Mulching

What is mulch?

Mulch keeps your garden looking tidy, reduces the need to water, suppresses weeds and it makes use of material that might otherwise be dumped in a landfill.

Mulch is any layer of material that covers and protects the top layer of soil. Mulching helps plants grow by keeping the soil moist and cool, and allows roots to grow more easily by reducing compacting of the soil. It also attracts earthworms and helps control weed growth, plant diseases and erosion.

Mulch is like insulation for soil, keeping it cooler in summer and warmer in winter. Mulch-covered soil retains moisture better in hotter weather, and it keeps temperatures constant around plant roots, reducing freezing damage in colder months.

What to use

A surprisingly wide range of materials can be used as mulch. Organic mulches add nutrients to the soil as they decompose but the decomposition process can make nutrients unavailable to your plants and it can sometimes burn plant roots. For this reason it is best to not add too much. Inorganic material can also be used as a mulch.

Organic mulches include wood and bark chips, leaves, grass clippings (preferably partly dried), pine needles, hay, straw, compost, mushroom compost, pine needles, sawdust, shredded paper and seaweed (but wash the salt off first).

Inorganic mulches include rocks, pebbles and gravel, plastic sheeting, and crushed glass or glass beads.

The Kapiti Coast District Council’s “Summer on the Coast” brochure (PDF 1.34MB) recommends suitable mulches for different types of garden:

• Coastal gardens – pebbles, builders mix or shells
• Rose gardens – shredded compost or seaweed
• Annual gardens – shredded compost or other fine mulches
• Native gardens – low nitrogen mulches like mushroom mulches, aged wood chips or straw
• Fuchsia gardens – mixtures of grass clippings, seaweed, chicken manure, mushroom mulch or aged wood chips
• Vegetable gardens – Straw or mushroom mulch mixed with compost
• Rhododendron gardens – pine based products or chicken manure mixed with straw and mushroom mulch
• Shrub gardens – mixtures of grass clippings, seaweed, mushroom mulch or aged wood chips

How to mulch

Mulch is best applied in late winter or early spring, when soils are well watered and before seeds germinate (though for frost protection, mulching should be done in autumn).

The mulch layer should be at least 10 cm deep. Keep the area around plant stems clear of mulch. Allow newly transplanted seedlings to become established on vegetable gardens before applying mulch.

Organic mulches should be topped up each year; otherwise the mulch layer becomes too thin to protect the soil. In vegetable gardens allow to soil to warm up in Spring before adding mulch.

Common mulching problems

• Collar rot – this happens when mulch is applied too close to plant stems, trapping moisture underneath and causing them to rot.
• Plant scorching – caused by actively decomposing mulches like young compost, grass clippings and fresh wood chips, which generate a lot of heat. Avoid this by not laying mulch too thickly and keeping stems clear of mulch.
• Over-mulching – can prevent air and water getting into compacted soil.
• Coarse mulch – very porous material like large bark chips can allow much moisture to evaporate. Prevent this by chopping material up finely.
• ‘Sour’ (low oxygen) soil – can result from using inorganic mulch, especially plastic sheeting. Make sure inorganic mulches have plenty of holes or gaps to allow air and water into the soil.
• Nitrogen depletion – some organic mulches like fresh sawdust can absorb nitrogen from plants as they decay – yellowing leaves are a sign of this.
• Mulch may provide moist shelter for slugs.

Despite these problems, in most cases the benefits of mulching far outweigh the disadvantages.