing the plants.
The only reason we can think of where removing the old mulch would be if there is disease involved, and the mulch may harbor fungal spores or similar.
A plant may become sick from external sources and end up dropping leaves that could possibly hold and further spread the problem. The removal of the mulch in this situation makes good sense.
The information in this post is generally pointed towards the grower who has gardens in a backyard, but the people who live in apartments and city units have access to balconies and it is possible to grow ginger in these locations. In these cases the mulch is usually replaced each growing season. “6 gingers to suit a balcony herb garden” goes into more detail for you.
When does mulch need topping up?
Mulch is best topped up when either the soil is starting to become exposed, or the coming season is expected to be a dry one. Fresh mulch can also be timed to coincide with a harvest.
We have very distinct seasonal changes where we live, and at the end of the wet season we try to get a good layer of mulch over the thin areas in the gardens.
This is also a time period when our turmeric and ginger are lifted to be processed in the kitchen. We time the mulch application to an area with the harvesting of these crops. Because we grow both turmeric and ginger together, it makes this process quicker and more efficient.
If you are short on time, this joining of processes might be worth thinking about. It works for us at times.
Will green leaves burn plants if used as mulch?
Green leaves can burn plants if this is the main material the mulch consists of. If the carbon/nitrogen ratio in the mulch is heavily skewed towards nitrogen, this creates the heat as it starts to break down.
The way to get around this issue is to apply the green mulch thinly over older mulch and this should provide a barrier to the plants roots and soil life that you want to keep. This process can slowly replace or rejuvenate older mulch as time goes by. We mention this method in “simple ways to make cheap mulch“.
We use this process a lot, and it is called chop and drop, or slash and mulch as some like to say. Tree branches directly above the garden are pruned and the branches and green matter is placed at that location as you work that area.
It saves time, avoids excess heat build up, and keeps the soil life stable and consistent. We have a dedicated article on this heat issue with mulch titled “Will too much mulch burn plants?“. We recommend it.
How to grow your mulch where you need it.
This is a subject that could have a complete article written on it. It is a basic under-pinning of the design science of Permaculture. As a plant grows, lets say a nut tree, it requires very different conditions as a seedling vs the fully grown tree.
This change is not a sudden event but a slow transition over time, and this is where we can intervene.
It is in the trees infancy where growing mulch beside the seedling comes into its own. The mulch plant can provide shade and a structure to tie the young plant to help support it.
Many times this nurse plant is a legume that has nitrogen nodules attached to the roots and when this nurse plant is pruned, some of these roots are dropped from the plant.
The nitrogen nodules are released and are now available to the seedling nut tree. As the nut tree grows, the nurse plant can be pruned until the seedling requires more light and exposure.
The nurse plant is then removed at ground level and chopped up as mulch and placed around the nut tree. It is a copy of natural plant succession that happens all the time throughout the natural forests of the world.
We just borrow the method and harvest the value through fruit, nuts, and other useful materials that trees provide us.
A simple question about replacing mulch is not so simple after all when you understand what is really going on in your garden. The information above has helped us move to an almost self-sufficient home life so we know it is based in fact.
We have several articles about mulch here on site, and you can find them all in 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.