Quite a pervasive myth this one. I'll just let a few quotes from a Horticultural Scientist - Dr Linda Chalker-Scott blow it out of the water. She is an Associate Professor in this field at Washington State University.
From a scientific viewpoint, there are few drawbacks to using arborist wood chips but many benefits. It mimics what you might find in the duff layer of a forest – which is really what we should be shooting for in many of our landscapes that are based on trees and shrubs. I can say definitively that if wood chips are used as a topdressing and not worked into the soil they will not tie up nitrogen. We’ve demonstrated this in laboratory research as have others.
.....many studies have demonstrated that woody mulch materials increase nutrient levels in soils and/or associated plant foliage. My hypothesis is that a zone of nitrogen deficiency exists at the mulch/soil interface, inhibiting weed seed germination while having no influence upon established plant roots below the soil surface. For this reason, it is inadvisable to use high C:N mulches in annual beds or vegetable gardens where the plants of interest do not have deep, extensive root systems
Another myth is that you must let wood chips age before you use them. Dr Chalker-Scott again;
Personally, I have never done this; I happen to love the smell of fresh wood chips and enjoy spreading them out over the landscape. Additionally, some of the nutrient value (particularly nitrogen if the chips contain leaves or needles) will be lost in the composting process. Using fresh chips ensures that some of the foliar nitrogen will feed the landscape rather than the compost pile
In the previous post we looked at some of the benefits of mulching. This time we will examine some of the things you need to consider when choosing which mulch to spread over your garden beds.
Firstly, if we look at nature we can gain some insight into the importance of mulch and it's composition in the natural environment. In dense bushland/forest areas, mulch is created naturally by pieces of bark, fallen leaves, twigs & branches that fall from the forest canopy and build up on the ground. This is why in those areas you'll always find a thick layer of organic material on top of the soil, keeping the soil moist and supressing weed germination. At the lowest layer of this organic material, the area where humus interacts with the soil surface, soil microbes and worms are continually breaking this organic matter down into a form that plants can use and mixing it into the soil.
What does all this mean to us in an urban environment? Firstly, the closest type of mulch to approximate the compostion of mulch in natural areas is the relatively cheap and sometimes free mulch available from your local tree lopper (or from me if I lop & prune your trees). Similar to the mulch in those natural environments it contains leaves, twigs, bark & chopped up branches. It's generally fairly chunky in its consistency which is important because it allows water to percolate through to the soil surrounding the roots of your plants. Another good choice is pine bark mulch which retains it's colour for a long period of time & has the added benefit of slightly acidifying the soil, which counteracts the effects of the mostly alkaline soils in Perth.
By contrast, fine particle mulches quite often pack down tightly over time, holding moisture above the soil layer forcing your plants roots upwards searching for that moisture. It also contributes to faster evaporation because that moisture is held in a layer closer to the drying effects of wind and sun.
The next post will be the last in this series on mulch and will deal with a prominent myth about using fresh woodchips. Does it tie up nitrogen or not?
Mulching provides a number of benefits to your garden beds. Firstly it moderates soil temperature, keeping it cooler in summer by shading it, protecting it from the harsh glare of the sun. It also keeps soil warmer in winter, acting like a blanket on those chilly cloudless nights. It reduces soil evaporation by between 30 to 50% which greatly reduces watering requirements. This is because mulch on top of soil reduces it's exposure to the doubly drying combination of wind and sun.
Another benefit of mulching your garden beds is a greatly reduced incidence of weeds in them. As long as the mulch is applied in sufficent quantity, it blocks light from reaching the weeds which they need to grow. In addition to this, any new weed seeds entering the bed, (e.g. dispersed by the wind blowing through your neighbour's weed infested garden) land on what is essentially bark or woodchip, not soil and so they can't germinate. If the actions of rain or reticulation eventually wash those seeds through the bark/woodchip layer to the soil beneath and germination does occur, those tiny weeds should expire before they reach light, which they need for further growth.
Yet another benefit is that mulch, as it is slowly broken down by soil bacteria, fungi and other organisms, releases nutrients into the soil making them available to your plants. Depending on the types of plants and mulches involved, a properly composted and mulched garden bed will rarely if ever need any artificial fertiliser inputs, which again goes toward recouping the initial cost of purchasing the mulch.
The next post will deal with choosing the best type of mulch for your garden.