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Lesson:
11

Non-organic mulches


In this lesson we look at non-organic mulches, which are mostly plastic sheeting. We consider when they can be useful, and when they are best avoided.

Clearing difficult weeds or mulching quickly

1. May: we planted squash in holes cut in polythene covering couch grass
May – we planted squash in holes cut in the polythene, which had been covering couch grass
1a. May: potatoes emerging during a cold spring, were planted through holes in the polythene
May – potatoes emerging during a cold spring; they were planted through holes in the polythene (see how this worked in ‘Potatoes’ section below)

Large covers which exclude light are a delightfully easy way to clear weeds, for little time and effort. Sometimes you can grow vegetables at the same time, and I illustrate this here with potatoes and squash.

Other vegetables that you can grow through holes in the plastic are any of those that need a lot of space per plant. This results in fewer holes, cut into whatever material you are using. Suitable vegetables include broccoli, cauliflower, kale and Brussels sprouts.

However there are downsides:

  • Vegetables grown through covers suffer an increased risk of slug damage, caused by moisture retention under the material.
  • If using the non-woven polypropylene (landscape) fabric, you will need to lay cardboard first to achieve 100% light exclusion.

Or mulch with no cropping

Sometimes it may be simpler just to cover an area of problem weeds and grow nothing. This ensures total light exclusion, and is appropriate where you have other space ready to grow plants, or are simply happy to wait a year.

Thinking ahead and mulching like this can save so much time and effort in the following year. The prerequisite, especially if using polythene, is that the ground is moist before laying it. This keeps soil organisms happy, and means they build soil fertility when eating and secreting any organic matter that you have applied before covering with plastic.

This table gives details of possible mulches to use.

Attributes of non-organic mulches

Mulching perennial weeds

However you go about it, be thorough and conscientious. Neither underestimate these weeds, nor be overawed by them. Don’t try to dig any roots out before covering, except for woody plants.

Here are two cases showing the attention to detail you need in order to clean soil of perennial weeds. They quickly regrow towards any chinks of light.

Example 1

At an allotment that I visited, I saw a plot that had synthetic carpet laid over couch grass and buttercups, but with gaps between the carpet edges. Weed leaves easily find these gaps and start to grow again.

There were also gaps between carpet and plant – wide ones. Any mulch to kill weeds should butt up to the stems of existing plants.

To be clear, and despite what you may read, it’s fine to sit mulches right against plant stems or tree trunks. I have never suffered or observed problems from doing this with cardboard, compost or polythene.

3. Globe artichoke where the mulching has been half-hearted and not thorough enough
A globe artichoke, where the mulching has been half-hearted and not thorough enough

The photo also shows another problem with synthetic mulches: some pieces of carpet have moved or been moved, and are starting to fragment and disappear into the ground. This sinking into the soil is a result of earthworms casting upwards, and debris landing on top.

The result is plastic debris buried in soil. A buried, invisible and possibly toxic barrier to both plant roots and soil organisms.

If this carpet were made of wool there would be no problem, because it would all be digested in the end and become food in the soil. Any synthetic carpet with nylon fibres will not degrade. Such carpets may also contain chemicals such as fire retardants.

Example 2

The photos below were taken at Homeacres in 2018. It’s an area with vigorous perennial plants, including brambles, cow parsley, thistles of different types, stinging nettles, couch grass and bindweed. I used a sharp spade to remove the main stems of brambles, then cut grass and weeds short, in order to facilitate laying the mulches.

Before mulching we planted the small trees of Amelanchier and Sea Buckthorn into small holes, and without any loosening of other soil. Then we spread 3 cm (1 in) of compost.

The next steps were to:

  • Butt thick cardboard up to the tree stems.
  • Spread any organic amendments – here it was a little more compost and mostly wood shavings on top.
  • Lay fabric over the top, and for each tree we cut a small cross-slit.
  • Push fabric edges into the soil using a blunt spade, both to hold it down and to make it easier to mow weeds right up to the fabric edge.
23. Homeacres far corner in June 2017 to give an idea of weed growth where we planted a hedge
The far corner at Homeacres in June 2017 – showing weed growth in an area where we planted a hedge
23a Mulching weeds after planting the hedge: Charles lays cardboard
Mulching weeds after planting the hedge – laying cardboard
23b. Charles pulls new tree through hole in landscape fabric
Pulling a new tree through a hole in landscape fabric
23c. 20th March: hedge trees planted and the mulch pushed in with a blunt spade
20 March – hedge trees have been planted, and the mulch has been pushed in with a blunt spade
23d. 10th September: edges had occasional mowing. Sea Buckthorn end mulch
10 September – the edges had occasional mowing; a mulch covers Sea Buckthorn at the end
23e. Six months of cardboard and thin landscape fabric did an amazing job for these trees
Six months of cardboard and thin landscape fabric did an amazing job for these trees

I used some old and slightly torn fabric, and here it was helpful to hold the pieces of cardboard in place. In addition it slowed entry into the strip of the adjacent perennial weeds.

