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Plant Care Plantcare - Using Mulch

Story and Pictures by Helen Williams and Margaret Chapman

Garden books, garden programmes, garden lecturers and now Everyone is talking about mulching.

Why, beginners often ask. Why is it so important to mulch, what does it mean and what benefit is there in mulching over your garden beds?

The short answer is, lots of benefit. Mulches are one of the gardener's first lines of defence in providing ideal conditions for plants. Mulch is a thick cover of organic or and inorganic material laid over garden beds. Mulches reduce exposure to wind, help to hold and build the soil, prevent weeds, keep water in the root zone and can provide an attractive design feature.

Incorporating any bulky organic matter into the soil will improve soil health, moisture retention and increase fertility. But mulches can be used to reduce the soil temperature in summer and the effects of cold and frost in winter. In summer mulches prevent the sun from reaching the soil and thus reduce water evaporation and moderate temperatures around the plant's root zone. In winter mulch regulates the temperature of the soil around the roots preventing freezing damage, frost heave and the delaying emergence of shoots.

There are some potential hazards such as nitrogen deficiency (first signs are yellowing leaves) when using very fresh organic materials such as sawdust and grass clippings that can be alleviated by addition nitrogen. Some mulches can starve the soil and over-mulching can result in water and oxygen being unable to penetrate to plant roots. But overall, the benefits of mulching far outweigh the disadvantages.

How Mulch Works
If you think of a playground drying up after rain, while an area under a pile of leaves stays moist and damp, then you can understand how mulch retains moisture in the soil, reducing watering and the impact of drought. Water some mulched soil and the water soaks into it. Water bare earth and the water runs off and is lost.

Soils drain better and hold more moisture when they have plenty of organic matter and/or a layer of mulch. Organic matter is much like a sponge in the way that it soaks water up. Plants can take up this water later. Inorganic mulch, and especially plastic sheeting, can result in 'sour' soil as the soil lacks the air pockets present when there is plenty of organic matter or humus, without these the soil cannot breathe and oxygen levels may become low.

By placing a layer of mulch over the soil you smother weed seeds, which cannot then germinate. After a year or so, the mulch can become thin and patchy and weeds will begin to break through. Laying a new layer of mulch will overcome this and any weeds that do emerge will be easy to remove and come cleanly out of the soil. Mulching can reduce herbicide use and is one of the best options for weed control in organic gardens.

Mulch is also good for erosion control, rain on bare soil can hit with considerable force, compacting soil and preventing good moisture absorption. Not only the rainfall but also soil washes away. A thick loose layer of mulch absorbs the force of the rain and lets water soak through to the soil instead. On very steep, stony banks, however, you may be better not to mulch, as the rain will wash the mulch away and it will end up at the bottom of the hill. There are plants, such as the Marlborough Rock Daisy, (Pachystegia insignis) that enjoy in these conditions with just a stone at their roots.

Soil Health and Mulch
In nature we can see how naturally occurring leaf mulches develop in woodland or native bush. Mulch slowly breaks down into organic matter or humus. Soil moisture is retained and woodland floors are damp underneath the leaf litter layer.

This decays to form soil and the profile of soil shows the different stage of decomposition. So, right at the top, you get the fresh mulch or organic matter, then partly organic matter (decayed mulch). Then there is darker soil where humus that has been incorporated into the soil and, finally a layer of clay-like material at the bottom.

The organic matter or humus is important to soil health because it provides air pockets in the soil and it provides places to bond with fertiliser. Most of the nutrients will be in the top, humus layer of the soil.

The organic material that develops in soil is a result of mulch decomposing, this also increases fertility. Trees and plants use any organic material in the soil and their roots lie in this soil layer. However there are some gross-feeding plants that will require additional fertiliser.

Types of mulches
There are mulches for almost any situation. See our table for a listing of commonly available mulches and how and where to use them.

Coarse mulch is best for weed suppression and water conservation.

Next Page
Gravel mulch
Gravel mulch can make an attractive cover

Clippings mulched with the mower, ready for use
Clippings mulched with the mower, ready for use

Readily Available Mulches
    Organic Mulches
  • Compost 5-10cm (2-4in) improves soil structure, fertility and water holding capacity. Make sure to use weed-free compost. The very fine compost will draw moisture up from the soil and is less effective than other mulches at keeping moisture in the soil. Renew annually.
  • Lawn clippings 2.5cm (1in) - best dried first, lets some water in but holds little and inhibits transpiration. Can become a little slimy, but a good source of nitrogen, and useful mixed with sawdust. Useful around new seedlings. Renew annually
  • Leaves/Leaf mold 5-10cm (2-4in ) - should be partially rotted before use, lets water in and retains it and improves the water holding capacity of the soil. Leaves gathered from under deciduous trees break down into fine organic material. Renew 1 to 2 years.
  • Mulcher debris 8-10cm (3-4in) made from domestic mulchers or by using the lawn mower from tree trimmings etc. - lets water in and adds to humus but may have uneven appearance & may cause nitrogen deficiency. Renew 2 to 3 years.
  • Mushroom compost 5-10cm (2-4in) Good source of humus but suitable only for use with lime-tolerant plants, do NOT use with rhododendrons, camellias and other califuge plants. Renew annually.
  • Newspaper 2.5cm (1in) or less, equivalent to 6-8 sheets and wet. Keeps some water in and controls erosion. Not very attractive. Renew annually.
  • Peastraw 10-15cm (4-6in) - lets water in, cools soil, and holds some water in the soil. (Available in the South Island only.) Obtainable also in palletised form, the pellets expand when wet. Renew 1-2 years
  • Peat 5-10cm (2-4in) Improves soil structure and is attractive but will soak up soil moisture when first applied so use only on moist soils and water well after spreading. Conservationists in many countries are campaigning against peat use, as valuable habitats are lost when it is harvested. Looses 15% of volume per year.
  • Pine needles 5-8cm (2-3in) - increases water holding capacity of the soil but hold little itself, increases soil acidity which helps acid loving plants such as azaleas, rhododendrons, and camellias. Renew 1-2 years.
  • Sawdust 2.5-7cm (1-3in)- Ue only when at least 12 months old. Decomposes adding humus to the soil and retains moisture, although it also soaks up moisture from the ground, which then evaporates. It also takes nitrogen from the soil and plants as it breaks down. Adding nitrogen, such as urea or other fertilisers will overcome this. Renew annually.
  • Shredded bark 8-10cm (3-4in) lasts well, less likely to wash away, let's water in and good moisture retention. No nutrient value. Renew every 2 to 3 years.
  • Woodchips 8-10cm (3-4in) lets water in and good moisture retention. Renew 1 to 2 years
Next Page for Inorganic Mulches

Bark mulch complements native grasses
Bark mulch complements native grasses
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Last revised 07 Jan '02