Robin Gale-Baker, from Sustainable Macleod, discusses mulch. She has also written articles for this website about growing broad beans, cauliflower, eggplants and capsicums, garlic, other vegetables, apricot trees, blueberries, persimmon trees, other fruit trees and herbs. Also articles on growing techniques and shade cloth.
With the drought has come high prices for organic mulch, that indispensable item for the home gardener. Pea straw and sugar cane mulch have risen steadily over the past few years and are now the most expensive they have ever been. What are we to do?
Mulch is important for several reasons. Most importantly, it stops the soil surface drying out and retains moisture in the soil. It also prevents weeds growing and seed germinating, and adds organic material to the soil, improving its health and aeration. It insulates the soil, keeping the soil temperature consistent. And it prevents compaction caused by heavy rain.
There are many materials that can be used for mulching. Some are suitable for the veggie garden which generally needs a pH of 6-7, some for trees, and some for acid-loving plants requiring a pH of 5.5 or below:
- Pea straw is high in nitrogen, significantly improves soil microbiology and provides excellent worm food. Spread it 4-10cm deep on new beds and 2-4cm if refreshing beds previously mulched.
- Sugar cane mulch has a low pH and should be mixed with some dolomite to balance it. It is quite fine and can mat, providing a bedding for creeping weeds such as chickweed but, as it breaks down rapidly, it does benefit the soil microbiology quickly. It is more expensive in the long run than pea straw because of its rapid decomposition. Spread it to a depth of between 4-10cm.
- Fruit trees do well with wood chip. At this time of year, pull back any mulch to the dripline of the tree and ring the dripline with 6-8cm width and height of woodchip. This encourages fungi, plus worms which take the mulch underground and shred it, thus creating a soil food web.
- Acid-loving plants, such as blueberries and passionfruit, require an acid mulch. Pine needles when aged are suitable but, when unaged, contain terpenes, which are chemicals that are toxic to many plants. Cedar chips also contain these.
- Kelp seaweed is my favourite mulch but unfortunately is almost impossible to get, and it is illegal to collect it from beaches. It provides sodium, boron (which soils in our area are deficient in) and other trace elements plus nitrogen and potash.
- Growing a green manure crop is a cheap and quick form of mulching. Once matured, cut the plants at ground level and spread them. Leave the roots in and they will provide runnels for water to enter the soil.
Other readily available mulches – often free – include grass clippings, leaves and coffee grounds and husks. All must be applied very lightly as they can mat and also produce an unpleasant odour.
- Grass, which will add nitrogen as it breaks down, should be dried and spread thinly. Avoid grass clippings with dog faeces and lawns that have been sprayed with pesticides to kill weeds. As a green material, grass clippings will support soil bacteria.
- Leaves break down slowly and, to prevent matting, they need to be spread very thinly. If green, they support bacteria; if brown, fungi.
- Coffee husks by themselves are a disaster turning into a heavy, rancorous mat but with coffee grounds rubbed through them by hand – a somewhat tedious job – they are usable and will improve the soil.
- Wood shavings and sawdust rob the soil of nitrogen (good for keeping paths weed free for that reason) and should be avoided, though they can be added to the compost bay.
Macleod Organic Community Garden has free wood chip mulch from Banyule City Council. Turn up at the garden with bags, tubs or trailers on Wednesdays or Saturdays between 1pm and 4pm and take as much as you like. Bring a fork or spade!