Tuesday 17 January 2017

The advantages of mulching

Much ado about mulching

Mulch is a thin cover on the soil surface. Mulch gets thin cover in the horticultural press too. Reporting is very shallow.
You probably know that mulching makes plants grow better, conserves water, looks good, insulates the soil and can suppress weed. You might add that it protects the soil from erosion in heavy rain and keeps the mud splashes off the strawberries.
For many gardeners that is sufficient but you won’t get away with so little today. As is my wont, I want to dig deeper!
Except I don’t dig. A  perceptive student once inquired why as I am so keen on minimum cultivation I don’t just put a mulch on top of the soil and get a cavalcade of further advantages. He was right of course, it was a very shrewd insight. Although I have often rather snidely declared that the main advantage of mulching is to prevent the over enthusiastic gardener disturbing the soil it might actually be true.
As I so often promote the benefits of none cultivation I will spare you that today!

I bark mulched last May. Three cubic metres delivered from a local nursery £120

Mulching materials
The actual properties and gardening merits vary with the material. The very best for water conservation are gravel and stones. Include in this list larger rocks and un-cemented paving. How often do you hear that clematis likes a ‘cool root run’ under stones. Partially true. The active principle is that loose stones and gravel do not intercept water and even light rain nearly all penetrates through. Like most mulches, as intended, they do reduce direct evaporation from the surface. Permeable plastics and substances such as polythene add to the list of good water conservers. 

Bark mulch gives all year round cover
Mulches are usually fairly long lasting surface materials although those such as bark and wood shreddings need topping up every few years as they slowly degrade. Discuss with the rabbits and moles the permanence of stone chippings.

Peter was mulching last week (I think this post reminded him!)
We also use the term mulching to include surface organic materials such as farmyard manure and garden compost. In actual fact these materials are usually intended to be incorporated by the worms and only remain at the surface for a while. Manure mulched rose beds annually topped up might last the whole year. Lawn mowings and Autumn leaves are perhaps a half way house in this crude classification. Mulching as a very effective means of adding organic matter is not really my subject today.

I will use my plastic scarifier to sweep the leaves off the the grass path on to my borders

Weed control 
Other than plastics, mulching against existing established perennial weed is almost useless. The weeds will love the mulch as much as the plants. We will say nothing about gardener’s attempts to smother perennials with newspaper and cardboard!
Against weeds coming from seed mulch’s measure of control is really quite useful. It seems to be generally agreed that mulches need to be about two inches thick to stop weed seed germinating. This cover will be enough to suppress light levels at the soil surface to inhibit germination and enough to prevent emergence of small seeds. It won’t help very much with seed that blows onto the surface or is shed in situ (shame on you).
In my own case the thinness of my wallet reduces the thickness of my mulch. My weeds are well enough controlled by my glyphosate spraying and I actually want many of my garden plant to sow themselves around. (This is why my mulches never overlay plastic - I want my garden plants to self seed).

Hand weeding anyone?

My Scilla biflora feely seeds itself around
This cyclamen would have not sowed itself if the gravel was under-laid with plastic
People tell me that weeds that do sow themselves in mulch pull out very easily!
This year I newly mulched a large part of my borders with bark. This was to pacify Brenda who can’t stand the liverwort! 
The freshened up borders looked very nice.

An insulating layer. Be careful what you wish for
In general a mulch’s insulation might be thought to be a good thing if it keeps soil warm in Winter and cool in Summer. 
Not all mulches are insulators and plastics are only thin layers. Some colours are good heat absorbers and/or emitters. The relationship between day and night temperatures might be just as significant as those between seasons. It might for example be a difference between crop failure and success if night air above mulch is too cold and fruit blossom is frost damaged.
It is really quite complicated - what do you want? If a Winter laid mulch keeps the soil colder this might be thought to be a bad thing. On the other hand if my frost delicate Dicentra spectabilis emerges a little later I will be very pleased.
In practice in normal UK conditions I ignore potential exceptions, lay mulches at almost any time of the year and in terms of heat transfer hope they are beneficial!

