How to mulch your garden and why

Mulching consists in spreading a layer of material called mulch over the soil either on bare land or around crops. Either way, mulching presents a number of widely recognised benefits.

Reduces need for watering

Covering up the soil limits the amount of water loss through evaporation. This helps to save the gardener both water and time.

Limits weed growth

Mulching helps to keep the sun from reaching the soil. This prevents seeds from germinating which will help to weaken and kill off young weeds. Mulching is the most effective, least expensive and most natural method to avoid weed growth. It also saves you the chore of weeding!

Protects the soil from the elements 

Mulching helps to prevent soil compaction and stops soil crust from forming on clay- or lime-heavy soil. It helps to protect plant roots and the soil from the cold and frost in the winter and heat in the summertime.

Improves soil fertility

Of course, this advantage only applies to organic mulch. Mulching using organic materials can help to make your soil more fertile by improving its physical properties and supplying it with minerals and humus.

Enhances soil life

Mulching helps to create a more favourable micro-climate for micro-organisms and insects by ensuring a more temperate soil climate. Soil life is essential for transforming organic material into minerals that can be taken in by plants.

Prevents slug movement

Most of us are aware that wood ash can help to stop slugs in their tracks but a finely ground organic material, such as hemp mulch, can play a similar role. The same goes for mineral mulches, pozzalana in particular.

Protects crops from disease 

Mulch forms a barrier which helps to shield plants from splashing water and earth when it rains. This can also make some fruit and vegetable crops (such as strawberries) cleaner when it comes to harvesting.

There are three main types of mulching materials:

• organic materials;

• mineral materials;

• synthetic materials.

Organic mulch

This is the most common type of mulch and consists of old plant matter. The rate at which the mulch decomposes – and therefore incorporates into the soil – depends on the ratio of nitrogen-rich fresh material and carbon-rich woody material. 

Fast-acting mulch

Mulches that decompose quickly can be broken down by soil life in a matter of weeks. Rich in nitrogen, these types of mulch produce nutritional humus which is quickly incorporated into the soil. These mulches are mainly used in vegetable gardens and for annual plants. The following materials will break down particularly quickly:

• dead soft leaves (from lime, hazelnut, ash, birch, hornbeam and black locust trees);

• lawn clippings;

• ferns;

• nettles, and so on.

Slow-acting mulch

Lignin-based mulch is slow to decompose and will take several months, or even years, to break down completely. While this type of mulch does not contain a lot of nutrients, it will improve soil structure. This type of mulch is mainly used for perennials, trees and shrubs, and flowerbeds with perennials. The following materials are slow to decompose:

• dead tough leaves (plane, oak, beech, maple, bay trees, and so on);

• pine bark;

• wood chipping;

• straw;

• hedge cuttings;

• cocoa bean shells;

• hemp or flax;

• any other locally produced material such as coconut shells, olive stones, etc.

Please note that pine needles and coniferous branches (e.g. cedar or cypress trees) do not decompose well and will increase the acidity of the soil. This type of mulch is best used for beds with perennial plants, shrubs or heath plants.

It is possible to find coconut fibre, jute or hemp matting in rolls or squares. This can be handy for mulching a large surface or a hedge.

Mineral mulch

Mineral mulch is not biodegradable and therefore has a very long lifespan. This type of mulch includes materials such as sand, gravel, stones, clay marbles or pozzalana. Most commonly used as a decorative feature, mineral mulch can also provide a splash of colour to your flowerbeds or plant pots. 

You can even enhance this effect by using two different types of material. To prevent the two varieties from mixing together, choose differently sized materials. This type of mulch is often used for rock gardens or around cactuses as it effectively retains heat in the soil. The honeycomb-like structure of pozzalana (a volcanic rock) makes it a great thermal insulator.

If you can get your hands on them, shellfish shells (e.g. mussels, oyster shells, etc.) can also act as a mineral mulch. The shells offer the added bonus of supplying minerals to the soil.

Synthetic mulch

Usually consisting of a plastic or textile cover stretched over the soil, synthetic mulch is rarely a very attractive option. These mulches are often used on sloped surfaces to hold the soil in place.

If you want to save on spending, why not learn how to mulch with materials you already have? Cost-free organic mulch can come in a number of forms:

• shredded plants and branches;

• grass clippings;

• dead leaves;

• weeds;

• old crops (e.g. tomato, courgette or bean plants and carrot tops);

• ferns (particularly bracken, which can easily be found by the roadside).

Mulching can be done at any time of the year with two exceptions:

• during the winter and any time when the soil is very cold: never mulch on frozen soil as the mulch will only work to retain the cold, making it slower to heat up in the spring. It is best to mulch before the cold season hits in the autumn and to leave the mulch in place all winter;

• at the start of the spring when the soil is still cold. Do not start mulching until the end of April or the beginning of May in order to allow the sun to heat up the ground first.

You can also mulch when your crops are in place. Simply spread the mulch between rows of crops such as leeks, cabbages, spinach, strawberry plants, and so on. Be sure to wait until your plants are developed enough so as not to cover them up.

All types of crops can benefit from mulching whether you're looking to support a vegetable garden, flowerbed, hedge, shrub, tree, rose bush, planter or even potted plants.

Avoid mulching any crops that do not like moisture such as garlic, onions and shallots. Similarly, rock beds should not be mulched with organic material but can benefit from the use of mineral mulch. 

Before mulching, you will need to weed the soil and take care to get rid of perennial plants such as quackgrass, bindweed and rumex. Remove as many roots and rhizomes as possible.

Next, spread a layer of mulch of a few centimetres over the soil (up to 10 cm depending on type). If you plan to leave the mulch in place all winter, do not hesitate to use a thicker layer. 

For hedges, the mulch should extend around 80 cm on each side. For a shrub, mulch around 1.5 metres around the trunk.

Leave a gap of a few centimetres around the plant if possible to leave the space between the roots and the stem clear. This will help to avoid any health issues caused by excess humidity.

Water your plants once the mulch is in place. 

Do not dig the mulch into the earth and let nature do the work instead. This is especially important when using a carbon-rich mulch such as straw as it won't decompose in the soil without the help of nitrogen produced by micro-organisms. As this process occurs, plants can suffer from nitrogen deficiency. In this case, you can apply a nitrogen-rich fertiliser.

Top up your mulch from time to time to ensure there is always an even layer.

If the mulch in your vegetable garden has not decomposed come spring, remove it at the start of the season to allow the sun to reach the soil. This will help to warm up the soil and prevent pests from setting in. 

Each time you sow seeds, rake the mulch into a small pile. You can then put it back into place once your plants have grown a bit.

If you are planting, you don't have to remove the mulch: simply clear a small spot to dig in your plants.