Guide written by:
John, Passionate gardener, Cambridge
Any plant around the garden or vegetable patch can benefit from mulching. Mulch boasts a range of advantages from reduced weed growth and watering to better protection from disease and the elements. Whether you go for organic, mineral or synthetic mulch, follow our top tips on how best to mulch your garden.
- Advantages of mulching
- Mulching materials
- When and what to mulch
- How to spread your mulch
The advantages of mulching
Mulching consists in spreading a layer of material (called mulch) over the soil on land that may or may not contain crops. Mulching the soil presents a number of widely recognised benefits.
Reduces need for watering
Covering up the soil limits the amount of water loss in the soil through evaporation. This helps to save the gardener water and time.
Limits weed growth
Mulching helps to keep the sun from reaching the soil. This prevents seeds from germinating and, in turn, weakens and kills off young weeds. Mulching is the most effective, least expensive and most natural method to avoid weed growth. It also saves you the task of weeding.
Protects the soil from the elements
Mulching also helps to prevent soil compaction and soil crust formed through the action of rainfall on clay- or lime-heavy soil. Mulching helps to protect plant roots and the soil from the cold or frost in winter and shields them from the heat in the summertime.
Improves soil fertility
Of course, this advantage only applies to organic mulch. Mulching 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
We all know 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, particularly pozzalana.
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 (e.g. strawberries) cleaner when it comes to harvesting.
Materials used for mulching
There are three main types of mulching materials:
- organic materials;
- mineral materials;
- synthetic materials.
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.
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 quickly:
- dead soft leaves (from lime, hazelnuts, ash, birch, hornbeam, robinia trees, and so on);
- lawn clippings;
- nettles, etc.
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 enhance the physical properties of the soil by improving its structure.
This type of mulch is mainly used for perennials, trees and shrubs, and perennial flowerbeds:
- dead tough leaves: plane, oak, beech, maple, bay trees, and so on.;
- pine bark;
- wood chippings;
- hedge cuttings;
- cocoa bean shells;
- hemp or flax;
- any other locally produced material such as coconut shells, olive stones, etc.
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 mulches are not biodegradable and therefore have 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 rockbeds or cactuses as it effectively retains heat in the soil. The honeycomb-like structure of pozzalana (a volcanic rock) makes it a good 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 a lot of minerals to the soil.
Usually consisting of a plastic or textile cover stretched over the soil, synthetic mulch is rarely an attractive option. These mulches are often used on sloped surfaces to hold the earth 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 range a forms:
- ground-up plants and branches;
- grass clippings;
- dead leaves;
- finished crops (e.g. tomato, courgette or bean plants and carrot tops);
- ferns (particularly bracken, which can easily be found by the roadside).
Type of mulch
Hemp and flax
Good moisture retention and provides good humus.
Wind can scatter mulch.
3 to 5 cm (water well after mulching)
1 to 2 years
Strawberries, lettuce, carrots, beetroot, rose bushes.
Cocoa bean shells
Rich in nitrogen and minerals, good hold even on sloped surfaces.
Expensive, exotic product (not environmentally friendly to transport).
Cocoa bean smell.
Fresh lawn clippings
Rich in nitrogen
Available in large quantities and cost-free
Risk of suffocating and rotting plants
Needs to be topped up regularly
Vegetable crops with short life cycles (lettuce, green beans, peas, etc.)
Dry lawn clippings
Available in large quantities and cost-free
Grass needs to be dried in piles and turned over for a few days
2 to 5 cm, topping up regularly
A few weeks
Tender or tough dead leaves
Provides a lot of humus
Provides good winter protection
Blows away easily
Potential for disease spread
10 to 20 cm
6 months to over a year
Annual or perennial flowers, shrubs and rose bushes
Mulched leafy shrubs
Rich in nitrogen and minerals
Can start to ferment if mulch is too thick
Hedges, shrubs, perennial plants, strawberry plants, raspberry plants, various vegetables
Compost + mulch
Applied alongside a thin layer of compost, straw is one of the best mulches available
Retains pesticide residue if product is not organic
5 to 10 cm
Vegetables with high nutrient demands: tomatoes, celery, aubergines, leeks, pumpkins and cucumber
Cost-free, good protection against cold and toxic to slugs.
Difficult to acquire in certain parts of the country.
3 to 6 months
Cabbages, tomatoes, leeks, pumpkins and courgettes.
Will increase soil acidity over time.
Heath plants, non-cultivated land.
Cardboard (no colour printing)
Cost-free, easy to use, very effective against weeds.
Need to remove all tape
Thickness of cardboard used
8 months to a year
Shrubs, strawberry plants, hedges and various vegetables.
Coco fibre, jute or hemp matting (in rolls or square mats).
Easy to use, biodegradable in a few years.
Thickness of mat used
2 to 4 years
Good for large surfaces, hedges, shrubs and perennials.
Easy to access in large quantities, rich in nutrients.
Risk of spreading seeds.
5 to 10 cm
4 to 8 months
Mineral materials: gravel, crushed slate, pozzalana, clay marbles, flat stones, etc.
Cost-free if sourced by gardener (e.g. slate).
Heats the soil up quickly in spring and retains warmth overnight.
Expensive, does not provide nutrients nor improve soil structure.
3 to 5 cm
Lasts for many years.
Ornamental flowerbeds, pots and planters: palms, bamboo, grassy plants and shrubs.
Heats up soil quickly.
Suitable for sloped surfaces.
Tricky to install, soil preparation required, expensive, unattractive as it ages.
Thickness of the material
3 to 6 months
Various vegetables: cabbages, aubergines, tomatoes, leeks, melons, etc.
Prevents weed growth, heats up soil quickly.
Negative environmental impact.
Thickness of material used, lasts for several years.
Planting hedges, perennial shrubs.
When and where to mulch
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 your rows of 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.
How to spread mulch
Before mulching, you will need to weed your land 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 in diameter 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 all the more important when you are using a carbon-rich mulch (e.g. straw) as it won't decompose in the soil without the help of nitrogen produced by micro-organisms. When this happens, plants can suffer from nitrogen deficiency. In this case, you can start by applying a nitrogen-rich fertiliser.
What to do after mulching
Top up your mulch from time to time to maintain 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 you to warm up the soil and avoid pests.
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.
Guide written by:
John, Passionate gardener, Cambridge, 61 guides
When I was young, I was already working in the family garden. Perhaps that is where my interest in plants and gardening came from. So, it was logical for me to study both plant biology and agronomy. At the request of various publishers I have, over twenty-five years, written many books on the subject of plants and mushrooms (a subject that is close to my heart).They were mostly identification guides at first, but shortly after they were about gardening, thus renewing the first passion of my childhood. I have also regularly collaborated with several magazines specializing in the field of gardening or more generally in nature. There is no gardener without a garden, I have cultivated mine in a small corner of Cambridge for the last thirty years and this is where I put into practice the methods of cultivation that will I advise you in as well.