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Organic mulches

Over the next two lessons we will explore mulching (covering) to smother existing weeds, so they die in darkness. In this lesson we focus on mulches of organic matter, and in Lesson 11 we look at non-organic mulches, such as plastic fabric and woven covers.

To consolidate what I have already shown and explained in earlier lessons, we’ll concentrate on three aspects:

  1. What makes an organic mulch successful in killing weeds: the principles and requirements.
  2. Time sequences of mulching while also cropping: how the two can happen simultaneously.
  3. Potential problems with edges, and the need and methods for keeping them clean and in the same place.

Anna Mcleish on Facebook, September 2018:

‘Thanks to no dig I am – for the first time since starting an allotment five years ago – spending this time of year making the allotment nice and tidy, pruning trees, seeing to compost and planning next year. Normally I’m hacking down six foot plants and weeds and ranting that I’m giving it up. It is so much easier it’s unreal.’

Organic mulches, year one

Be thorough

It’s essential to be methodical. Any ‘weak point’ in your mulches will be found by weed leaves searching for light, and some of them can move a long way, horizontally and under a mulch. Hence the importance of overlapping any cardboard sheets by 15 cm (6 in), plus, if there are adjacent weedy areas, of maintaining a light-excluding mulch along its edge.

Membranes and carpet in the soil

You may be unlucky enough to discover these under surface level: a mulch that someone laid as little as five years ago. As organic matter lands on the surface, they sink out of sight. Sadly it’s a digging job to remove whatever you find. It cannot be left there or it makes an impenetrable layer for worms moving up and roots growing down.

Level the surface

It’s easiest to make beds and mulch paths on a level surface. If it’s uneven, use a spade to move soil from the humps into the hollows. Remove any rocks and wood, including sides of old beds, old carpet, and any waste materials that you see. You can then apply mulches evenly and consistently, and in subsequent years you won’t turn an ankle when gardening!

Decide mulch thickness

How much mulch you need depends on the amount of weeds present, and how many of them are perennials. Below are guidelines in relation to this starting point (also bear in mind the descriptions of beds and paths in Lessons 7 and 8).

Five starting points in terms of weeds

Tamsin on forum topic: ‘Parsnips for the first time’, 14 October 2018:

‘Hello Charles. After 15 years of struggling with weeds and digging on our allotment we gave it up in despair. Luckily I discovered your methods and now have a no dig veg plot in the back garden. It has been remarkable in its first year and so easy to maintain while juggling lots of small children. And for the first time ever, we have harvested parsnips! A tiny patch produced a huge amount. This is on undug pasture, with six inches of compost on top, first year. I am so surprised, I can grow parsnips! The children and I all say a big thank you.’

These five categories of weed numbers and types are to help you decide your process of mulching in year one. Have a good look before starting; some areas of a new plot may need different mulches.

1. Few weeds and mostly soil visible

The easiest starting point. It might be ground which has been growing vegetables and flowers already, and was well looked after. Best to remove the few larger weeds, and then there are two possibilities:

  • If the soil looks in good condition, cover the beds with 5–7 cm (2–3 in) of compost, enough to create a soft and fertile surface. That is all you need to do.
  • If the soil looks stony and weed growth is not lush, apply a thicker layer of up to 15 cm (6 in) of organic matter and compost. This is my approximate upper limit for starting out, although I applied 20 cm (8 in) in the greenhouse, where cropping is more intense.

2. Many weeds, mostly annuals

Compared to the above, this simply needs a layer of cardboard to be laid before applying the compost. Cardboard slows weeds that are growing upwards as they strive to reach the light, and makes it unlikely that any annual weed leaves arrive at the surface before dying.

If weed growth is lush and tall I would cut them with a mower, strimmer or scythe, and then card over the cut leaves and stems. If you expect slug problems (in damp climates for example), you could remove the cut growth to your compost heap.

Watch for any regrowth, mostly of perennial weeds if there were some present, and pull any stems as soon as you see them.

A question on the forum from Zoe in Somerset:

‘If I am mulching onto neglected beds of seeded annual weeds, for instance a carpet of speedwell and chickweed, will a generous autumn mulch of rotted manure see it off by Spring?’

