Using wood chip as a mulch

Early last year, I started working with an Arborist, and we offer our tree Clients the option of keeping the woodchip which we produce, to be used as a mulch in their garden.

Here is an example, left: woodchips spread at the base of espaliered fruit trees, allowing the gardener to prune and attend to the trees, without having to step on earth, but without the expense of installing hard landscaping such as paving.

We like to do this, because it is about the most ecologically-sound thing you can do - for many reasons!

Firstly, it's the product of your own garden, so you are keeping the nutrients within the garden, instead of extracting them and, in effect, throwing them away. This is an extension of composting, where we strive to reclaim all the nutrients and minerals, rather than paying the bin men to take them away, or having to waste petrol going to the tip with bags of garden waste, or - worst of all - burning them.

Secondly, birds love mulch! It is quickly colonised by a myriad of bugs, beetles and other critters beginning with 'B', and the birds have a lovely time, flicking it about and scoffing to their hearts' content.

Thirdly, it means we don't have to charge our Client for taking it away - which reduces the cost of the job. 

Fourthly, it's free! Why buy expensive bags of chipped bark, or other mulch, when you can re-cycle your own trees and hedges?  Plus, if  you buy-in bags of chipped mulch, you never know quite what you are going to get, whereas when it comes from your own trees and hedges... well, you know exactly what's in it!

Some people, though, are reluctant to put chippings on their beds, for various reasons, so let's run through a few of those.

Before we get into the science of it, let's be clear about the terminology here: woodchip is applied as a mulch, which means that it sits on the surface of the soil. It is not "dug in", the way that compost, manure, 'organic matter' (posh word for manure) or soil conditioner are. 

So, what are the main concerns, then? 

Nitrogen stealing:

This is to do with the theory that wood chippings will take nitrogen from the soil, during the act of decomposition.

Not the case: as the mulch sits on top of the soil surface, and is not dug in, there is only a very small amount of nitrogen extraction being carried out by the chippings, and that all occurs on the interface between the underside of the mulch layer, and the surface of the soil. This means that any small transfer of nitrogen from the soil only affects the surface, and has no impact at all on the soil below, which is where the roots of the plants are living.

This, by the way, is possibly one reason why a thick layer of mulch suppresses weeds: although personally I think it is more to do with it being a physical barrier to prevent weed seeds making contact with the soil and by excluding light from the seeds already lying on the surface, before the mulch was applied.

Pathogens:

If the tree has any "germs" in it, will they not be transferred to the plants around which the mulch is spread? Often, the Arborists are called in because a tree is damaged, or diseased, or dying: not just to make it a bit smaller, neater, or tidier. It's a fair point, but again, chippings are used on the surface of the soil, not underground where the roots are.

Allelopathy:

This is a variation on the theme of Pathogens: there are some trees which contain chemicals, which  suppress the growth of competing plants. Walnut is allegedly one (although I have never yet found a Walnut tree with nothing growing underneath it), Rhododendrons is another, as is Cherry Laurel. So would wood chip (Digression: "how much wood, would a wood-chipper chip, if a wood-chipper would chip wood?" Answer: we can fill a one-ton builder bag in about ten minutes of intensive chipping!) therefore not kill everything, where you spread it? The answer is basically "No", because mulch sits on top of the soil, it is not dug in: meaning that any allelopathic chemicals do not get incorporated into the soil, where the roots of the plants are.

Cleanliness: next to godliness, so they say.

Chippings are bulkier than bought mulch: the chips form a matrix which creates a whole ecosystem for bugs, worms, and even small mammals to live, work, eat, and die in. Importantly, this matrix also allows air and water down into the soil, and the larger size of the chips means that soil and "dirt" gets washed down between the chips every time it rains, so the surface stays clean and mud-free. This makes it easier to walk across the area without getting muddy feet, which is nice for the gardener, and nice for the lawn as well!

Smothering the plants:

You often read the phrase "apply mulch in a deep layer, 4-6" deep."  I have never yet, not once, seen mulch spread that deeply. Good heavens, it would make the beds look ridiculous, if you piled 6" of mulch on top of the bed!  I would say that 2" is a more reasonable depth to aim for: enough that you can't see the soil through the mulch, if you casually stir it with your foot.

And personally, when applying mulch on "my" gardens, I always spread or rake it out by hand, and I tend to draw it back from the stems of shrubs. Herbaceous perennials generally find their own way up through a layer of mulch, but I would never put mulch over Peonies (because they don't like their tuber to be buried, it adversely affects the flowering) and Bearded Iris, because their rhizomes need to sit on the surface, and be baked by the sun each summer, in order to promote good flowering the following year.  

Bulbs, of course, will push their way up through any amount of mulch, even through this pile of bricks - right.

And I am very fussy about not piling wood chippings, or any sort of mulch, for that matter, up around the trunks of trees. I will always draw the mulch away from close contact with the bark of a tree.

That deals with the usual objections: there are also a couple of really good, positive reasons for using wood chippings as a mulch.

Soil surface preservation:

A thick mulch of chippings slows down the effect of rain on the soil: during heavy rain, instead of the soil being battered and waterlogged, the mulch absorbs and slows down the passage of the rain, allowing it time to be absorbed gently into the soil below. This prevents the creation of a surface pan, or "capping" where excess water dries and creates a crust: it also prevents soil erosion and the washing out of nutrients.

Temperature regulation:

A thick mulch of chippings protects the soil - and therefore the roots of the plants - from extremes of temperatures. In summer, it takes the worst of the baking heat - ah, remember baking heat? Did we have any, last summer? I can barely remember being hot... *sigh* - which keeps the soil below cooler: and in winter, it obviously provides a snug anti-frost covering.

Long lasting:

Chippings last a lot longer than bags of bought mulch, which quickly "composts" down and disappears into the soil. Sometimes this is what you want, but sometimes you want a mulch to remain on duty for longer, and chippings are ideal for that.

Where to use them:

On beds and borders: in a fairly thin layer. Don't worry about bulbs, they will push right up through the mulch with no problem.

Under hedges: as thickly as you like, which will help to cut down on weeds, and will provide a lovely ecosystem of bugs to feed birds and small mammals.

Under and around trees, for a natural woodland look

As a top surface for paths: wood chip makes a good all-weather surface which isn't dusty in summer, and doesn't make your shoes muddy in winter.

To absorb moisture from areas which have been trampled and worn out: just spread it around on the muddy areas, and keep topping it up until it's no longer muddy!

So there you go, many good reasons to use wood chippings as a mulch on your garden!

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