Covering or mulching the soil surface can reduce weed problems by preventing weed seed germination or by suppressing the growth of emerging seedlings. Mulches are generally ineffective against established perennial weeds. A mulch may take many forms: a living plant ground cover, loose particles of organic or inorganic matter spread over soil, and sheets of artificial or natural materials laid on the soil surface. Residues from preceding crops may be used to form a mulch but this is discussed in more detail in the use of cover crops to suppress weeds. With mulches consisting of organic materials, crop stand and vigour, particularly of direct-seeded small-seeded crops, may be reduced by chemicals released from the decomposing residues.
It is most practical to use mulches in well-spaced crops, particularly transplants. Plastic sheeting and straw mulches have long been used in soft fruit such as strawberries. In perennial crops and some other situations mulches may be intended to remain effective for many years. Mypex, a black, woven, polypropylene mulch, is expected to last for up to three crops (9-10 years). These mulches may be expensive but labour costs are reduced in the long term. Other uses for mulches include as an alternative to cultivation to clear vegetation before cropping by leaving them in place for 12 to 18 months. In freshly prepared seedbeds, short term mulching can be used to manipulate or reduce weed seedling emergence, by for instance, laying black plastic on the seedbed 2 to 8 weeks and then lifting it before planting brassicas or other crops.
The high cost of mulching makes it economic only for high value horticultural crops unless there is another reason for its use. In addition to weed control, mulches may be used: to prevent soil erosion, reduce pest problems, to aid moisture and to prevent nitrate loss. In strawberries, rain splash dispersal of disease spores like those of black spot (Colletotrichum acutatum) is reduced by straw mulch. Mulches can also moderate soil temperatures. Organic mulches in particular reduce heat loss from the soil in cold conditions and help to prevent frost heave. In hot weather the mulch slows down the warming of soil.
Types of mulches include:
Living mulches: consist of a dense stand of low growing plants established prior to or after the crop. The undersowing of cereals with clover and grass could be seen as forming a living mulch. It has been argued that annual weeds would provide a natural ground cover if managed properly. Living mulches are sometimes referred to as cover crops, but they grow at least part of the time simultaneously with the crop. Cover crops are generally killed off prior to crop establishment.
Often, the primary purpose of a living mulch is that of improving soil structure, aiding nutrition or avoiding pest attack, and weed suppression may be just an added benefit. Maintaining vegetation cover is important for preventing soil erosion, nitrate leaching and weed emergence in slowly developing crops like maize. An investigation of the influence of different mulch species on weed density and diversity indicated that weed numbers were reduced and maize yield was not affected where growth of the mulch was reduced by cutting or flaming treatments. When the growth of a living mulch is not restricted, or when soil moisture is inadequate, even a relatively vigorous crop like potato may suffer competition and loss of yield. Studies have been made of the use of living mulches to suppress weed emergence in horticultural crops but there are many different factors to take into account and it is difficult to get the balance between crop and mulch right. Living mulches are well suited to use in perennial crops such as fruit where self-reseeding is an advantage. However, even in established apple and apricot orchards a living mulch growing along the planted row may depress crop growth. In amenity situations, ground covering plants are established to form a dense canopy and suppress weed germination and growth.
Particle mulches: may be organic or inorganic. Loose materials like straw, bark and composted municipal green waste provide effective weed control but the depth of mulch needed to suppress weed emergence is likely to make transport costs prohibitive unless the material is produced on the farm. It has been shown that a 3 cm layer of compost was needed to prevent the emergence of annual weeds and weed control usually improves as the thickness of the organic mulch increases. Weed seeds in the mulch itself can be a problem if the composting process has not been fully effective or there is contamination by wind blown seeds. In straw mulches, volunteer cereal seedlings are a particular problem due to shed cereal grains and even whole ears remaining in the straw after crop harvest. With particle mulches like straw that consist of light materials there is the possibility of them being blown around by the wind. Organic mulches like straw with a high carbon to nitrogen ratio may deplete the soil of nitrogen as they decompose. Mulch improves water filtration into the soil and prevents the compaction and erosion that heavy rainfall can cause.
Before applying a particle mulch weeds should be removed, dry soil should be moistened, and compacted soil loosened. Old mulch should be removed or incorporated to prevent a build-up. Most mulch is applied 7.5 to 10 cm deep. Coarser textured materials require thicker layers. On sandy soils, the mulch layer needs to be deeper than on heavy or wet soils. The mulch should be raked periodically and topped up if necessary. Machinery has been developed for applying/spreading particle mulches. Bark blowers are widely available. Self-feeding straw blowers can handle 1-2 bales per minute and can cover an acre per hour. Flail-beater chains break the straw into 5-10 cm lengths. For green waste there are all-in-one collectors, shredders and spreaders.
Sheeted mulches: a layer of material such as plastic, paper of woven fabric covers the soil surface. Black polyethylene mulches are widely used for weed control in organic and conventional systems in the UK and elsewhere. Clear mulches are better than black for warming the soil but do not control the weeds. Plastic mulches have been developed that selectively filter out the photosynthetically active radiation (PAR) but let through infra red light to warm the soil. Infra red transmitting (IRT) mulches have been shown to be effective in controlling weeds. Various colours of woven and solid film plastics have been tested in the field. White and green coverings had little effect on the weeds, brown, black, blue, and white on black (double colour) films prevented weeds emerging. There are indications that mulching films, like white on black, with a higher rate of light reflectance are beneficial to the crop. Light reflectance may also affect the behaviour of certain insects, and plastic mulches in a greater array of colours are likely to become available. The woven and non-woven polypropylene films or geotextiles (like Mypex) are sometimes referred to as weed barriers and landscape fabrics. They are more durable than polyethylene films permitting multi-year use and are permeable to water. There are advantages both in reduced laying and disposal costs compared with single season materials.
Sheeted materials are relatively expensive and are usually laid by machine. Machinery has been developed that will raise the soil into beds and lay the plastic mulch, securing it at the edges. Beds can be prepared in advance of crop planting. Heavy duty plastic is used for long term crops such as perennial herbs. Woven polypropylene fabrics allow water to penetrate and are less likely to scorch crops when temperatures are high. Non-woven black fabric mulch may not be sufficiently opaque to prevent weed growth completely. After cropping, lifting and disposal may be a problem with plastic and other durable mulches and this adds to the overall costs. Even the degradable plastics may break into fragments that litter the soil. Sheeting made from paper and other natural fibres have the advantage of breaking down naturally, and can be incorporated into the soil after use. Paper mulches have compared favourably with black polyethylene in trials with transplanted lettuce, Chinese cabbage and calabrese in the UK although tearing and wind blowing can be a problem.