The answer:

‘Yes, for sure. These annuals are easy weeds to mulch because they have little energy in their root systems. Your mulch needs to be a level 5 cm (2 in) thick, or more if you have enough compost, with no weed leaves visible after spreading, so that no leaves can photosynthesise. Within a few weeks, and certainly by late winter, you will have clean soil underneath, with perhaps a few new weed seedlings to remove which will be growing in your manure or compost.’

If she had been faced with more than a few perennial weeds I would have recommended thick cardboard first, before applying her manure on the cardboard, see below.

3. Many weeds, and say a third or more perennials

This starting point needs more care, to be sure of not having perennial weeds coming back all the time. I suggest thick cardboard first, and 10–15 cm (4–6 in) of organic matter.

There will probably be some appearance of new leaves, straining for light and fuelled by energy stored in the weakening parent root. Don’t panic! Just keep removing any new growth, by pulling or using a trowel to remove the new shoots. You can’t lever out the parent root, but a time arrives when it just dies from having no more food.

How long this takes depends on the types of perennial weeds, and the vigour of their roots.

14. May: membrane rolled back shows cow parsley and couch grass, in a new bed made one month earlier with no cardboard under 10cm compost
May – the membrane is rolled back, showing cow parsley and couch grass in a new bed made one month earlier, with no cardboard under 10 cm (4 in) compost
14a. Using a trowel to lever out bindweed
Using a trowel to lever out bindweed
The view from Zino’s drone after Croatian TV filmed us creating a new bed on pasture, with cardboard under the compost
14c. We planted salads and spinach into the new bed on 10th October, and they all grew fine until flowering in the spring, with almost zero weed growth
We planted salads and spinach into the new bed on 10 October – they all grew fine until flowering in the spring, with almost zero weed growth

4. Woody growth and weeds underneath

Brambles are a common example (see Lesson 9 for my advice on dealing with them). For all plants with woody stems – including shrubs, honeysuckle, lavender and buddleia – you need a sharp spade. Perhaps even a pruning saw or axe, to cut or chop the thickest roots just below surface level.

The main non-woody weed that I find worthwhile to cut out with a spade is dock (see Lesson 9). Occasionally I take a spade or fork to the roots of stinging nettles, where they are overgrowing an edge for example. Otherwise I cut nettles to ground level, and mulch with thick cardboard.

15a. Using a spade to loosen the nettle roots
Pulling out a wire fence, which had stinging nettles growing through the wire
Using a spade to loosen the nettle roots

Here are my tips on how to remove enough root of large woody plants:

  • First cut diagonally into the soil and through the main roots, in a circle around the main stem or clump of stems. The circle’s diameter depends on the size of the plant, say from 15–45 cm (6–18 in).
  • Once the lateral roots are severed, push firmly on the main stem one way then another, to loosen soil from deep roots underneath and to enable you to push the spade onto them to cut through. This part may be easier with two people.
  • When the plant is loose, lift it out and knock as much soil as you can off its roots. It’s fine to leave many of the smaller roots where they were growing.
  • Use any nearby soil, or compost, to fill the hole and make it level. Then proceed as above, according to how many weeds there are.
16. Bed half cleared with tops cut, next I pull ivy roots and use a spade to remove roots of bramble
A bed half cleared with tops cut – I then pulled ivy roots, and used a spade to remove roots of bramble
16a. Bramble's new roots: the stems spread a long way and then root. Best pull out at this stage
A bramble’s new roots – the stems spread a long way and then root; best to pull them out at this stage

5. Mostly perennial weeds of notable vigour

Using only organic mulches on these weeds can kill them, but not before you may have spent a lot of time removing the regrowth, while parent roots are gradually losing their reserves of food. In year one you may need to weed repeatedly, until you see no more regrowth.

In the next lesson I explain about using polythene/non-organic mulches, which is easier for thick growth of perennial weeds. The drawback is that it’s less easy to grow vegetables in year one.

For those in the USA who suffer Bermuda grass (Cyonodon dactylon) I offer sympathy, because its root system is deeper and stronger than couch grass. However, I have heard from a few people on You Tube that mulching killed their Bermuda grass completely. Mulching, or total light exclusion needs to be:

  • Probably for a minimum of one year, and preferably with plastic.
  • 100% thorough, with nothing growing through holes in year one.
  • Over a larger area than is planned for growing, to reduce subsequent invasion from edges.