Another option, other than using fabric, would be to weigh down the card with say 10 cm (4 in) of wood chip. However this would allow more invasion of weeds from the sides.

During the summer we needed to pull significant amounts of bindweed off the tree stems, where it had found light through the planting holes. Plus the edges needed mowing every two to three weeks. Otherwise the tall weeds would have been able to grab a lot of moisture from soil in the strip, before the slower-growing tree roots could have found it. 2018 was a dry summer, and we did not water the trees.

I pulled the fabric off in September, and the only surviving weed was bindweed, in a much weakened state. Even after decades of mulching powerful weeds, this method still amazes me. The hedge was established, and we could now grow without mulching.

Clear difficult weeds and crop at the same time

This is a lovely combination: three worthwhile things are happening simultaneously, and in a short period of time too.

  1. 100% of weeds are killed, except the most persistent ones, in this case bindweed.
  2. Soil organisms are fed, leaving soil more fertile than when we started, and ready to grow more great vegetables.
  3. A harvest is taken.

Below are a couple of examples of clearing and cropping: potato and cucurbit. Which of these you choose to grow depends on climate, timings, and what you wish to grow as a succeeding vegetable.

For example, you can plant potatoes earlier than squashes, and their harvest is often in summer rather than autumn. This allows time for replanting a wide range of vegetables. Squash, on the other hand, usually finish towards the end of a growing season, though not always.

Polythene and moisture

Gardeners sometimes worry about how polythene, as opposed to woven fabric, does not let any rain pass into the soil. In practice this rarely, if ever, matters:

  • The aim is to lay polythene on moist soil, which then holds that humidity.
  • Moisture under the polythene is replenished when it rains, because much of the water runs into planting holes.
  • Often, and as in my examples, you are re-using old polythene which has a few slits. They are not large enough to allow weeds’ upward growth, but rain can pass through them into the soil.

Potatoes

This area had a small bonfire in my first spring, after I had cut down a wild cherry close to where the polytunnel was going to be. The bonfire’s heat killed many pasture plants, except for couch grass and bindweed. The compost I used, on an area about twice the size of the bonfire, was two-year-old cow manure. It had some straw bedding still visible, but was mostly well decomposed – dark and soft.

19. 20th Feb: couch grass partly cleared by a bonfire. Now spreading compost manure
A month after the bonfire – spreading some old cow manure compost on the surface
19a. Steph then plants potatoes on 1st April
1 April – Steph plants potatoes, after covering the compost with polythene
19b. 30th June: strong potato growth with little weed growth and the potatoes flowering
30 June – strong potato growth with little weed growth, and the potatoes are flowering

We planted the potatoes shallow because it was firm soil, and difficult to make any more than a small hole. As a result many tubers developed near the surface, and the polythene was not thick enough to keep all light off them. Therefore I laid cardboard on top of the polythene, between each potato plant, to prevent the potatoes from going green.

19c. Mid July and I removed polythene and pulled potatoes, one plant Estima
Mid-July – I removed the polythene and pulled the potatoes; this is one plant of Estima/ a second early, waxy potato
19d. Two hours later and the soil is mostly clean of weeds, plus we had all those potatoes
Two hours later, and the soil is mostly clean of weeds – plus we had pulled all those potatoes
19e. Same day as potato harvest I planted brassicas into the compost with net to stop pigeons
On the same day as the potato harvest I planted brassicas into the compost, with a net to stop pigeons

Lifting potatoes is easiest after you have cut all surface growth of the stems. I removed them to the compost heap, and then pulled back the polythene. This revealed many potatoes lying near the surface, easy to gather.

  • Even if there is some damage to potato stems and tubers caused by late blight (Phytopthora infestans), it’s fine to put everything on a compost heap. Blight spores can’t survive in soil and compost.

If, when removing polythene, you see only a few weeds still growing, there is no need to put it back. When soil is already almost clear of weeds, you can carry on without extra mulching. In this case I transplanted new vegetables into the now uncovered ground.

19f. By 24th September we had a beetroot harvest and broccoli damaged by slugs
24 September – we now had a beetroot harvest, and broccoli has been damaged by slugs
19g. A wet autumn in 2013 and the newly mulched ground resulted in many slugs
A wet autumn in 2013 – the newly mulched ground resulted in many slugs
19h. October 2014 and crops are clean!
October 2014 – crops are clean!

My polythene mulch had certainly given home to many slugs. They were already there in the old pasture, and you can see the result of a high slug population in the photos above. The plants had been fortunate to have had a dry summer for establishing, but once the rain returned in autumn, so did the molluscs!

Things improved the following year, when the mulch was only compost. With the grassy edge mown regularly, slugs found less habitat and their numbers decreased to manageable levels.

