Mulch

Mulch is a broad term for anything that covers and protects the soil. Usually it refers to organic material that is fairly finely cut and spread over gardens to prevent water loss through evaporation or runoff.

Most folks think of the various types of straw – pea, barley, lucern, wheat -when they think of mulch. Sugarcane and chipped woods are other popular organic mulches carried by local hardware stores and garden centres.

Mulch can be inorganic too. Gravel can count as a mulch as can recycled and granulated rubber or plastic. Plastic sheets are used as mulch in commercial strawberry operations too.

That's a lot of mulch!
3 cubic metres of mulch…Ouch! My back!

Why mulch?

Mulch is used for several purposes. Some mulches are aesthetically pleasing while others are more ‘functional’.

Mulch helps reduce extremes in the root zone, helping both plants and the complex web of microbial life that lives there from too much Sun and heat. Mulch helps soil retain moisture so the plants have access to it for longer. . Many mulches are also full of nutrients which become available to your plants as the mulch breaks down. I’d estimate that a mulched garden bed uses at least 50% less water than an un-mulched one.

Mulch also encourages mychorrizae, those white threads that you see under older mulch and compost, these are absolutely fantastic for your garden and are the organism of which mushrooms are only the fruiting bodies. Mycelia are how the fungi and plants communicate and they break both inorganic and organic materials down for plants to uptake. They’re so good that I’ll be dedicating whole page about them later.

Mulch can be used to cool your garden and home in Summer. It keeps the soil cool and the moisture in it evaporates, cooling breezes that pass over it. Also, it absorbs some of the heat and reduces heat being reflected from the ground onto your plants or home.

Of course, mulch can be used to suppress weeds, but, after you’ve learned about all of its other benefits, you might find that aspect is kind of boring.


Things to look for in a mulch

You will be looking for different things depending on your purpose for your mulch. You can look at aesthetics, water retention or nutrient supply.

Shredded wood mulch
Finely shredded wood mulch
Fine wood chips
Fine wood chips

River pebbles and big bark chips are great for aesthetic purposes, as is grape marc (the leftovers from grape processing at wineries) but For more practical applications, you’ll want something that’s not too green, too moist or too fine. That rules out masses of fresh, uncomposted grass cuttings which tick all of those boxes.

Grass clippings
Fresh grass clippings are not the best mulch. Compost them first!

Anaerobic breakdown

Everyone’s had that experience of making a pile of grass cuttings and promptly forgetting about them or just putting them out of their mind for a bit…then forgetting about them. When you return and dig into the pile, most of the time only to find that the base of the pile has matted together and gone soggy, black and smelly.

This is because the clippings tend to break down anaerobically. ‘Anaerobically’ means ‘without oxygen’ and is a very slow method of breaking down organic material. It takes a special kind of bacteria to live and work in this kind of environment. We’ll deal with this more on our ‘composting’ page.

The clippings are too fine and moist and too densely packed to allow either air or water to pass through. They’re even dense enough to stop some insects from getting through. If you’re going to use fresh clippings, mix some fine straw with them to allow air flow to the materials.

Most straw mulches are coarser and the pieces are harder than grass clippings, so they’re ideal for mulches. If you buy your bales fresh from the farm though, look out for insects and weed seeds (we’ve had mice in ours!) and you might have to reduce the size of the pieces a little, This is easy to do with a whipper snipper or lawn mower.

At Ligaya Garden, we take a slightly different approach and all fresh organic material arriving on the block for the garden goes into the chicken pen for a while to be mixed in with the deep litter there. The girls love picking through it and eating the bugs and seeds while breaking it into smaller pieces ready for the garden. They also poo in it and mix that in so that the mulch has a special kick to it. It gets mixed with remnants of other food scraps too, so is really a superfood for the garden.

Chooks do all the hard work
Chooks do all the hard work

Compost vs mulch

Compost, not mulch
Beds filled with compost, then with mulch on top of that.

Size is the main difference between compost and mulch. Mulch has much larger pieces in it. It also hasn’t been broken down as much as compost and isn’t as readily available to your plants. It doesn’t mix into the soil as well as compost , being used more on the surface. If you sieve your mulch, you’ll end up with fine compost sized pieces.

You can use compost as mulch but mulch doesn’t make good compost. Don’t dig your mulch into the soil. It’s too coarse and will take forever to break down. In ideal situations, mulch will become compost, but that can take a while.


Many mulches, composts and potting mixes contain a lot of fine dust and even bacteria and fungi. Inhaling any of this can be bad for your health. Try to remember to wear a mask when handling any of these. If a mask isn’t available you can go ‘cowboy’ and tie something around your face.


I always dampen down anything like this that I use. It reduces the amount of dust flying around and the chance of me or others breathing it in.

