Greenhouse growing is possibly the most popular way to grow sun-loving plants, fruit and vegetables in our mild British climate.
But with British weather being notoriously unpredictable, sometimes growing plants in an unheated greenhouse can be a gamble.
Although a greenhouse will act as a type of solar unit, trapping the sun’s heat during the day, at night temperatures can plummet so low that tender plants can be damaged.
Therefore it’s during the night that many people turn to greenhouse heaters to maintain the ambient temperature.
Passive and Active Methods
There are principally two greenhouse heating methods – active and passive. The active method basically involves using an electrical power source that provides the energy that pumps heat into the greenhouse. This is usually in the form of an electric heater, which can sometimes prove quite complicated and inefficient in terms of energy usage and cost, even when just used overnight.
The passive method usually employs what we could consider as more natural heat sources that absorb heat throughout the day, and then disperse the heat during the cooler nights.
For the passive method to work, it needs a heat sink. This is a material or vessel that absorbs the heat, allowing it to ‘sink in’ during the day. As the air temperature around the object or material cools, the heat trapped within is slowly released during the night, stabilising the ambient temperature and preventing it from dropping too low.
It’s worth noting that passive methods are most efficient and successful during the summer ‘growing season’ months, when the heat of the sun is at its most powerful. British winter days, even with clear skies, may not generate enough sunlight or warmth to enable passive heating methods to be successfully used during the colder season.
Here are some ideas of how you can implement a more ‘natural’ passive heating method:
- Create a heat sink using rocks, paving slabs or concrete blocks (in a metal cage)
- Create a heat sink using large plastic water/ disused food barrels
- Create a ‘hot bed’ using compost or well-rotted manure (so as not to attract flies etc.)
Improving Heat Sink Efficiency
Coating the materials or objects with non-reflective black paint will increase their ability to soak up and store the heat during the day. Also make sure that you site your heat sinks somewhere where they will catch the maximum amount of sunlight during the day. For people in Britain, a south-facing aspect is ideal. You should also make sure that your heat sinks are situated inside the greenhouse, slightly away from the sides. This is because if the heat sink is in contact with panes of glass, once the outside air temperature has fallen, the windows will absorb the trapped heat fairly quickly.
One of the disadvantages of using a heat sink is that they can take up a considerable amount of space in the average sized greenhouse. Heat sinks that use compost or manure will also need to be regularly topped up and turned in order to maintain the production of heat, and can prove messy if not adequately contained.
However you could try to scale your heat sinks to suit the size of your greenhouse – but remember that the more packed your greenhouse is, the less energy you’ll need to heat it. To help with natural passive heating, you could also try to insulate your greenhouse with bubblewrap. This can be done throughout the greenhouse, or if preferred, restrict your bubblewraping to just on the north facing side. Also by making sure that the greenhouse is as air tight as possible during the night (with plenty of ventilation during the day), you’ll help to retain warmth.
It may be that passive heating is only really suited to the summer growing season in Britain. However, it’s a method that has been in use for many years, and proved successful in many a large stately Victorian hothouse. Why not give it a go and see whether natural heating methods work for you?