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How to mulch

A mulch is a layer of material applied to the surface of the soil (usually a minimum of 5cm deep, ideally 7-10cm deep) for one or several of the following reasons:

  • Suppress weeds.
  • Retain moisture.
  • Insulate the rooting area against sudden changes in temperature.
  • Improve the appearance of bare soil.
  • Deter some pests.
  • Provide additional organic matter to improve the soil structure.

There are a few disadvantages of using mulch:

  • With many mulches it’s difficult to get them to stay in one place and you often find they ‘drift’ to other parts of the garden, such as the lawn.
  • Mulches may need to be topped up on an annual basis, which can become expensive over time.
  • Some mulches risk bringing toxins, pests or diseases into the garden, or altering the soil’s pH levels.

Many different materials can be used as mulch, a selection of which are discussed below. An alternative to mulching bare soil is to grow ground cover plants, which provide most of the benefits of a mulch without some of the disadvantages.

Before applying a mulch, ensure that all perennial weeds and their roots have been removed as these can work their way through the toughest of mulches. Don’t apply the mulch when the ground is cold or frozen (otherwise the mulch will keep the cold in and prevent the soil warming up) and ensure the soil is moist before applying it; it’s best to apply mulch between mid spring and autumn.

When you apply the mulch, ensure that you don’t smother existing, low growing plants and that the mulch doesn’t touch the stems of woody perennials (particularly organic mulches) as they can make the stem too wet and soften it. Don’t mulch around seedlings until they are well established so you don’t smother them or increase the humidity to the extent that it encourages damping off. If you are mulching around a single tree or shrub, the width of the mulch should be as wide as the canopy of the plant.

Once the mulch is in place you can continue to water plants through the mulch (they may require more watering in dry spells to account for water taken up by the mulch) and apply fertilisers through it (using a liquid fertiliser if you’ve put down a plastic woven membrane).

Organic mulches

These mulches are made from plant or animal waste products and can be effective mulches with the added benefit that they are biodegradable and therefore will add organic matter to improve your soil (either that season or over a longer period, depending on how long it takes the material to break down). On the downside they will require topping up more frequently than most inorganic mulches and may not be as effective in supressing weeds. When topping up or replacing an organic mulch, rake through any remaining mulch to loosen it before adding the new mulch, otherwise it can compact and form a hard layer over the soil.

Bark chippings/composted bark

These are a popular choice for mulches and come in various sizes. Bark chippings are larger pieces, whereas composted bark is a smaller sized material which is partially decomposed. Bark is a relatively cheap material which makes an excellent organic mulch, however there are a few downsides:

  • As the bark decomposes it can draw nitrogen out of the soil, therefore reducing nitrogen levels available to plants. This is less of a problem with composted bark where a proportion of the decomposition process has already been completed.
  • Barks from coniferous plants (eg pine) can reduce the pH of the soil as they decompose, thereby making the soil more acidic.
  • There is a small risk that you may introduce a toxins, pests or a disease such as honey fungus with the bark, although this shouldn’t be an issue if you purchase the bark from a reliable source.

Bark should be laid at least 5cm deep – a 50 litre bag will cover 1m² to about this depth.

Cocoa shell is a similar material to bark chippings but has a neater appearance (and can be more expensive).

Farmyard manure

Farmyard manure is great organic matter for improving the structure of your soil and it makes great mulch which is pretty cost effective. However, there’s a risk that you may introduce weed seeds into your garden by using it (many seeds will survive their journey through a horse’s gut!) and it can create a topsoil environment that encourages weed growth.

Garden compost

This is a cheap way of getting an effective, organic mulch for your garden. Once you have made your garden compost, layer it onto the soil at least 7.5cm deep and water it if it’s dry.

The disadvantage of using garden compost is that it can provide an excellent environment for weeds to grow and may actually contain weed seeds to further add to the problem. You also need to be careful not to incorporate any diseased materials or toxins into the compost heap, otherwise you will be transferring these to the soil when you use it.

Leaf mould

While leaf mould can take a while to produce (thicker leaves should be shredded first to speed up the process), it is well worth the effort as it provides an effective mulch which makes great food for earthworms and has an attractive dark colour. If you have a lot of trees then it’s definitely worth collecting the leaves to produce your own leaf mould. Oak, beech or hornbeam leaves make the best leaf mould. Evergreen and conifer leaves are best put into the garden compost as they will break down much more quickly mixed in with other materials.

Leaf mould can pack together and become a solid mass over the soil, so you may find it best used mixed with another type of organic mulch.

Spent mushroom compost

This is compost left over from mushroom farming which can usually be purchased relatively cheaply. At its base is composted straw, it provides a good level of organic matter to condition the soil as well as being a mulch. However, it can contain chalk (from the packing material used during the mushroom farming process) and therefore be alkaline in nature (sometimes unused mushroom compost is available which has far less chalk). Therefore, unless you need to raise the pH of your soil, you should mix it with other mulches (such as garden compost or farmyard manure) or alternate applications. You should avoid using it altogether around ericaceous plants or fruit trees.