Three case studies about soil temperature
*In my earlier post about gravel mulching a lady wrote in that in her tropical climate gravel got too warm in the hot Summer sun. I must say that on our subsequent holiday in Costa Rica I did not notice this effect on the weeds growing in the hotel’s gravel roads! On the other hand the dry sand on the beach was too hot to walk on. I imagine in such climates the soil is much cooler under stones and although hot for a plant to clamber over keep roots happy?

*A neglected exploitation of mulch’s insulation is putting an extra layer over dormant tender plants such as dahlias in Winter. The mulch might even be extra soil!
This year we had five months of spectacular colour of dahlias overwintered in the ground - it had been a very mild Winter. To increase my chances next year I have carefully folded over the large frost-dead tops of the dahlias and supplemented that with a layer of my miscanthus prunings and scattered amongst it mole soil from the lawn! I will remove it all in March! It does look a mess but the dahlias are worth it.

 Dahlias advertise my vulgar taste next to the road
The mulch will have confirmed my reputation

This mulch was an accident
I have only limited hopes for my effort! Insulation might be successful if Winter cold comes in short sharp spells.
If we get a prolonged period of penetrating frost there will only be a very small benefit. With only a very small heat source from deep soil my mulch will be of very little value in stopping the soil freezing.
As an analogy in our extreme 2010 Winter the inside of my unheated greenhouse was as cold as outside. Your own Winter coat only keeps you warm because of the heat from your body. Blog sleuth and myth buster Robert Pavlis writes about this phenomenon when he tested soil insulating cones.

The best insulation for your dahlias is to plant them very deeply at original planting!

 *When I went to see Peter he showed me the graphs he had drawn when he wrote his PhD thesis. They showed how soil surface temperatures fluctuated throughout a sunny Summer’s day. It is really quite astounding. Most mulches would dampen such fluctuations although thin plastics might increase them.

Drawn by hand before computer graphics, Peter measured soil surface temperatures on successive Summer days.
(Ignore the two horizontal lines)
Water retention
My old Head of Department, a soil scientist, contended that mulches contributed very little to soil water conservation and I used to argue that they did.
I thought before finalising this section I would have a word with soil scientist Peter Williams and found that he was not very enthusiastic either. He emphasised that never-the less he thought mulches in the round were a very good thing.  He went on to mention that he had recently assessed the soil moisture under the mulch in his garden after heavy rain that had followed a dry period and found that very little water had penetrated through. Mulches reduce water evaporative loss from a soil surface but they sometimes intercept rain before it can penetrate in.

At that point I scrubbed my text for this section and started again!

I think a key attribute of a water conserving mulch is that it should let water pass through without soaking it up! I have long argued that none absorptive materials such as gravel and to a lesser extent coarse bark act like a one way valve for light rain that otherwise would evaporate away.

A significant thing to understand is that an un-mulched wet soil surface becomes dry very quickly in drying weather. It rapidly becomes its own mulch in terms of water retention and soil surface evaporation stops. It will start again only when the soil is re-wetted and in the case of light showers the water will be very soon gone. 
Gardening books will correctly tell you that the best time to put on a new mulch is when the soil is already very wet. I put this to Peter and he explained that although the water retained by such a mulch would be a very useful equivalent of perhaps half an inch of rain that this was not very much in relation to the soil’s total available water content.
Leaving gravel and plastic aside, I think a mulch’s greatest contribution to soil water relations might be to keep the soil surface ‘in play’ in dry weather. When a dry un-mulched soil has reached the stage of ‘self mulching’ it is dry several inches down. In that zone root water absorption and such as mycorrhizal activity will temporarily shut down. Considering that the most fertile soil is usually at the surface this is not a good thing. Peter added that such dry surface soil can also get harmfully hot on a warm Summer’s day (see his graph).

If roots come right up to the surface and into a mulch so much the better

My Sternbergia lutea is growing in the gravel

Cosmetic value  - disguise. I have my own problems when I try to grow ferns on my hydrophobic sandy soil! I plunge ten litre pots to give me a watering lip. 
Things people ask.
Do mulches such as wood chips deplete the soil of nitrogen?

