When to use mulch

When to use mulch / mulching is a much misunderstood practice, the amount of misinformation out there is astonishing and the many forms available confusing.

There are not many gardeners out there who fully understand the importance of a good thick many layered mulch. This is a process that is continuous spreading layer on top of layer year in and year out.

Well tended soil is a thing of great beauty. To put the spade in and see the layers of wood chip in various stages of decomposition is very rewarding, the lower layers should be white with mycorrhizae indicating garden soil that is truely alive. Never ever use fungicides. If plants suffer from fungal infestations they’re growing in the wrong place, try something else. Fungicides kill mycorrhizae.

Pea straw, sugar cane and other soft mulches are good for annual vegetable crops, once the crop is harvested the mulch is forked in and a fresh layer added. In other parts of the garden they decompose very quickly and leave little residue. Pine bark is ok but should be partially decomposed and fine. Large pine bark takes way too long to break down, the chips are usually of a consistent size and unless constantly applied does little to improve soil health.

The best mulch by far is chip from an Arborist. The various textures of leaf, bark. twigs and heart wood means the process of breaking down is staggered, the wood being last and this helps to open up the. soil. allowing for drainage and air circulation. This mulch should be applied thickly 15-20cm and frequently. At least annually. I don’t mind what species the Arborist is giving me.

Wood chip has much misinformation attached to it and I’ll give you my take on some of it.

When to use mulch. Hydrangea love a cool root run, a thick layer of mulch goes a long way towards achieving this
Hydrangea love a cool root run, a thick layer of mulch goes a long way towards achieving this. Don’t worry about pushing the mulch hard up against their trunks they will just grow roots into it.

Nitrogen leaching.

This is the most common. I’m not a scientist so I have no idea if this is a thing or not but I can say that after 20 years of spreading wood chip here and in other gardens I have never seen any evidence of a nitrogen deficiency. I have on many occasions spread it straight off the back of the truck green and steaming, with no ill effects. I do sometimes spread fertiliser under the mulch but only to feed my plants and with no thought of replacing leached nitrogen. If you have the luxury of being able to age the chip this is good practise in that the ageing process encourages microbial activity. Here I’m generally in too much of a hurry to spread it so this doesn’t often happen.

Becoming hydrophobic over summer.

I haven’t experienced this but can imagine in a drier climate this could occur. My response would be to either add a fresh layer or loosen the mulch with a fork.

Its ugly.

This one never fails to floor me. In my garden the soil surface is not about aesthetics it is about creating a soil structure in which the plants I grow can flourish. In a newly planted garden the sea of wood chip can look dreadful but it has a vital role. As your garden matures it will become less obvious.

I have seen a look of horror spread over some peoples face when they see the soil surface here. I am a great believer in wood as an agent for soil improvement so sticks and branches are not cleaned up but rather left on the beds to be covered with mulch in the future. Likewise with prunings, I often just roughly chop them then leave them scattered under the shrubs. Larger chunks of wood play a vital role in opening up the soil and creating a light airy structure.

A bit about autumn leaves.

My garden has many mature deciduous trees, when I moved here I finally understood why the Americans call this season fall. The cleanup starts in mid Autumn and lasts for 3 months. All leaves go straight on to the garden beds, piled up very deep and in tact. I know some people compost them which is terrific but a separate process from mulching. I like them whole as the decomposition takes longer so they open up the soil. Woodland plants are very accustomed to pushing through leaf mould. Imagine the depth of litter in the deciduous forests they come from.

The leaves themselves don’t really constitute mulch, their tenure is too brief, by mid summer if left alone they have vanished. Over the spring and early summer I put a layer of chip on top of the leaves and so they gradually work their way down and become yet another layer of humus. This is my way of developing what’s described as leafy soil. Much is made of Oak leaf and there is no doubt its good but Beech is my favourite. The big leaf deciduous species such as Plane trees will require a good load of chip on top of them to speed up their decomposition. They will if given this treatment still make fine left mould.

When to use mulch. The soil has a light open structure and its hummus content is very high.
After many years of chip and autumn leaves. The soil has a light open structure and its hummus content is very high.

Compost is not a mulch, it is a fertiliser and soil conditioner. Compost is spread prior to mulching so it sits under the chip where it can work its magic. Used as a mulch it dries out so most of the goodness is lost and some composts form a crust and become hydrophobic.

So in summary, mulching is a layering process. A fresh layer spread on the old, at a minimum annually. As each layer works its way down it slowly decomposes and opens up the soil, retains moisture and provides nutrients for your plants. In gardens with difficult soils, like sand or heavy clay apply mulch twice yearly Autumn and late spring and spread it very thickly, 15-20cm. You will be amazed how quickly the soil structure improves. I have never encountered collar rot but can imagine for some plants its an issue, citrus springs to mind as do many Proteaceae. The vast majority of plants however have no issue with mulch piled against their trunks.

Develop a relationship with a local Arborist so they know yours is a good place to dump excess chip.

When to use mulch. Aucuba japonica var. honshu in deep soil rich in hummus
When to use mulch. Aucuba japonica var. honshu in deep soil rich in hummus
Plants that grow in the shade of others are accustomed to deep soil rich in hummus. In this case Aucuba japonica var. honshu

One thought on “When to use mulch

  1. Louise says:

    This is an excellent, informative article. Very helpful and answered all my questions. I wish I’d read it years ago. Thank you for writing it.

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