To finish this section on a positive note, I saw this comment on Facebook, from Dawn Canham in London:

‘Bindweed has been completely manageable with cardboard and no dig – we just keep pulling up the shoots and it’s giving up.’

Types of organic mulches

The table below summarises common mulches of organic matter. I have not mentioned them all in the text, because of the values of compost and cardboard as main mulches for clearing weeds and cropping straightaway.

Mulches such as hay and straw are best for dry climates, where slugs will not be common. Like wood chips, they decompose in situ, and turn to compost at soil level in months or years. They have value for paths as well as beds.

  • Preferably use organic hay and straw, to avoid the chemicals used by farmers to grow them.
  • Hay usually has a lot of weed seeds, so what starts as a nice solution can create more work.

For annual vegetables, some mulches make cropping difficult. You can’t sow carrots in a mulch of wood chips for example. Hay mulches stick to lettuce leaves when you are picking them. Straw mulches are light in colour so reflect sunlight, which keeps soil cool through spring, just when you need it to warm up (especially in cooler climates). Which mulch or combination of mulches you use depends on:

1. Which are most available to you

2. Your climate

3. The weeds currently growing

4. How soon you want to sow and/or transplant (the table below gives some pointers)

Mulches for killing weeds by light exclusion


These sequences show how there is no fixed time between making a bed on top of weeds, and  then sowing or planting. A big advantage of using compost mulches (rather than plastic) is the ease of sowing and planting straightaway.

Because it’s possible to make beds at any time of year, sometimes you will find yourself sowing and planting on the same day as you create them. If that is the case, be sure to tread the compost firm. We call it the no dig dance: you can’t compact compost.

Creating new beds to double crop in year one

Winter is a great time to create beds, even when the ground is frozen (only not if snow is deep). Just look for any time in the quiet season when you can start. Then you will be ready to plant in spring, which, in my experience, always comes in a rush. So it pays to be ahead.

The photos below show new beds that we laid out and filled in November, almost at the end of Homeacres’ growing season. In one corner we popped in a few spare broad bean plants, and they gave harvests by late May. For the rest of this area I waited until March, the right season for the plantings I wanted to make.

1. I measure the new bed to 1.2m/48in between the insides of the sides
Measuring the new bed to 1.2 m (48 in) between the insides of the sides
3. Tipping compost which is old and lumpy cow manure, and best as a bottom layer
Tipping compost which is old and lumpy cow manure, and best as a bottom layer
2. I use a rake to level the first batch of lumpy manure before we spread finer compost
Using a rake to level the first batch of lumpy manure, before spreading finer compost
4. Homemade compost was most of the surface
Homemade compost made up most of the surface

The March plantings were helped to establish by having fleece over. It’s a soggy area with squelching paths at this time of year, so the raised beds were helping plant roots stay aerated.

  • In subsequent years the drainage has been noticeably better and, when the soil is saturated, these paths no longer squelch.
4a. New plantings on 27th March - coriander and dill. Beyond are overwintered salads under fleece
New plantings on 27 March – coriander and dill; beyond are overwintered salads under fleece
4b. We laid fleece over the new plants as soon as they are in the ground
We laid fleece over the new plants as soon as they were in the ground
4c. So much changes in spring and this is the wider area on 27th May. The herbs are still growing on the right
So much changes in spring, and this is the wider area on 27 May – the herbs are still growing on the right

By May the coriander and dill were showing signs of flowering, and in early June we interplanted module-raised plants of beetroot between them.

By that time the cabbages were making hearts, and we popped in French bean plants as soon as they had finished, after twisting out the cabbage stumps. The beds stayed full in summer and autumn, and sometimes even into winter. For example, after the French beans finished in October, we planted chervil and spring onions.

The beetroot harvest was in December. One beetroot weighed 1.9 kg (4.2 lb); large roots grown like this are perfectly tender and juicy. They are also good to store over winter – we ate the last ones in late April.