Some brands of packaged mulch, such as sugarcane mulch from the hardware store or garden centre have had the dust extracted as they were packed. It still pays to err on the side of caution though. When I get the occasional bag of sugarcane mulch, I take an extra precaution – I make a hole in the plastic bag and fill the bag with water. By the time I’ve had a cup of tea or done something equally useful, its soaked enough and I tip it out where I’m going to work with it. That way, there’s no dust.


Bacterial vs Fungal

This is one for the professionals! Different mulches can help to encourage different populations of organisms. Briefly put, green mulches such as shredded leaves or a light sprinkling of grass clippings will encourage bacterial dominance in the soil (which is great for annuals). Brown mulches such as straw and wood chips will encourage fungi. This is a good thing for trees and perennials. I’ll be writing lots more about this topic on its own page soon (by the end of 2020 at the latest).


How to mulch

You can, of course, just throw your mulch around. It’ll get to the soil eventually anyway, but I’ve found the following to be a good guide to applying mulch –

Take the following steps and your mulching efforts will surely pay off –

  • clear away any weeds and generally tidy up the area to be mulched
  • wet the soil to be mulched
  • add some fertilizer to the cleared soil to help balance possible nitrogen loss (nitrogen drawdown).
  • lay out any weed mat that you might be using if you want that extra layer of protection (see ‘sheet mulching‘)
  • if you are using cardboard or newspaper as a weed mat, make sure it is thoroughly wet before adding the mulch
  • lay the mulch to a depth of 10 cm and wet it well
  • don’t mulch right up to the base of your plants, leave a 3 -5 cm gap around them to stop rotting of the stem

There you go. Mulched.


Yellowing and Nitrogen drawdown

The nitrogen rich fertiliser isn’t essential but some mulches will draw nitrogen out of the soil (or rather, the bacteria decomposing them will), which means that it will be unavailable for the plants until the mulch has broken down a bit or you fertilise again. This is the reason that sometimes, after they mulch, gardeners sometimes find annual plants that they’ve recently mulched turning yellow. In most cases, that’s simply a lack of nitrogen and is easy to fix with a little fertilizer. This has the technical name of ‘nitrogen drawdown’ and I like to fix it with a little liquid chook poo extract.


Sheet Mulching

Sheet mulching is when you cover a large area with some fairly large pieces of material. Newspaper and cardboard are the most popular materials to use. It can make a very effective weed barrier and save a lot of money on mulch.

Sheet mulching
Athena inspecting our sheet mulching efforts

When you sheet mulch with paper products, try to use non-coloured ones and don’t use glossy paper. The colours often contain toxic dyes and the glossy ones take forever to break down (if they ever do). Remember to take the sticky tape off of things too. It’s annoying to find it in your garden a year later. Pull off those little plastic windows on envelopes too. They’ll not break down in your lifetime.

It’s OK to leave staples in the paper. They’re pretty small and usually made from steel which will rust. Just think of them as a little iron supplement for your garden.

As with other styles of mulching, wet the soil first and make sure you wet the mulching material first. This also helps prevent it from blowing around and making a mess. Usually other mulching materials such as straw or compost are applied on top of the paper products.


If your mulch becomes water repellent

If you apply bone dry mulch or let mulch you already have on your garden dry too much, it will become hygrophobic (or hydrophobic…they’re two words for the same thing). This means that they will repel water.

New bales of dry straw are notorious for this. The only way to remedy it is with frequent applications of water. Don’t just pour water on it for an hour though, most of it will simply run off. Keep your sprinkler or water gun putting out a fine spray and work it over the dry mulch then come back in half an hour or so and do it again. Repeat this until the water can be seen soaking into the mulch and not running off.

One trick that I’ve learned when using the packaged bales of sugar cane mulch is to open the bag at one end and fill the bag with water. Leave while you have a cuppa or do something equally useful and then empty the contents onto the soil. Much of the water will have soaked into the sugar cane. It will also reduce the dust that come from some brands.

If you have very thick mulch, the solution is not as drastic. Thick mulch rarely dries out completely in a garden situation. It is often still damp at the bottom. If the top has dried out and become water repellent, turn the mulch over with a fork and mix the repellent with the absorbent. Then water well. The absorbent will hold the water and let it soak slowly into the dry material.

A third solution is to wet the mulch, then cover it with plastic or a tarpaulin. Leave it overnight then come back in the morning, remove the cover and give it a bit more water. The humidity and moisture trapped in the mulch by the covering will have worked its way into the dry material.


Inorganic (permanent) mulches

Organic mulches are great for plants in so many ways but sometimes they’re not always aesthetically pleasing (to some folks anyway). The good news is that lots of other things can be used to protect the soil. Some folks call these ‘permanent mulches’.

