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
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.
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.
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.
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.
- 100% of weeds are killed, except the most persistent ones, in this case bindweed.
- Soil organisms are fed, leaving soil more fertile than when we started, and ready to grow more great vegetables.
- 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.
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.
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.
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.
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.
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.
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.
Further viewing:
- My video on YouTube: No dig, two ways to clear weeds