One might counter with the question, how could a layer at the surface do so? 
It can! Most of the decay of woody mulch is a result of fungal action. Fungus mycelium grows down into the soil to draw up nitrogen to maintain a suitable carbon/nitrogen ratio.
The good news is that the nitrogen extraction is very small and for most practical purposes can be ignored.
Not so if woody mulch is worked into the ground. If it is a problem it can be corrected by a light dressing of a nitrogen containing general fertiliser.

Peter’s recent three inch mulch of composted mowings and leaves won’t deplete nitrogen 
Can fresh wood chips, bark or shreddings be used without composting?

There might sometimes be slightly toxic content of fresh material that might damage very delicate plants.
In practice with my well established sturdy perennial borders if I find an arboriculturist anxious to rid himself of a lorry load of his shreddings I ask him to dump them and I use them straight away.

Is a gravel mulch suitable for herbaceous borders?

Not if your border is the kind where you are aways transplanting. I personally prefer bark for a herbaceous border
On the other hand I love gravel mulch around free standing specimen herbaceous plants - and shrubs too


My previous post on mulching
Hardcore gardening


  1. We use lots of wood chippings to,cover the weed control fabric on our plot as the council deliver as much as we want free. Really it's use is to hold the fabric in place. If wood depletes nitrogen why do commercial composts often get away with using it as a major ingredient. I have often wondered whether this is a factor when plants fail to thrive in a given compost.

    1. Good question Sue. I wish there were more!
      You know I am dubious about a lot of the composts offered these days.
      Our local recycling plant accepts lots of wood with which they make 'municipal compost' The local farmers seem quite keen to take it.and I have recently said yes to a couple of loads!
      Although wood absorbs nitrogen when it decays the nitrogen is not lost from the organic cycle because in later stages of decay it eventually comes available - over the years for coarser wood.
      When shredded for compost and properly put through a composting process and nutrients added(?) the final compost from woody ingredients should have a suitable carbon/nitrogen ratio.
      Any compost for garden centre sales fortified with fertiliser should not be defficient.
      You might be right to suspect that compost is not always properly made.

    2. It soon seems to run out of steam unlike the peat based alternative. Trouble is it seems anything can be foisted off on gardeners as I found out when we had the manure problem. People like Trading Standards have no interest.

    3. Oh for peat based composts!
      Not really and as you know I am very keen on my soil based compost and/or my charcoal mixes from the weathered fortified remains of my bonfires.
      Peter, featured in the article, is very pleased with his homemade potting compost too - made from the mulching material shown, but more decayed and supplemented with fertiliser

  2. I am reassured by your comments about dahlias. I have thought that if they are deeply planted, and if the soil is built up of covered quite deeply during the winter (perhaps with straw) then there should be sufficient insulation to protect them from frost (in the UK if not in Siberia!). I think the same should apply for gladioli. These are good sources of cut flowers on the allotment and storing them out of the soil over the winter is a bit tedious.

    1. Deeply planted dahlias do seem to make their new tubers at the same depth each year which helps.
      Undoubtably people lose dahlias after a very cold Winter but I have had both dahlias and gladiolus regularly surviving - but it does tend to be a bit hit and miss.
      A common story is that on less well drained soils they don't like it.
      I have now several clumps of gladiolus doing very well in their fifth year

  3. I put down bark mulch a few years ago, on top of my usual cardboard, it looked great. But unfortunately my neighbour's cats thought I'd made a litter tray :-(
    I've tried Strulch, and were it not for the cost, I'd use more of it. It's very light so the bags are easy to move, and it fluffs up and goes a long way. So far the cats haven't shown any interest in it, probably because it smells rather strange.
    When I rule the world there will be a law that says that cat owners need to make a sand pit in their own garden. *now hiding under the table while cat owners tell me where to go*

  4. Belated happy new year Sarah
    Trust you to come up with an interesting angle!
    Cats also seem to like fluffed up un-mulched soil too.
    'Smells rather strange' does not endear me to the thought of 'strulch' - but I have no experience of it.

    1. Thank you Roger, a happy new year to you too!
      Strulch smells a bit like the yeasty smell of a brewery. Not especially nice, but not especially horrible either. And a lot nicer than the bark smelt after the cats had been visiting it for a couple of weeks!!!


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