Some types of mulch used by landscapers.
Some types of ‘mulch’ used by landscapers.

Many things that are used as an inorganic mulch are used to tie the garden’s parts together visually. Gravel, river pebbles and crushed bricks can provide a pleasing visual effect while protecting soil from the extremes of the weather and they can help retain moisture in the soil. For those reasons, I’ve included them in this article. There’s even recycled plastic and rubber available but I’ll let you make up your own mind about them.

I’m this very broad category of ‘inorganic mulch’, I also include gum nuts and the really coarse, coloured, wood chips that are available. They don’t add any benefit to the soil in the short to medium term.

Often folks will put a layer of plastic or weed mat under these inorganic mulches to help stop weeds coming through. River pebbles and such are very expensive and you don’t want nasty weeds thumbing their noses at you through spaces between the stones. One of the great gardening stand bys in this area is old carpet underlay. This is easy to find and thick, making it a good backup. Carpet too is OK but be careful of snagging its threads and getting entangled and embarrassed by dragging long ropes of the stuff as you walk.

These materials generally go on around 60 mm thick. Maybe a little less if you use weed mat or plastic underneath.


Seasonal mulching

While mulching can be done with the blanket approach of a thick layer a couple of times a year, gardeners who are more in tune with their gardens will quickly learn that each season has its own needs and will begin to mulch accordingly. Seasonal mulches can be dug into the soil at the turn of the next season. They’re well broken down by then.

Winter

Mulching for Winter protects bare soil (should there ever be such a thing?) and helps protect plants from frost. For a while at the start of the season, it helps keep the temperature in the soil. It can be used on a larger scale to prevent runoff and and having soil washed away.

A thick layer of mulch applied at the beginning of the season will see you through until Spring. It should have been broken down a lot by then too, providing nutrients for the life in the soil.

Spring

Mulching for Spring is done from a different perspective. It is also part of the run up to the ever more severe extremes of Summer. Mulching for Spring is about letting the new season’s warmth get to the soil while putting a warming blanket back on the soil for the still cold nights and helping the slowed down the nutrient cycle kick off again. It is different to mulching at other times of year because, we are only adding a thin layer of organic material, stuff that will break down quickly while allowing and protecting new growth that is starting to pop up.

Summer

Mulching for Summer is all about protection. Protecting the soil and its inhabitants from the extreme heat while protecting and conserving precious water in the root zone. As part of a climate sensitive garden design, it can be about reducing reflected heat reaching the house as well.

Summer is about thick layers of mulch. Hefty, insulating blankets of the stuff applied early in the season can see plants comfortably through the hot dry times ahead. It can help to greatly reduce water use over the season, conserving a valuable natural resource and saving you a few dollars at the same time.

Autumn

Autumn mulching is similar to Spring. We want to allow the warmth of the Sun to reach the soil on the still warm days and we want to provide enough goodness to the soil to see the life in it through the cold of Winter.


Tip:

Mulch with the rain. Mulching while it’s raining or just after can help the rain reach the soil better, providing a better home for microbes before the mulch is added. Rainwater on your mulch will always be better than tap water.


There are other useful water saving ideas on this website that you can check out with a click – Gardening in the heat and Water saving approaches to gardening

The different types of straw

Walk into a fodder store and you’ll see a bewildering array of straw bales. They all have a purpose to agriculturally minded folks and can have different benefits in your garden.

The most nutritious of the mulches is Lucerne hay. It has more of everything than the other types and has a high level of protein, making it a superfood for worms. It’s also known to stimulate root growth.

Pea straw is good to use in the drier weather, it has a lot of good stuff in it but I’ve found that in the wet weather, it can attract a black mold. I wonder if that isn’t on the straw all of the time but just inactive. The stalks are hollow and can make a nice home for earwigs. A big bonus is that you’ll even get free peas from it as it gets wet and they germinate.

Sugar cane mulch is whats left over after all the goodies are extracted from the sugar cane. These goodies are, of course, sugar and molasses, so its not rich in nutrients. As a mulch, its benefits are that it comes in a convenient pack, some brands have extracted the dust and debris from its processing and also its very finely shredded, meaning that it breaks down quickly and there’s no spaces for earwigs to hide in.

Wheat straw is, to me, the worst of all of the straws that are available. It’s coarse, and relatively low in nutrients. All of the goodness went into making the grains. It is cheap though.

Oat straw has it’s own benefits. Even though the oats are long gone, the straw itself is full of minerals, making it a great way to boost them in your garden. They’re not at the same level as rock dust but are already in an organic form, ready for assimilation by the critters in the soil. Oat straw is used herbally as a way to help people suffering from exhaustion as its minerals are easily assimilable.

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