Sunday, 15 January 2017

Too much mulch?

It is self evident that if you cover a plant completely with mulch you will kill it through lack of light.  This is one of the benefits of mulching - weed control.  However, what is the correct amount of mulch to apply to crop plants? 

There is a lot of advice to suggest that heaping relatively large amounts of compost around fruit tree trunks could cause the trees to die.
There is a reduction of oxygen diffusing through to lenticels and reaching respiring cells below bark in the trunk.

Now I went along with this until I though, hang on a minute.  When I put woody cuttings into soil or compost, there is a similar reduction of the free flow of oxygen to growing cells in the woody stem of the cutting.  Lenticels are covered in the same way.  So how do cuttings survive this treatment? 

My rooted willow cuttings from stems just pushed into the soil. I put quite a bit of woody
mulch on these too.
I have just been turning the composts today and found some woody stems I threw in  from the willow.  They were right in the middle of the compost and merrily throwing out new growth. 
Layering woody shrubs would not work if this were correct and we would never get Irishmen's cuttings. 
For an explanation to have credibility it must be consistent with all the data collected. Inconsistencies must lead you to question sloppy explanations. 

So here is another piece of data that does not really fit in to the " not too much mulch" hypothesis.  I tend to heap compost, manure and woody chippings around my soft fruit bushes.  They do not die throwing out lots of adventitious roots into the compost.  A little irritating because this means that the roots need to be covered again when the mulch decomposes and they are exposed. 

Blackcurrants and rhubarb with a thick woody shredding mulch.

Blackcurrants with a thick horse manure mulch.

I think that much more tree and shrub death occurs from poor planting practices than the addition of too much mulch. 

Brussel sprouts with a thick woody mulch to stop them from falling over.
Mulching brassicas with compost or hoeing them up does not kill them.  I do this to stop them from flopping over.  The Brussel sprouts produce adventitious roots that root into the mulch or soil. 

Having said all that, I have found that it is unnecessary to add more than a couple of centimeters of woody shreddings to get all the benefits associated with mulching.  Adding more does risk substantial nitrogen drawdown.  This immobilisation of nutrient does have an affect if excessive woody shredding material is used for mulching.


  1. Although I agree with your observations I would always try to avoid changing soil levels at the base of an old tree unless I knew it to be tolerant of bark being buried.
    It is most unlikely normal mulching will ever harm woody plants though I would tend to taper off the depth of mulch at the base of a tree.
    By coincidence I am currently blogging about mulches!

    1. Hi Roger,
      I was also thinking about the amount of litter that is lost from the canopy of a wood. There is a constant rain of leaves and dead wood. This can be seen on the forest floor as a thick layer of mull organic matter. Although this is constantly being decomposed and incorporated into the soil by invertebrates, there is still quite a noticeable layer over the bases of trees. If this is a normal occurrence in a common tree habitat then surely there would be an adaptation which would prevent it becoming a problem. I would appreciate your comment on this Roger.