Mulches

What is a mulch?  It’s a layer - which can be made of a variety of materials - put on top of the soil to protect or improve it.

Organic growers use mulches for a number of reasons:
1. To suppress weeds – by depriving weeds of light a mulch can weaken and kill them
2. To improve the soil – a layer of organic material, such as compost or grass cuttings,  will break down and feed the soil life as well as improve soil structure
3. To protect the soil – acting as a blanket, the mulch prevents the soil drying out in wind and sun as well as protecting it from frost.

Mulches can be made of various materials, which come in two types: a blanket of light excluding membrane (plastic, woven fabric, cardboard etc), or a layer of organic material (compost, straw, leaves or grass cuttings.)  Here are some examples:

Mulch membranes
Membranes are most useful at suppressing weeds.   This can be for permanent weed exclusion – such as in paths – or to clear a patch of weeds ready for growing. (See No Dig and preparing your allotment growing area.)   In both cases, you need to remove weed foliage down to ground level (put it on your compost heap), before laying the membrane. You can also improve the soil by putting some compost underneath the membrane.

• Geotextiles: Geotextiles are synthetic water and air permeable membranes that are woven or spun. They will last an average of 15 years, when covered with a loose mulch such as wood chips. Secure them at regular intervals with wire pegs.
Pros: Provide excellent long-term weed control. Geotextile membranes can be planted through.
Cons: Expensive. Can't feed through it; worms can't work in organic matter covering the membrane. A non-renewable resource.
• Black plastic film (400 - 600 gauge): Black plastic will only last for one to three years. To secure it, bury the edges at the borders of the planting beds. Silage sheet is cheaper for large areas.
Pros: It is useful for clearing weedy ground prior to planting. Applied to clean soil it can be planted through and covered with a loose mulch. Vigorous growing vegetables such as potatoes or courgettes can be planted through the membrane whilst trying to clear weedy ground. Warms up soil.
Cons: As it is not air or water permeable it is not recommended for long term use. Will degrade quickly if not covered, leaving plastic residues in the soil. A plastic non-renewable resource.
• Tree mats: It is important to keep 1m² weed free at base of a tree for the first three to five years. Tree mats are made of wool, geotextile or black plastic. It is also possible to make your own using newspaper. Secure mats by burying the edges or pegging down.
Pros: Wool mats and geotextiles are air and water permeable. Tree mats can also be used for large shrubs.
Cons: Wool mats can be destroyed by birds using them for nesting material. Black plastic is not air and water permeable. Synthetic membranes are best covered with a loose mulch. Synthetic membranes are a non-renewable resource.
• Cardboard: Flattened cardboard boxes make an excellent membrane which will last for one growing season. It can be held down with bricks, planks, or straw.
Pros: Free. It is useful for ground clearance and can be replenished when weeds start to grow through. Vigorous growing vegetables can be planted through it. Biodegradable.
Cons: Degrades quickly.
• Newspaper: Newsprint, not coloured magazines, is excellent for short term weed suppression that will last one growing season. Use a whole opened out newspaper at least eight pages thick. Hold it down with a degradable mulch such as grass mowings, hay or straw.
Pros: Free. A thick layer will suppress perennial weeds. Use it around the base of fruit bushes and raspberries (remember to remove it in the autumn and replace it in the spring). It also makes good tree mats and can be used as a mulch in the vegetable garden. Biodegradable.
Cons: Degrades quickly. May further acidify soils with a low pH.
 
Loose mulches
Loose mulches are excellent at protecting the soil from extremes of weather, as well as feeding the soil. They can also help hinder weed growth. Before applying, make sure weeds are cleared, and the soil is warm and well-watered.

• Woodchips: Woodchips are chipped wood from a variety of sources. It is cheaper but less attractive than ornamental bark.  They can be used as weed suppressants.
Pros: Excellent for informal paths. Biodegradable. Recycling waste material.
Cons: Although it will give some suppression of perennial weeds soil must be clear of all weeds before application. Wood chips can cause nitrogen shortage in the soil.  So if you are using them in growing area, mix with an organic nitrogen rich fertiliser (such as grass cuttings or nettle foliage) to counteract this.
• Ornamental bark: Ornamental bark is composted and graded conifer bark. It is more expensive than woodchips but it is visually attractive.
Pros: Excellent for decorative beds. It does not cause nitrogen loss in the soil and actually conditions the soil. Biodegradable. Recycling waste material.
Cons: Although it will give some suppression of perennial weeds soil must be clear of all weeds before application.
• Shredded prunings: Woody prunings and other woody material produced on site can be chipped or shredded for use as a mulch. These are best heaped up to compost for a few months before use on planted areas. Composting will darken the colour of the mulch, giving it a more attractive appearance. The addition of nitrogen - in the form of grass mowings, nettle liquid or nitrogen rich manures for example - to the heap of shredded material will speed up the process.
Pros: Can be used fresh for paths. Biodegradable. Recycling waste materials.
Cons: Home-made mulches, which are likely to contain more green material, may degrade more quickly.
• Straw/hay: Straw and hay will make a good mulch for one growing season. For the most effective weed control use over a membrane such as newspaper. It is preferable to use semi-rotted material.
Pros: Hay is a source of potash and nitrogen. Straw also supplies some potash. This mulch is extremely useful for fruit bushes. Biodegradable. Recycling waste material.
Cons: Hay introduces grass and weed seeds.
• Sawdust: Best used as a mulch in ‘wild’ areas or to cover tree mats. Do not use sawdust from treated wood.
Pros: Biodegradable. Recycling waste material.
Cons: Causes nitrogen robbery (takes vital nitrogen from the soil) so incorporate organic nitrogen fertiliser before application. It is not suitable for highly decorative beds or where it might be dug in