A mulch is a protective layer added to the surface of the soil to improve plant growth in several ways:

  • They control weeds by blocking the sunlight necessary for germination.
  • They help the soil to retain water by slowing evaporation.
  • They adjust soil temperature by helping soil retain more heat in spring and autumn, and by keeping soil cool and levelling out temperature variations during the summer. This keeps plant roots warm in winter and cool in summer.
  • They can add organic matter and nutrients to the soil through the gradual breakdown of the mulch material.
  • They can help to repel insects.
  • They can provide a clean, dry surface for ground-lying fruit such as marrows.
  • They can improve growing conditions by reflecting sunlight up towards the plants.

A wide variety of natural and synthetic materials may be used for mulching.

Organic Mulches

To be successful, an organic mulch should have an open structure that allows rain to pass through it easily. It should also be long-lasting and not easily dislodged by the weather. If the mulch rots too rapidly, it can form a barrier that will block air and water, causing damage to the plants.

There are many organic mulches available, including:

  • Organic residues such as grass clippings, leaves, hay, straw, sawdust, wood chips, shredded newspaper, course bark or wool. Coarse bark is particularly successful in preventing weed seed from germinating.
  • Garden compost, peat or peat substitutes. Although composted material can greatly improve the structure of the soil, they also provide an ideal medium for the germination of weed seeds.
  • Organic sheet mulch; a biodegradable alternative to plastic mulch.

Inorganic Mulches

Inorganic mulches are generally sheets of plastic, paper or fleece fibre that may simply be laid over the soil surface. The sheet should be secured with pegs and the plants allowed to grow through slits or holes. An inorganic mulch raises the temperature of the soil, which can be useful when growing crops. If the soil is very dry or waterlogged, the sheet should be perforated to let rain penetrate the soil or to allow water to evaporate.

An alternative to laying the sheet over the soil is to apply a 'floating' mulch, in which a light plastic or fibre fleece sheet is laid over the plant. This acts in the same way as a cloche, raising the soil temperature, insulating the plant from harsh weather and providing a barrier against pests. The sheet should be perforated or permeable to allow air and water to pass freely to the plants and soil below.

Application of Mulches

Mulch is usually applied in early to mid-spring to warm the soil by helping it retain heat. This allows early seeding and transplanting of certain crops, and encourages faster growth. As the season progresses, the mulch will stabilise temperature and moisture, and prevents sunlight from germinating weed seed.

Never apply a mulch to cold or frozen ground; the mulch will merely serve to keep the soil cold. Always wait until spring when the soil has had a chance to warm up.

All perennial weeds should be removed before mulching; if slugs are a problem you may also want to scatter a few pellets that will be concealed by the mulch.

Scatter organic mulch in a 10-15 cm (4-6 in) thick layer around the plants to the full extent of the foliage canopy. Avoid dropping it onto the plants and, unless you have a retaining edge, try not to mulch right up to the edge of the border otherwise to prevent it from spilling out onto the path or grass. Loose organic mulches will gradually rot down and become incorporated into the soil, so you will need to top them up the following spring.

A sheet mulch may be positioned over the soil and held in place with pegs. You can also anchor the sheet by sinking its edges into 5 cm (2 in) trenches dug into the soil.