Part 1: Is it ethical to heat a greenhouse in the UK winter?
Houseplants need warmth to survive. Left in freezing conditions many, if not all varieties of houseplants will perish. As a sustainable UK grower, the question of how best to heat the greenhouse over winter has been one that has kept me up at night. This two part blog post will explore the different methods of heating a greenhouse, my current practices and my future plans for Harriet’s Plants. Sometimes even the words sustainable business seems to be at complete odds. A lot of the time, the best sustainable practices makes no business sense and the best business decisions are awful for the environment. At Harriet’s plants I will always put the planet before profits. I pledge to always educate myself on sustainable practices and to share my knowledge and this is what this blog post is all about. Whether you are growing in your own greenhouse, a competing business or an ethical consumer I want you to know how I operate so together we can share knowledge and do the best we can.
The first step in heating a greenhouse is to limit the heat that escapes. By their nature, greenhouses are draughty places, so step one is to seal cracks, replace broken panes and ensure vents and doors fit snugly. It is expensive to heat a greenhouse whatever the method so don’t let that heat escape! You can also add a layer of (recycled!) bubble polythene to insulate the greenhouse however, it does have its drawbacks, as every layer of plastic cuts out a significant amount of light (about ten percent). This will have an impact on the growth of your plants long term but is sufficient enough to give your plants a frost free environment. Fun fact: This also applies to double-glazing, which reduces light transmission and affects houseplant growth!
Choosing the right temperature can help save fuel.
A minimum of 3ºC (37ºF) is sufficient to sustain many tender plants but it is a bit risky if they get damp, and most greenhouses have cold corners. Damp and associated moulds and diseases can be very damaging in winter.
The lowest practical temperature is 7ºC (45ºF), and 10ºC (50ºF) will give greater peace of mind. However it is recommended that gardeners use fleece and other means to protect plants at these fuel-saving temperatures.
These temperatures won’t encourage healthy growth but they will enable the plant to survive the colder months without heating heavily.
The most common fuels for greenhouse heating are natural gas, propane, and fuel oil, but several “alternative fuel” options are available that might be a good option for a grower. The most common heating system in commercial greenhouses is the multi-purpose tube rail. Especially in vegetable crops, the tube rail heating system has been widely applied as it has a significant logistical advantage. Another common hot-water circuit in the vegetable production is the grow tube.
Diesel is a common method of heating a greenhouse as it is an extremely stable fuel – it’s not flammable and is safe in many environments – so it has long been the go-to choice for fuel to heat. But it’s made from fossil fuels and its use is dramatically risking the environment, burning CO₂ into the atmosphere.
Government figures released in 2018 show that electric heating is better for the environment than gas. This revelation is due to investments in renewable energy, plus a reduced reliance on coal-fired power stations. There has been a huge increase in the installation of wind and solar farms, leading to greener energy in the UK. In fact, the average CO2 from the UK’s electricity grid has halved in six years. This means the carbon footprint of electric-based heating is more favourable than other methods, like gas.
In comparison, gas is a ‘fossil’ fuel, formed from the remnants of living matter from millions of years ago. While it is the cleanest of the fossil fuels, it’s still the UK’s biggest source of greenhouse gases. Gas systems produce damaging carbon emissions and with climate targets increasing, gas could become obsolete in a matter of years.
Electric heating is clean and respectful to the environment. It does not emit polluting gases or use heavy metals. It is worth remembering that not all electricity providers are equal. Don’t be swayed by environmentally friendly images, designs and phrases, if they’re not backed-up with factual information. With more and more consumers looking to make a green switch and reduce their carbon footprint and with the UK having more than quadrupled its renewable electricity generation since 2010, the government wants to ensure consumers signing up to a green tariff know their energy is coming from green sources of electricity generation. The government is therefore reviewing how energy retailers market ‘green’ electricity tariffs to consumers, amid concerns that some are overstating how environmentally friendly their products are. Greenwashing is within EVERY industry and lots of companies use trigger words and colours on their sales packaging to trick people into thinking they are doing better for the environment.
Ground source heat pumps also offer an alternative, more sustainable solution for heating greenhouses. A ground source heat pump is powered by thermal energy. Everything around us contains thermal energy – or heat. Heat naturally flows from a warmer place to a colder place. To provide the heat energy in a home when outdoor temperatures are colder, we need heat to flow in the other direction – from a colder place to a warmer place. But how does it do it? When the pressure of gas increases, the temperature also increases. When the pressure decreases, the temperature decreases. This relationship between pressure and temperature is the key to how a heat pump works. Heat pumps are more efficient than other heating systems because the amount of heat they produce is more than the amount of electricity they use. The amount of heat produced for every unit of electricity used is known as the ‘Coefficient of Performance’ (CoP). So, if a heat pump has a CoP of 3.0, then it will give out three units of heat for every unit of electricity consumed. They offer the hope of more environmentally friendly options in the future. Unfortunately, for now, they are not cheap, easy to install or particularly suitable for modest greenhouses.
As you can see, the information on efficient energy use can be very overwhelming as there is so much choice and often vital statistical information, regarding each options sustainability credentials, can be misleading. It makes you wonder, is it ethical to heat a UK greenhouse in winter?
The next blog post will go into detail on Harriet’s Plants energy use and plans for the future. I cannot wait to share all the businesses heating information so you can see what I am doing to create a more sustainable nursery for our planet.
