Article by David Marks
A garden mulch is a layer of material which is placed on the ground surface. The key purposes for using a mulch are:

• To suppress weeds and other unwanted plants.
   
• To conserve moisture in the ground.
   
• To raise the temperature of the ground.
   
• To add nutrients
   
• To improve the texture of the soil

TYPES OF GARDEN MULCH

There are many different types of mulch, they key ones are listed below with some notes on how effective they are:

GARDEN COMPOST

Anything that is on your compost heap and is well rotted can be used as a garden mulch. This is, without a doubt, the best general purpose form of mulch. However, it is also often in short supply. An 8cm / 3in layer will conserve water and raise the temperature of the soil surface by a degree or two. It’s not the most attractive mulch but it very quickly merges into the soil.

Most weeds (but not all) will be smothered by a layer of garden compost and new weed seeds will find it difficult to establish themselves. Over a year the layer of compost will break down even further and at the same time will be pulled into the top soil by worms and general weather action. This will add long-lasting nutrients and greatly improve the texture of the soil.

HORSE MANURE

Well rotted (six months plus) horse manure, including the straw / saw dust bedding, makes a great mulch but it will naturally compost into the soil (and therefore improve it) more quickly compared to other mulches.

If the manure is not well rotted, it can be spread over unused ground in autumn and will be fine by spring time the next year. Do not add manure where you want to grow potatoes and other root vegetables (carrots, turnips, swede etc.) the next year.

CHIPPED BARK / WOOD CHIP

Both of these can be bought from garden centres and online. Often, tree surgeons and other garden companies will give away wood chip for free. It can cost them to dispose of wood chip and you can save them that cost.

In years gone by there was a view that placing new wood chip on garden soil would significantly leach it of nitrogen. This has lately been scientifically proven to be untrue to any significant degree.

An 8cm / 3in layer will conserve water and raise the temperature of the soil surface by a degree or two. Bark chips especially are an attractive form of mulch, even wood chip looks good.

Both do an excellent job of suppressing weeds and preventing new weed seeds from rooting. Over the long term they will add a low level of nutrients to the soil and improve its texture. It will take several years for this happen.

BLACK PLASTIC

Any black plastic will do the job but the rolls of it sold by the garden centres and online are by far the most convenient method of using this type of mulch. Without a doubt this is a particularly unattractive mulch and it will need to held in place by stones or earth. It can be removed and stored away for use the next season when it has done its job.

It does however warm the soil better than all the other mulches, so much so that it is the preferred mulch used by professional growers who want to warm the soil. It will also do an excellent job of suppressing weeds but you will need the heavy duty, more expensive weed control fabric for this job.

Black plastic will reduce evaporation of water very well. It won’t however improve soil texture or add any nutrients. Unless you know different, always pierce several holes in black plastic mulch to allow water to get into the ground below.

NEWSPAPERS / CARDBOARD

A free and commonly used form of mulch especially for larger areas. It is principally used for suppressing weeds and conserving moisture. Ensure that the layer is thick enough to prevent weeds growing through it. Where newspaper is used we would suggest about 10 layers of newspaper at the minimum.

Both newspaper and cardboard will need to be weighed down with stones and or earth to prevent them blowing away. We would strongly recommend a thin layer of earth to ensure the paper is not shredded by high winds.

The nutritional value to the soil of cardboard and newspaper is negligible. It will improve the texture of the soil to a minor degree over a couple of years. As far as we are aware it will not raise the temperature of the ground by any significant amount.

GRASS CLIPPINGS

Normally in plentiful supply, these are useful for localised mulching around individual trees and plants. It will suppress weeds, conserve water and provide a low level of nutrients. As it rots down it will also improve soil texture to a very small degree. Grass clippings won’t raise the soil temperature.