4cc. 15th July in a dry summer and the new beetroot right have had very little water
15 July – a dry summer, and the new beetroot (on the right) have had very little water
4d. 7th October and we recently planted chervil plus spring onions, and harvested beetroot
7 October – we had recently planted chervil plus spring onions, and harvested beetroot
4e. Two weeks later in the early morning. Fennel leaves top left
Two weeks later in the early morning – fennel leaves are top left
4f. In a mild December, still many vegetables to harvest. Bed on right now mulched with 5cm compost
In a mild December – there are still many vegetables to harvest; the bed on the right has been mulched with 5 cm (2 in) of compost

Whenever cropping finishes for the year, we spread compost to protect and feed the soil throughout the coming year. In the case of the middle bed this was in October, after we had cut the French bean plants on their stems. Most roots remained in the soil, and we spread 3 cm (1 in) of old, fine compost, before the new plantings of chervil and spring onions. They were cropping until the following May, so it made sense to spread the compost mulch in September.

There is a myth that legume vegetables leave a lot of nitrogen in the soil, for the benefit of subsequent plantings. As with so many statements, this needs qualifying:

  • Legume vegetables which have finished cropping do not add much nitrogen to the soil, because most of it (up to 97%) has been used by them to grow. They are not natural philanthropists.
  • For soil to have a nitrogen boost from legumes’ pink nodules, you need to cut off the plants before cropping, when in full flower.
  • Nonetheless there is valuable organic matter, and other nutrients, in the roots of legumes and all vegetables. Old roots are food for microbes, and indirectly feed new plants.

After the beetroot harvest in December, we spread 5 cm (2 in) of slightly lumpy homemade compost, which was eight months old. In volume terms, it’s probably an equivalent depth to 3 cm (1 in) of finer compost. The lumps shatter in frost, and after a light rake in early spring the bed is ready to start again. Preparation is so quick with no dig.

Mulching and planting undercover

Year one, greenhouse – mulching cleared 100% of the couch grass

Before the greenhouse was built, its whole area was full of couch roots. In February, the first thing we did was lay some cardboard, when the wall was in place and before the greenhouse builders arrived. After that, my final compost spreading happened in March.

  • Only a few blades of grass made it to the light and, by August, there were no more spears of new grass. Thick cardboard and 20 cm (8 in) of compost were more successful than I had dared to hope, after the amount of rhizomes I had seen when the builder took out soil to build the walls.
Year one, polytunnel – erection, mulching, cropping

We put up the polytunnel before doing any mulching, on pasture with weeds. It was an old tunnel and I dug a trench to lay the polythene in. The photos below show  the subsequent mulching and planting in year one.

In January 2013, as soon as the initial mulch of cardboard and 10 cm (4 in) of old cow manure had been spread, I planted some of Steph’s spare pea and broad bean plants into the mulch, during weather of frost and some snow. They provided welcome food by late May. By that time their roots were into the soil, below the compost and decaying cardboard.

In mid-April I planted multi-sown Boltardy beetroot, which had also cropped by the end of May, to everyone’s amazement after a cold spring. Polytunnels are wonderful things, and great value per area covered.

5. I had already banged in the “foundation tubes”, then it needed just two hours to erect the 11 year old frame, from my previous garden
After banging in the ‘foundation tubes’, it then needed just two hours to erect the 11-year-old frame, taken from my previous garden
6. Once we had the polythene secure in January, I began mulching the many perennial weeds, using no polythene mulches
In January (once the polythene was secure) we began mulching the many perennial weeds, without using any polythene mulches
7. How it looked in February, even with carrots sown bottom right, and there was some soil under the composts
How it looked in February – with carrots sown bottom right; there was some soil under the composts
8. Bindweed pushing through the first mulch in early spring, the first time I knew it was there as it’s dormant through winter
In early spring, some bindweed pushing through the first mulch – this was the first time I knew it was there as it is dormant through winter
9. All finished mid April and still some night frosts, now waiting for warmer weather before planting frost-tender plants
All finished by mid-April, and there were still some night frosts – now waiting for warmer weather before planting frost-tender plants
9a. Harvest May 24th of carrots, sown January on a new bed of manure and some soil
May 24 – a harvest of carrots, sown in January on a new bed of manure and some soil
9aa. Polytunnel May 30th, all tomatoes planted, legumes cropping and the beetroot to right
30 May in the polytunnel – all tomatoes have been planted and legumes are cropping; beetroot are on the right
9b. By mid July, vegetables are well grown and I did hardly any weeding all year, mostly bindweed
By mid-July vegetables are well grown – I had done hardly any weeding all year, and it was mostly bindweed
sow & propagate
Transplant - Size, time of year, Spacing, support
container growing
Prune and train plants/thin fruit
Harvest times and method
Potential problems