There are many benefits to growing vegetables in a polytunnel, especially here in the Irish
climate where it can be used to create much better growing conditions for most of the
year, especially for the warm season crops.
But there are costs associated with this type of structure.
They can be relatively expensive to purchase and put up, but the increase in yield and
quality of what you can grow in the shelter that they create, means that a polytunnel
can essentially pay for itself in one growing season.
There are other non-financial costs, many of them I would broadly term ecological costs,
with pollution, resource extraction, energy use and eventual disposal of them being potentially
problematic issues.
And I think that we should recognise and account for or at least acknowledge these externalised
costs where possible.
These days there's a lot of focus on the problems with plastics, and there’re plenty of good
reasons to question their use.
And I’ve been wondering if I should really be relying on the use of plastics in my growing
spaces or is it something that I should avoid as much as possible because the material is
so ecologically problematic.
Or can the considered use of this valuable material in the gardens have such significant
benefits that it can outweigh the costs or problems, and perhaps even significantly reduce
the use of plastics and other resources elsewhere in our societies.
Of course I’m not the first person to be concerned with this, and decades ago Eliot
Coleman explored this issue in one of his books.
He did an excellent analysis of the energy required to produce the plastic for use on
a polytunnel to grow just one crop of lettuce on his farm in Maine in the United States.
He then calculated that it actually took more energy to transport the same amount of lettuce
from California.
And because the plastic would last many seasons he could easily produce at least 6 crops of
lettuce for less total energy than it would take to transport just one crop across the
country.
He concluded that although fossil fuels were used in the creation of the plastic, it was
substantially less than the energy needed to transport the food in from far away, and
therefore justified its use in our gardens and farms.
I’ve always liked this type of analysis and I've been meaning to do similar calculation
for my context, only using greenhouse gas emissions as the main unit for comparisson.
Two years ago I purchased and put up a fairly large polytunnel, and the single sheet of
polythene or polyethylene plastic that covered the steel frame weighed 70 kg.
According to the research I’ve done there is about 6 kg of carbon dioxide released for
every kilogram of plastic, through its whole life cycle, including the incineration of
the plastic at the end of its life.
Making plastic requires energy from fossil fuels, as well as the carbon contained in
even more fossil fuel, as the feedstock or main ingredient of the plastic itself.
I’ve calculated that incinerating 1 kg of plastic will release about 3 kg of carbon
dioxide, but until then this fossil carbon is sequestered or held in the plastic itself.
This leaves about 3 kg of carbon dioxide released in the process of manufacturing 1 kg of plastic.
So that mean that 210 kg of carbon dioxide emissions were released into the atmosphere
to manufacture this big sheet of plastic for my new polytunnel.
This particular plastic was manufactured in Italy about 3000 km away and there will have
been even more emissions getting it to me in Ireland.
Using estimations of emissions for road transport in Europe this would add another 30kg of carbon
dioxide emissions to get the plastic to me.
That brings the total to about 240kg CO2 to make and deliver the plastic, and another
possible 210kg when plastic ceases to be useful, hopefully in many years.
But it’s not just about the plastic, of course, as there is also a steel frame.
This skeletal structure is made from a total of 425kg of steel pipe, and there is apparently
1.83 kg of carbon dioxide emissions released for the manufacture of each kilogram of steel.
This would mean that 780 kg of carbon dioxide was released for manufacturing the frame of
this polytunnel.
The steel pipe was produced in the UK, probably about 600 km away, and there would’ve been
approximately an additional 40 kg of CO2 released in transportation.
That brings the total to 820 kg of CO2 emissions to produce the steel frame and get it to me
in the middle of Ireland.
That is more than three times the emissions that there was from the plastic, which is
really interesting, as so much emphasis is placed on the plastic as being the problem.
In total I have estimated that there was just over 1 tonne of carbon dioxide emissions to
manufacture the polytunnel frame and plastic, and to transport it all to me.
I have no doubt that the actual emissions were higher than this, as this type of calculation
doesn’t include everything, but I think that 1 tonne would account for the bulk of
the emissions involved.