This area of the garden has now been absorbed into new beds.

Cucurbits

Many cucurbit plants ramble to cover a large area, with extensive root systems from trailing stems. Only a few plants are needed to pull value from a large area. Yields are even higher if polythene is not used, because roots from the trailing stems can then feed into the soil. However polythene is effective against perennial weeds, and cucurbits grow well through holes in the cover.

4. May 17th: this polythene had been laid on weedy pasture just five weeks earlier
May 17 – this polythene had been laid on weedy pasture just five weeks earlier
4a. We found slugs, leatherjackets and weeds still alive but weakening
We found slugs, leatherjackets and weeds – still alive, but weakening
5. The near end had most couch grass and we laid some cardboard as extra mulch
The near end had the most couch grass, and we laid some cardboard as extra mulch
6. We spread a 7cm/3in mulch of compost, then relaid the polythene over once again
We spread a 7 cm (3 in) mulch of compost, then laid the polythene over once again
7. 25th May: we made holes and planted 5 Crown Prince squash for autumn harvests
25 May – we made holes, and planted five Crown Prince squash for autumn harvests
9. Growth by July, the squash are on right
Growth by July – the squash are on the right

I am often asked by worried gardeners if there is a remedy for their white and mildewed squash and courgette leaves in late summer. The answer is not to worry, it is natural dieback as summer cedes to autumn. Powdery mildew (of which there are various types) can be found on old leaves, and helps plant energy go into ripening the fruits. There are still new leaves and a high level of photosynthesis, until early or mid-autumn.

In the polytunnel, we cut mildewed leaves off cucumber plants. This makes it easier to see and harvest the fruits, and to prune the side shoots. Outside we leave courgette and squash plants unpruned, because the mildewed leaves are not in the way and do not harm new growth.

10. Growth by August and the first natural die back of squash leaves
Growth by August, and the first natural die back of squash leaves
11. Mid October and we can now see the fruits with just a few bindweed leaves
Mid-October – we can now see the fruits, with just a few bindweed leaves
11a. Over 50kg of squash taken for winter, polythene not needed any more, weak growth of bindweed
Over 50 kg (110 lb) of squash was taken for winter; the polythene wasn’t needed any more, and there was weak growth of bindweed
11b. Same bed after adding compost, and broad beans sown
The same bed after adding compost and sowing broad beans

After harvest, we rolled up the polythene to use again, and used a trowel to lever out some roots of surviving perennial weeds (here it was bindweed). Then I spread another 7 cm (3 in) of compost on what became a wide bed of 2.1 m (7 ft).

Combination of polythene and organic matter to kill weeds

In another area of the garden there was a lot of couch grass and bindweed. I used polythene initially, then compost, cardboard and fabric.

I used several mulches, partly to check their relative merits, but also to be sure of killing all the couch grass, and to reduce the growth of bindweed. The strong growth of courgettes/zucchini also helped to suppress weed growth.

In early September, we pulled out half of the courgette plants and planted endives. There was still some bindweed, but all the couch roots had decomposed and subsequent plantings needed little weeding.

12. We laid polythene on the pasture in February after spreading 5cm compost, now it’s 7th April
7 April – we had lain polythene on the pasture in February, after spreading 5 cm (2 in) of compost
13. See what was happening under the polythene, light-deprived roots of couch, while bindweed is yet to appear
See what was happening under the polythene – light-deprived roots of couch graass, while bindweed is yet to appear
14. Still 7th April and new beds have thick cardboard, compost over, and sides for their first year
15 cm (6 in) of compost – mostly old manure with homemade compost on top – after we had covered the whole area with cardboard
15. Three days later and the beds are finished, plus rain has darkened the surface
Three days later and the beds are finished, plus rain has darkened the surface – they are ready to plant, but we are waiting for warmth
20 May – as weed roots were still growing, I put black polythene over and planted just a few courgettes; we could have managed without the polythene, but it was a timesaver
16a. Three weeks later and growth is strong, little chance for the couch grass there
Three weeks later and growth is strong – little chance for the couch grass there

The following summer we found it easy to keep removing occasional bindweed, as it continued to weaken. By summer 2018 there was none at all in that area, just some in the adjoining old pasture (which we keep removing when seen as it spreads back in). You can reduce this reinvasion by keeping the edge cut short, to reduce vigour in roots.

No dig garden Charles Dowding September
Mid-September – one bed of courgettes have gone, and endives are now growing (on the right hand edge)
17. From the other end, how this area looked by December, normal beds and no more polythene, cardboard or wooden sides.
How this area looked by December, a view from the other end – normal beds and no more polythene, cardboard or wooden sides

Further viewing:

1
sow & propagate
2
Transplant - Size, time of year, Spacing, support
3
water
4
container growing
5
Feed
5
Prune and train plants/thin fruit
6
Harvest times and method
5
Potential problems