Why mulch your garden?

The main reasons we mulch are:

1. Keep moisture in the soil. More specifically it can reduce the loss of water by surface evaporation by 70%.
2. Smother or at least suppress existing weeds. Not only do weeds look unsightly, they compete with your plants for moisture nutrients, space and light, and if left to flourish they will out compete many young plants.
3. Reduce weed seed germination. Seeds that do not touch the soil have less chance of germinating.
4. Mulching protects the soil. A good layer of mulch helps to insulate the soil and keep it cooler in hot weather. This has two benefits – one the plants’ roots don’t stress, and secondly the earthworms and other beneficial soil microbes are kept happy too.
5. Slow the flow of water across the soil to increase absorption and decrease erosion.
6. Keep fruit of crops such as cucumbers and strawberries off the soil to help prevent disease or spoiling.

So what do you choose?

That is a personal choice usually based on looks. In the vegetable garden the best mulches are straw based mulches which break down more rapidly and add organic matter to the soil. Not everyone wants this to happen elsewhere in their gardens. Many prefer the bark based mulches which will stay there and look good for a year or two. In my garden I love using pea straw as it is from local SA farmers and I know that it is improving my soil each time it does break down. The peas that shoot don’t worry me as I know that being a legume, they too are improving the soil and if I just nip them off at ground level, the pea stays in the ground and keeps fixing nitrogen in the soil. I am not a fan of sugar cane mulch which travels from Queensland and as a result has a high carbon footprint, although when speaking to gardeners in Northern NSW or Queensland I do promote it (in other words use what is local!). You can also use inorganic products such as gravel or crushed brick as a mulch. These are great in high fire prone areas or around plants like succulents which do not like to get too wet.

How much mulch is needed?

This depends entirely on the type of mulch – a heavy bark mulch should not be as thick as a light straw based mulch which is full of air. I use pea straw from bales and apply it in biscuits around 10cm thick but if I use crushed brick, it only needs to be around 2cm thick. As a very general rule of thumb, apply a 5-7cm layer of mulch to your garden and your pot plants. Remember that mulch is not there for purely cosmetic reasons to cover the soil. A thin layer is not effective but by the same token put it on to thick, especially near the trunks of plants and you will cause problems.

More about Mulch including the difference between Compost and Mulch

There is still a bit of confusion between compost and mulch. As a general rule compost is finer and goes in the soil, and mulch is coarser and goes on top. Compost is incorporated into the soil either by digging, scratching or even just by the earthworms and soil microbes mixing it in for you. Its role is to significantly improve the soil’s water holding capacity, as well as feed the living organisms in the soil which improve the soil and feed the plants. So the compost holds water in the soil and the layer of mulch on top stops or at least slows the moisture evaporating out of the soil. As a general rule mulch has less nutrient value to the plants than compost. Just to confuse the issue further, organic mulches do break down and when they do, they become compost and become part of the soil.

Can I use compost as mulch?

The reason that we do not use compost as mulch is that it is composed of fine material. Studies have proved that using fine compost based products as a mulch actually dry your garden out – the opposite effect from what the gardeners were expecting. The reason for this is that wind passing over fine mulches draws moisture up from the soil through wicking. Also, if there is a light shower of rain, the fine products actually absorb moisture within themselves rather than letting it pass through to the soil. Finally weed seeds germinate readily in these fine materials, just as if they were seed raising mix.

Does every plant need mulch?

It is also important to understand that not all plants need to and in fact should be mulched. Some plants require baking to flower successfully. These include bulbs such as belladonnas (also known as naked ladies or Easter lilies) and Josephine lilies. Mulching these bulbs may result in reduced flowering. Bearded or flag iris are a rhizome which grow on top of the soil and it is often suggested that these shouldn’t be mulched as their rhizomes like baking and covering them will reduce flowering. However their rhizomes can actually suffer if they get baked too much so a light layer of mulch over our harsh summers is beneficial.

Cacti and succulents come from desert areas where there is very little natural organic matter to act as mulch and in fact the rocks and stones act as mulch. These plants are best mulched with inorganic mulches such as crushed brick or gravel. Mulching them with organic material can cause them to rot off.

Finally, if you want plants such as flowering annuals or vegetables to self-seed in your garden, do not apply mulch until after the seeds have germinated, as not only will it suppress weeds seeding, it will also suppress your desired plants.

Myths about Mulch

There are several old myths about mulching that I have heard that are incorrect.

Myth One : mulch = slime

One is that by mulching the ground goes slimy. This is not true. If anything, using organic mulches is likely to increase earthworm and soil microbial activity, which improves the soils friability and aeration under the mulch, which would decrease the hard compacted and water logged conditions required for algae and slime to grow. There is a growth called a slime mould which can sometimes appear on bark mulches after rain. It may be mustard coloured and spongy. Slime moulds are harmless organisms that grow on the wood and bark in the mulch, and is one of the many organisms involved in natural organic breakdown. It will proliferate for a while and the visible signs will disappear after a few days to a week. There is no need for alarm – this nature in action.

Myth Two : mulch = pests

Another myth about mulches is that mulches attract insect pests to the garden. Certainly when you look underneath any mulch you will see many different types of insects, including worms, slaters, millipedes and earwigs, and these would not be present if you were practicing the scorched earth policy instead of mulching. These creatures are not necessarily destructive. They are also part of the natural breakdown process of nature, involved in breaking down the mulch. In most cases they do not cause a problem. In some instances if their numbers build up beyond the natural balance, they may cause some damage and generally the plants which are most at risk are young seedlings which have just been planted out. Protect these with physical barriers around them (such as plastic pots with their bases cut out pushed into the soil for the first few weeks until they get bigger and also treat them with seaweed based plant tonics to toughen them up.

Myth Three : mulch prevents required digging over

While mulching can prevent us from digging over the soil, this means that we no longer need to cultivate the soil on a regular basis. In the past, gardening practice involved digging over soil to help with soil structure, aeration and water penetration. If the soil is covered by a good mulch this is no longer necessary for a number of reasons including increased earthworm activity which aerates the soil and the addition of extra organic matter improving soil structure and preventing a crust forming on the soil surface.

Handy Hint

One common complaint about using mulch is that the birds scatter the mulch and make a mess, especially on paths and driveways. I have seen people use gutter guard, tin strips (which in themselves can pose a significant hazard) and a number of other physical barriers to prevent this. Personally I think that these measures look ugly and greatly detract from the whole purpose of having a garden as a thing of beauty.

The best and easiest solution is to use plants as a border around the garden bed. This is a basic design principle for any garden that serves both an aesthetic and practical purpose. If these plants grow up as little as 15 cm high they will prevent the birds flicking the mulch onto the path or driveway. Not only can these borders be used in the ornamental garden but also in a vegetable garden. I use borders of parsley, thyme, oregano, rosemary and chives in my vegie patch and plants like catmint and society garlic (Tulbaghia) elsewhere. If you are using low groundcovers, they will be acting like a living mulch and further mulching will not be necessary.