This a significant amount and would have contributed to the increase in greenhouse gases in the
atmosphere, adding to the problems of climate change for many years to come.
To put this one tonne into context, one return transatlantic flight from Dublin, Ireland
to my home in Toronto, Canada, would produce the equivalent of 1.5 tonnes of CO2 emissions
per passenger.
Or looking at this another way, the same amount of emissions would be caused by transporting
about 2.5 tonnes of vegetables from Spain to Ireland, where we currently get a lot of
our vegetables from, especially the warm season crops.
In a polytunnel of this size I can easily grow 1 tonne of vegetables a year, which can
be seen as replacing a similar amount of food that would otherwise have to be imported from
Spain, and even further away.
So the emissions caused in manufacturing this polytunnel would easily be paid back within
three years, and there would be a net benefit from a carbon emission standpoint after that,
for as long as I am growing in this polytunnel.
This of course doesn’t take into consideration the substantial emissions in transport infrastructure
of roads and the truck itself, and ignores the likelyhood that substantial amounts of
plastic would be used in growing the food in Spain, and it also ignores the inevitable
plastic packaging that would have been involved.
If I was able to do a whole lifecycle analysis of all of these factors, I suspect that I
would end up finding that there would be a net climate benefit to growing vegetables
in a polytunnel here in Ireland after just one season.
The longer I grow food in this polytunnel, and the more productive it is the greater
the benefit, or another way of looking at it is that the ecological costs can be seen
as spreading out over many years.
I expect the frame to last a least 20 years, but the plastic will probably not last that
long.
This quality of plastic is supposed to last about 7 years, and I would theoretically need
to replace the plastic twice for the polytunnel to last the full 20 years.
So that is three sheets of plastic in total, plus one that I already needed to replace
due to massive damage by fire.
Ignoring the fact that I be reusing a lot of this plastic for other purposes in gardens,
and assuming that all four sheets of plastic are incinerated at the end of their life,
the emissions from the four sheets of plastic and from the steel frame would total 2.6 tonnes
of carbon dioxide.
Averaged over the 20 years, this would be only be about 130kg CO2 emissions per year.
This is about the equal to the emissions from transporting less than 300kg of vegetables
from Spain each year, again not counting all the other emissions related to their production
or packaging.
Looking at this another way 130kg of carbon dioxide emissions a year is equivalent to
about 1000km travel in a relatively efficient car.
So one return trip a week to the local town to buy groceries would produce more emissions
than this polytunnel would, if averaged over its full lifespan of 20 years.
Of course it is possible to grow vegetables here in Ireland without the use of any plastics,
or any kind of protected cropping at all, and perhaps that's something that we should
be aiming for.
Personally, I think the benefit of growing with the assistance of a polytunnel makes
this ecological cost more than worth while.
I think I would probably eliminate plastics from all other parts of my life and reduce
carbon emissions from a lot of other activities before I would give up this type of protected
cropping.
Fundamentally I believe that plastic is a very useful and amazing material, we just
use way too much of it for things that are so unimportant.
But again, it’s not just about the plastic.
We should be mindful about all the resources that we use to produce our food, and to take
care of them so that they can last as long as possible, and to get as much benefit out
of them as we can.
And I think it important to do this type of ecological accounting with all of the methods,
tools, and equipment that we use to produce our food, without getting too caught up in
the details.
At the end of the day, I think it’s just so important to simply grow food for ourselves
and our communities, and to try to grow as much as we can throughout the year if possible.
And if we can do that in ways that reduce the negative externalities, the pollution,
waste, and the greenhouse gas emissions, all the better.
An important part of this RED Gardens project that I’m working on is to better understand
the issues and benefits of growing your own food, regardless of what method you use, compared
to importing your food in from elsewhere.
And I plan to continue to share all of the data and analysis that I develop on this YouTube
channel, so that it may help to develop and change policy and encourage more people to
grow more of their own food, for themselves and their community, for the benefit of everyone,
and the planet.
If you’d like to support me on this project please visit my Patreon page linked here or
there’s other suggestions in the description below, but most importantly thank you for watching.