Types of mulch and how to use them

Learn about different mulches and when and how to apply them.

Mulching is a must-do for all gardeners. It is one of the basic techniques in gardening and landscape design. Many people question whether they should use it or not and ask which type is better: organic or inorganic? The answer, depends on your soil type, on what you wish to achieve and your personal preferences. This blog post will help you to decide which type of mulch is best for you and your garden, and how to apply it.

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What is garden mulch?

Mulch is simply a loose covering of material placed over the soil in a layer, and around the plants.

They can be used in your garden borders and to top-dress soil in containers. Generally, they are used to help to retain water, suppress weeds, protect roots in winter and for aesthetic purposes. You may not know that mulches not only include biodegradable matter such as garden compost, but also decorative aggregates and woven landscape fabrics. 

What are the reasons for mulching soil? 

There are many reasons to use mulch in your borders or container. Different kinds offer different solutions for a variety of garden concerns.

Here are the main reasons to begin mulching in your garden:

• To suppress weeds - A layer of mulch that is two to three inches deep is enough to stop most weeds. It smothers seeds that are yet to germinate and prevents sprouting seeds from pushing through because it blocks the sunlight so that they don’t have enough energy. 

• To retain water - Mulch acts as a physical barrier to drying winds, and they lock in moisture in their air spaces.

• To reduce time spent watering - Many mulches retain moisture in the soil, which means that you will not need to spend so long watering your plants.

• To improve soil fertility - Organic mulches break down over time and add nutrients to the soil. Worms take them down into the soil which makes it more fertile too.

• To warm up the soil - A mulch can help to insulate the soil. Dark coloured ones absorb and retain the sun’s heat. 

• To keep the soil cool - Light coloured mulches (e.g. white gravel) reflect light and keep plant roots cooler in strong sunlight.

• To protect roots - Mulches protect tender plant roots against extreme heat and freezing temperatures.

• To encourage worms and wildlife - Organic matter provides food and habitat to worms and other beneficial soil organisms. Worms help to improve the soil structure which aids in moisture retention, drainage and fertility.

• To improve the appearance - Some mulches are purely decorative or help to give your borders and pots a neat appearance.

• To deter pests - For example, Cedar or Cypress wood chips contain natural chemicals and oils which help to repel some non-beneficial insects. Seaweed repels slugs.

• To stop edible crops coming into contact with the soil - Mulches can be used to provide a barrier between your fruit/vegetables and the soil. This stops them from sitting on wet soil and rotting. It also helps to deter ground-living wildlife which may eat your crops e.g. slugs, mice and rats.

Types of mulch

There are two main groups of mulch – biological and non-biological. Biological mulches are also referred to as organic and are made from dead plant material. Non-biological mulches are inorganic and include everything else including plastic sheeting.

Biological, organic mulch 

These break down gradually and release nutrients into the ground. They also help to improve the soil’s structure. As they decompose, organic mulches will need replenishing once they have completely rotted away.

 Examples of biological mulch -

• Garden compost (rotted down) - retains moisture, suppresses weeds, improves soil structure and fertility, darkly coloured compost looks attractive.

• Wood chips - breaks down slowly to improve soil structure, improves drainage, retains moisture, decorative.

Photo by Paul Green 

• Conifer bark (processed) - as above, breaks down slowly to improve soil structure, improves drainage, retains moisture, decorative.
• Leaf mould - low in nutrients, retains moisture, improves soil structure, suppresses weeds, increases fungi in the soil, including mycorrhizal fungi (which helps plants to take up water and nutrients).

Photo by Annie Spratt

• Manure (must be well-rotted) - high in nutrients and improves soil fertility, retains moisture, the dark colour looks attractive, makes use of waste material.

Photo by Hansjörg Keller

• Straw - retains moisture, breaks down quickly, is light-weight, protects the soil’s surface and fruit (e.g. strawberries) from the impact of raindrops, makes use of waste material.

Non-biological, inorganic mulch  

Generally, inorganic mulches do not break down (except for plastic sheets and some weed membranes). They do not release nutrients and can sometimes actually block nutrients from reaching the soil. Non-biological top-dressing can help to suppress weeds and they need replacing less often than organic types. Some of these have the added benefit of looking attractive too.

Examples of non-biological mulch -

• Slate and shingle - suppresses weeds, retains moisture, decorative, warms up/cools down plant roots. Dark colours help to warm the soil up, light colours reflect the sunlight to keep it cooler.

• Gravel, pebbles and decorative aggregates - suppresses weeds, retains moisture, available in many colours (decorative), improves structure as it gradually sinks into the soil, warms up/cools down plant roots. 

• Seashells - suppresses weeds, retains moisture, is decorative, cools down plant roots.

• Tumbled glass - suppresses weeds, retains moisture, is available in many colours (decorative), cools down plant roots.
• Plastic sheets and landscape fabric - suppresses weeds, can block nutrients/water/air from reaching soil, long-lasting, some disintegrate (leaving plastic in the soil).

Photo by Zoe Schaeffer 

When to mulch?

Generally, mulches can be applied two times throughout the year. Apply either in spring before annual weeds have germinated and herbaceous perennials are dormant, or in autumn when plants are dying back. 

Where to mulch?

• Border and containers - Apply around new plants in borders and containers, on established plant beds and around specimen plants. It can be particularly beneficial to apply a mulch around a newly planted plant, at any time of the year, to help suppress competing weeds and to retain water. 
• Bulbs - Apply around spring bulbs when they have finished flowering and the foliage is dying back. This will feed the bulbs and help to retain moisture.
• Hedges - Feed hedging plants and help to lock in water by adding a layer of organic matter around the base. These plants grow closer together and can benefit from the added nutrients.
• Fruit trees and bushes - This helps to retain moisture around their roots, which is essential in fruit production. It will also help to keep weeds to a minimum and therefore reduce the number of available habitats for pests and diseases to thrive in.
• Herbaceous perennials - Perennial plants which die back will benefit from being mulched with an organic material. Apply it before they start growing (mid to late spring) to give them the nutritional boost they need. Dark-coloured ones such as well-rotted garden compost looks attractive and helps to set them off wonderfully.

How to mulch?

• For optimum results, prepare the soil before beginning to mulch a border.
• Remove weeds.
• Rake the soil to level it out and fill in any dips and hollows.
• Soil should be moist and not frozen.
• Water the soil if it is dry.
• Apply organic mulches (e.g. homemade compost) in layers that are two to three inches thick. Use your hand, a spade or a shovel. For shorter bulbs, two inches is sufficient as they may struggle to grow through very deep mulch.
• Distribute it over the entire bed or border.
• Carefully spread it around the base of the plants.
• Take care not to pile it up against the stems of shrubs and woody plants. 
• Don’t smother or squash very small plants. Ground-cover plants may be left unmulched.
• Mulch trees and specimen shrubs to the radius of the canopy.
• Rake or hoe lightly to disperse it evenly.
• Apply mulch around container-grown plants, in the same way, making sure not to smother plants or pile it up around their stems too much.

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Mulching is a great way to help your garden flourish and look great at the same time. It gives your garden a tidy, finished look and helps to improve the health of your soil and plants too. 

These are just some of the benefits and drawbacks of using each type of mulch, but, of course, if you have a specific question, please send me an email at hannah@talkingofplants.com or leave me a comment below.

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