As temperatures drop, every tunnel owner has a fight on their hands to keep things growing undercover. There is always a trade-off between managing a productive environment and doing it sustainably. We look at different ways of keeping your tunnel warm, to make it a usable space even as the weather gets colder.
Hotbeds
Image courtesy of growveg.co.uk
The heat generated by the decomposition of organic matter is a great way to start your spring crops early.
If you’ve ever been close to a manure pile or even dug in one, you’ll know how warm temperatures can get.
Sometimes we have to look back for a little bit of inspiration, while many people associate hotbeds with the Victorians, it is also believed that the Romans were also au fait with the heat from decay and used it to keep their melons going despite the season. Regardless of where the technology began, hotbeds can reach 65°C which can have a significant impact on the temperature inside your tunnel.
The hotbed generates more than just heat. It can keep your ambient temperature up for weeks through the coldest weather, help jump-start your seeds and then after all that it is still great compost.
Insulation
Putting on an extra layer is a familiar concept as it gets colder, so why shouldn’t it apply to polytunnels?
Insulating with any material is always going to be a trade-off that any tunnel owner is well acquainted with at this time of year – light vs warmth.
While fleece may be a better material to line with, its lack of light often sees it being left by the wayside in favour of the more transparent bubble wrap.
There are hot debates over bubble wrap vs sheets of PVC (how much impact do bubble wrap bubbles have?) and the size of the bubbles in question (larger bubbles are harder and the overall sheet is lighter so they are usually the choice) however it is worth investigating.
There are some drawbacks though, namely ventilation, condensation and chances of mildew so make sure you keep regular ventilation available.
Segregation and Inner Tunnels
Taking on a divide-and-conquer attitude can be the route to winter warmth undercover. You won’t be growing at the same rate as summer so do you need your whole tunnel to grow in? If not, then heating smaller areas is easier.
There are many different ways to divide your tunnel up, some people put cloches, cold frames and even smaller tunnels in their main polytunnel. This works twofold, an added layer of insulation between tender plants and the great outdoors and reduced area to maintain temperature. While polytunnels can be a large investment, smaller hoop houses or portable greenhouses can be endlessly useful for keeping plants above freezing.
Heating
The original and best option for maintaining a constant temperature within your polytunnel is to install a heater, be it electric or gas.
Many heaters have thermostatic controls meaning that once the desired temperature is reached, the appliance stops heating until the temperature drops, they do however have the ability to keep the fan moving to aid an even temperature throughout your polytunnel and best of all, come the warmer months, the electric heaters can be turned off whilst you use the fans for added ventilation.
Heaters can usually be hung from the crop ridge bar/crop bars or stood on the ground/bench, whichever you prefer.
