Some of the most effective gardening jobs are also the simplest. Mulching is a good example.
Spread a layer of organic material over the surface of the soil and it can help retain moisture, suppress weeds and gradually improve the ground beneath. It also protects bare soil from the effects of heavy rain and extremes of temperature.
In nature, soil is rarely left completely bare. Leaves fall, plants die back and organic material gradually decomposes on the surface. Mulching takes its cue from this natural process.
Whether you’re caring for a vegetable plot, herbaceous border, roses or newly planted trees and shrubs, a well-chosen mulch can make a noticeable difference.
What is mulch?
Mulch is simply a layer of material spread over the surface of the soil.
Organic mulches include garden compost, well-rotted manure, leaf mould, composted bark and woodchip. As these materials gradually break down, they contribute organic matter to the soil.
Other materials, such as gravel, slate or stone chippings, can also be used as mulch. These don’t improve the soil in the same way, but can help cover the surface and reduce moisture loss.
The right choice depends on what you’re growing and what you want the mulch to achieve.
Help the soil hold on to water
One of the greatest benefits of mulching is its ability to slow the loss of moisture from the soil.
Bare ground exposed to sun and wind can dry surprisingly quickly. Covering the surface reduces evaporation, helping the soil remain moist for longer and potentially reducing the amount of watering needed.
This is particularly useful around newly planted trees, shrubs and perennials while they’re establishing their root systems.
For the best results, mulch when the soil is already moist. Applying a thick layer to very dry ground won’t magically rehydrate the soil beneath it.
Keep weeds in check
A good layer of mulch can make weed control considerably easier.
By covering the soil surface and reducing the amount of light reaching it, mulch can suppress the germination and growth of many annual weeds.
Established perennial weeds are a different matter. Brambles, bindweed, couch grass and other persistent weeds should be dealt with before mulching rather than simply buried beneath it.
Mulch won’t eliminate every weed, but those that do appear are often easier to spot and remove.
Feed and improve the soil
Organic mulches do more than cover the ground.
As compost, leaf mould and other plant-based materials decompose, they’re gradually incorporated into the soil by earthworms and other soil organisms.
Over time, this can improve soil structure and its ability to hold both moisture and air.
On heavy soils, adding organic matter can help create a more workable structure. On lighter soils, it can improve moisture retention.
Rather than repeatedly digging organic matter into established borders, applying it to the surface allows natural processes to do much of the work.
Protect the soil surface
Watch what happens to bare earth during a heavy downpour and you’ll see another reason to mulch.
Large raindrops strike the surface, breaking up soil aggregates and potentially causing the surface to compact or wash away. A covering of mulch helps soften that impact.
It also provides some insulation from rapid changes in temperature.
This doesn’t mean that mulch will prevent the ground from freezing during severe winter weather, but it can help buffer the soil from sudden fluctuations and protect shallow roots.
Support life beneath your feet
Healthy soil is full of life.
Earthworms, fungi, bacteria and countless other organisms play a part in breaking down organic material and cycling nutrients through the soil.
Applying organic mulch provides material for this soil ecosystem to work on and helps create the moist conditions in which many soil organisms thrive.
Lift a layer of established compost or leaf mould and you may find plenty happening underneath.
It’s a useful reminder that some of the most important work in a garden takes place where we can’t see it.
Which mulch should you use?
There’s no single mulch that’s right for every part of the garden.
Garden compost is a useful all-round choice for beds, borders and vegetable plots, adding organic matter as it breaks down.
Well-rotted manure can be particularly useful on hungry soils and around plants that benefit from rich growing conditions. Make sure it is properly composted before use.
Leaf mould is an excellent soil conditioner and particularly useful where you want to improve structure and moisture retention.
Composted bark provides a longer-lasting surface covering and can work well around established shrubs and trees.
Woodchip can be useful on paths and around established woody plants, although fresh woody material needs to be used appropriately.
For Mediterranean plants and those that prefer leaner, free-draining conditions, a mineral mulch such as gravel may be more suitable than rich organic matter.
Let the plants and the soil guide your choice.
When should you mulch?
Spring and autumn are both good times to mulch.
In spring, wait until the soil has begun to warm and make sure it’s moist before applying the mulch. This can help conserve water as the growing season gets under way.
Autumn mulching can help protect the soil surface through winter and gives organic material plenty of time to begin breaking down.
Avoid mulching frozen, waterlogged or exceptionally dry soil.
Before you start, remove troublesome weeds and water the ground if necessary.
How much mulch should you apply?
A token scattering won’t achieve very much.
For many organic mulches, aim for a layer roughly 5–7.5cm deep, depending on the material and situation. This is generally enough to help suppress weeds and retain moisture without burying the plants themselves.
Keep mulch away from the stems and crowns of plants.
This is particularly important around trees and shrubs. Avoid piling mulch against trunks in a mound – sometimes called ‘volcano mulching’ – as persistently damp material around the bark can encourage problems.
Instead, spread the mulch across the root area while leaving a small clear space immediately around the trunk or stem.
Don’t dig unless you need to
Once the mulch is down, there’s usually no need to dig it into an established border.
Earthworms and other soil organisms will gradually incorporate organic material for you.
Leaving the soil relatively undisturbed also helps preserve its structure and the networks of organisms living within it.
Simply top up the mulch when the previous layer has broken down.
Let nature do some of the work
Mulching isn’t glamorous, and the results aren’t as immediate as planting a new border or watching a favourite flower open.
But its effects build quietly.
The soil holds moisture for longer. Fewer weeds emerge. Organic material disappears into the ground and contributes to healthier soil. Plants have better conditions in which to establish and grow.
A few barrowloads of compost spread across a border can therefore be one of the most worthwhile investments of time you make in the garden.
Cover the soil, look after what’s beneath your feet and let nature take care of much of the rest.
Further Reading: My Garden Journal & Notes
