Using Plastic Sheets In Gardens Is Controversial. What Are The Pros, Cons & Science? Are Sheets /Or Tarps Useful Or Damaging To Soils & Plants? Thats What This Article Examines.

Plastic is everywhere. In supermarkets where it wraps and double wraps food and other products; in our vehicles, white goods and homes. If in the past we made it of wood or metal then someone has invented a plastic alternative. It fills our streets, washes down our drains and rivers and contaminates our seas with huge floating plastic garbage patches. And today we use a lot of Plastic In Gardens.

Reading the above you’d think I’m against plastic. I’m not. I’m not even against single use plastic and many people will read this far, condemn me and read no further.

But it’s not the plastic that is at fault. It’s how we use and dispose of plastic that is at fault.

A Quick History Of Plastics

I remember the advent of synthetic plastic. It wasn’t that long ago. It started with products such as bakelite. Bakelite was invented by Leo H. Baekeland (1863–1944), a Belgian-born American chemist. Bakelite became popular in the 1950s when it was used to make electrical components including telephones. Other products included bakelite mugs, which gave drinks a very distinctive flavour! I remember the first time I tasted it. I was 50-60 miles from land on a small yacht taking part in an Ocean race. But more on that another day.

Before the early plastics we only had natural “plastics”. Gums are an example, but let’s leave that for another day.

Since the advent of oil based synthetic plastics humans have created over 9 billion tons of plastic. And the real problem is that most plastics are nearly indestructible. Even the compostable products require heat to decompose quickly, and usually fail to break down in domestic composting systems.

The few plastics that can be recycled tend to get lost amongst those that can’t be, as they all look the same to most consumers.

Plastic Use In Farming & Gardening

A lot of plastic is used on farms. From use in ensiling grass and other vegetable products, to growing crops, the volumes used are huge. There is no doubt in my mind that used sensibly they can help us feed the world. The questions is at what cost.

Plastic Sheets In Gardens and on farms

When it comes to gardens and allotments I often see people sheeting down land for the winter. But when asked why, the answers are often vague or make little objective sense. The general idea seems to be around keeping the soil dry and preventing weeds from growing due to the exclusion of light.

But soil needs moisture. Rain is the natural way to provide soil moisture and it has so many other benefits over artificial watering systems.

As for weed suppression, few weeds grow much over winter. So why do we need to sheet down to prevent them when there are so many other options such as cropping, green manures etc.?

And let’s not forget, a weed that grows today is one that can’t grow in the future. So, in some senses, it is good if weeds germinate, provided we can control them and prevent them damaging a crop and/or seeding.

How To Hoe Gardens In Wet Weather

Those practising No Dig experience far fewer weeds simply because they are not turning the soil and bringing weed seeds up out of the seed bank. However traditional gardeners can take heart as a few weeds germinating over winter can be easily dealt with, whatever the weather, by following a simple system that has been practised by experienced gardeners for centuries. It is to use an extra long handled hoe.

The problem in winter, with soil being very wet, is that if we attempt to hoe we need to walk on the soil. This leads to soil compaction. Soil compaction then leads to poor plant growth, puddling and poor drainage. The answer is to extend the handle on the hoe and reach across wide veg beds without walking on them. In my own case I used to bolt a 10 foot long piece of wood on to the my handle. I was able to use that to lightly hoe very large areas of soil, without walking on the soil. Done on drier or frosty days during the colder months this dealt with weed seedlings before they established deep roots. With a 10 foot handle extension it was possible to reach from both sides of a 25-30 ft wide veg bed without causing compaction.

The Downsides Of Plastic In Gardens and Allotments

Plastic left on the land all winter denies the land of much needed rainwater. It also limits the effect of frost on the soil and the creation of a “frost tilth”. Frost tilth are created by the freezing and thawing action on the soil in winter.

Traditionally some farmers and gardeners would plough or dig to leave a rough surface. Frost, snow and rain would act on the surface and break it down to a fine tilth in time for spring sowing. Alternatively they would cover bare soil with farmyard manure, which would be weathered over winter and pulled down by worms. Both these actions created a wonderfully organic rich soil. However, plastic sheets limit the weather effects.

Plastics also potentially leach chemicals into the soil. The worse of this is when carpets are used to sheet down land, as the plastic tends to be shed as micro plastics as well as being leached in to the soil.

On occasion gardeners have complained that the soil is too dry in spring. After being covered by impermeable plastic sheets that doesn’t surprise me.

Another negative is that to cover a significant amount of land a lot of plastic is needed and this can become very expensive.

The Upside Of Plastic In Gardens and Allotments

Plastic, used as a mulch for a limited period, reduces evaporation and keeps the soil moist.

Depending on temperatures, heat from the plastic kills weeds and/or helps seeds germinate. Plastic can be used for short periods to kill weeds in a process called occultation or soil solarisation . The heat generated by the sun in spring or summer can literally roast the weed seedlings.

Black plastic used to cover established cover crops or green manures can be used to kill the crops via the occultation process. This can take several months if the crop is well established but can be speeded up if the crop is first crimped. Crimping is the action of bending, folding or creasing something. In the case the weeds so that they die or are severely inhibited. Farmers sometimes crimp cut grass so it dries out faster when making hay.

Growing Crops Through Plastic

Though I am hesitant to advise any gardener they should use plastic in the garden there are exceptions to every rule.

One is where a very weedy new piece of land is being brought into cultivation and the space is needed to grow crops whilst the land is being cleaned up.

In this case the weeds can be cut off very short, the land covered with black plastic, and crops such as courgettes or pumpkins planted though holes cut or burnt in the plastic. This can be very effective but needs to be managed carefully if the crop is to yield well whilst the weeds die.

Plastic Fleeces And Other Growing Techniques

Where I live in Devon I often sometimes see whole fields covered in plastic. The technique being used is to lay a floating mulch or fleece over the planted crop where it protects the crop and land from weather extremes. Crops grown this way can be harvested several weeks earlier than unprotected crops. It is an expensive process but can pay dividends if a premium price is received for early, and perhaps better quality, crops.

In the past the crop cover was often single use plastic and recycling wasn’t always as good as it should have been. Today there are statutory recycling schemes for commercial growers. However I would much rather see a product that can be reused over several seasons being used, if it is deemed necessary to cover a crop. Plastics have improved in recent years and where, say, polytunnel sheets only lasted two years in the past they now last 7-8 years or more.

The Plastic In Gardens Debate: Conclusion

There can be good reasons to use plastic sheeting in gardens and allotments. However I can see little point in covering land just because it is winter. Sadly many gardeners don’t understand the growing and gardening cycle, or the principles behind growing, well enough. They cover their plots just because their neighbours do it! Done correctly, for the right reasons, I can see it can be a useful technique. Though I have to confess to not embracing it myself as I feel there are few instances when it makes sufficient sense for me.

Research sources

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