Spent hops

A by-product of the brewing industry, spent hops are often available very cheaply (or even free) from local breweries and make a good soil improver. They are usually supplied moist (from the brewing process) and can be a bit pongy! As they dry out they can become light and may be moved by the breeze, so you might find it best to combine them with other mulching materials to weigh them down. Hops are poisonous to dogs so shouldn’t be used if you have a dog or have dogs visiting your garden.

Grass clippings

If you have a lawn, this is a very cheap way to mulch the rest of your garden. However, you must be careful that you haven’t used a herbicide on the lawn, which may kill the plants you mulch. It also has a high nitrogen content and can burn seedlings. It is very light and can harden to form a solid mat over the soil, mixing it with other mulches can reduce these problems.

Seaweed

Seaweed is often used in coastal areas to provide a winter mulch (since it can attract flies if used in the summer) with a good level of nutrients to release into the soil, often combined with other mulches. Care should be taken to observe local conservation laws if you wish to collect seaweed yourself. Generally the salt levels in seaweed aren’t problematic, but you may wish to rinse it in fresh water briefly before use. Apply the seaweed directly onto the soil without waiting for it to dry or compost.

Straw

This is frequently used as a mulch around strawberries, tomatoes and other crops since it has weed suppressing and moisture retention benefits, plus it helps to keep the fruits off the soil and, therefore, makes them less likely to rot.

Inorganic mulches

Inorganic mulches won’t help the structure of your soil (and some may risk compacting it), however they are long lasting (since they don’t biodegrade) and effective mulches. The initial cost is likely to be higher than organic mulches, but this should be offset over time by not having to replenish the mulch as often and the time saved from less weeding being required.

Plastic woven membrane (aka weed matting)

These sheets provide one of the most effective weed control methods of mulching, creating a water permeable barrier across the soil through which weeds can’t grow. They are often used under other mulching materials (since they don’t look very attractive if left bare and don’t provide much insulation by themselves), particularly under gravel which otherwise would work its way into the soil and need topping up more often.

It is important to ensure the soil is moist before applying the sheeting, and the sheeting should be pegged down at at least 1m intervals. Cut crosses into the material to create planting holes. Use a liquid feed to provide nutrients (assuming the membrane you’re using is water permeable).

While a quick, effective and relatively cheap mulching option, the sheeting does have some disadvantages. It is difficult to add organic matter to your soil and, therefore, the structure of your soil underneath the sheeting could deteriorate as a result. Accumulations of soil and organic matter on the surface of the membrane can lead to weed growth on top of it, particularly if you have a bark covering on top. You may, therefore, find it best to lift and relay the sheeting every few years, adding organic matter to the soil and forking it over while it is uncovered.

Gravel/shingle/pebbles/slate

Gravel is a commonly used inorganic mulch which is effective in suppressing weeds (so long as it is deep enough) and retaining  moisture in the soil. Gravel is available in all shapes, sizes and colours so you can find the perfect fit for your garden. There are many different types of stone which can be used as gravel, including:

  • Coarse-grade stone chippings (limestone or granite).
  • Slate chippings, which are available in greys, blues, greens and plum colours.
  • Pea shingle, which has rounded edges.
  • Washed shingle, which is sharper and therefore not great for areas children will be in.
  • Horticultural grit, which is ideal for alpine beds.

The smaller sizes will easily be washed into the soil, so common practice is to lay a plastic woven membrane underneath the gravel. The gravel should be laid to at least 2.5cm deep, you’ll need about 50kg per m².

Seashells/Tumbled glass/Crushed CDs/Clay balls

These are much more expensive that other mulches, but equally effective. Their high cost, coupled with their attractive appearance, make them a good option for mulching containers.

Recycled tyre mulch

Rubber crumb mulches are highly effective since they take many years to break down, plus they come in a wide range of colours. Being recycled, they get a ‘green’ stamp of approval, although the problem still exists of what to do with the mulch if you want to replace it.

There have been some concerns raised about health issues related to recycled tyres being used in this way. There doesn’t appear to be any tangible evidence to suggest it is unsafe, but if you have children (or pets) you may wish to purchase certified rubber mulch and teach them not to put it in their mouths (pets are unlikely to eat it as it isn’t organic).

Foil/polythene/plastic sheeting

Before the advent of plastic woven membrane, foil or polythene were commonly used as a cheap solution for suppressing weeds and insulating the soil. The ground must be well watered before it is laid and holes should be made in the sheet where water puddles to ensure that water reaches the soil.

The polythene or plastic used should be black, as clear sheeting will allow light through and stimulate weed growth.

Foil used as mulch, or shredded and mixed with another type of mulch, is said to deter pests such as aphids from the plants growing through it.