As the cold weather has now arrived, it is a good idea to think about insulating your mini greenhouse as it will keep heating costs down. One of the pitfalls of adding insulation is that it can reduce the movement of air inside the mini greenhouse, causing damp problems. For this reason we normally only recommend the insulation of the end glass and the roof. With the front glass left un-insulated, the plants can easily be accessed, the doors can be slid open for ventilation on brighter days, and the clear glass will allow the maximum daylight to enter the greenhouse.
The best material to use when insulating your mini greenhouse is bubble wrap, which can normally be purchased off the roll at garden centres. The end glass can be easily insulated by cutting pieces of bubble that are about 1″ (20mm) bigger than the panel of glass. The glass sheet can then be carefully removed, the bubble wrapped around the glass with the bubble on the inside. The glass can then be replaced and any excess of bubble trimmed off. Be very careful when removing the glass, as if the corners are damages the toughened glass will shatter.
Once the 4 end sheets are insulated, the top glass can be removed and the bubble placed carefully on the front and back rails before the glass is replaced, trapping the bubble in place. If you have a shade cover, this can be placed over the top of the mini greenhouse to provide an extra layer of insulation.
If you have any old polystyrene plant trays, place these under any plants that are on the floor, as this will keep the roots off the cold earth. For more delicate plants, on cold nights wrap the leaves in some fleece, this will keep the warmth that is held in the pot and the soil close to the plant, stopping in getting so cold. A tip on a recent Alan Titchmarsh show was to add glass bottles full of water to the mini greenhouse. The idea is that they absorb the warmth during the day and then release it back during the night. If the mini greenhouse is against a brick or concrete wall, this will act in a similar way.
Insulating your mini greenhouse will slow down the loss of heat from the mini greenhouse, which is fine if the days are warmer and the frost is just over-night, but when it is very cold some supplementary heat in needed to keep the greenhouse above freezing. Invest in a max-min thermometer to keep an eye on the temperatures in the greenhouse – a wireless unit will allow you to have a sensor inside the greenhouse and the display inside the house – some heaters will also feature an alarm to warn you if the temperature drops too far.
Once you have finished insulating your mini greenhouse, you need to think about adding heat. In terms of heating there are three main options: electric heater; paraffin and for occasional heat candles.
An electric heater has the advantage that it has a built-in thermostat, so it is only ever being used when the temperature nears zero. The fan will move the warmed air around the greenhouse, reducing cold spots. It is important to leave some space in front of the heater otherwise delicate leaves can be damaged by the heat. It is also important to keep an eye on the compost and make sure they pots do not dry out too much – normally hand watering with a small watering can is best at this time of year.
If there is no electricity in the mini greenhouse, then an alternative is paraffin. The modern paraffin grades run very cleanly and their main by products are CO2 and water vapour. CO2 is good for the plants, but it is important to ensure that the water vapour does not cause damp problems – open up the mini greenhouse at every opportunity to allow the inside to dry. It is also important to ensure there is some ventilation inside the mini greenhouse, as the flame needs to be fed with oxygen.
For more occasional frost protection duties, in addition to insulating your mini greenhouse, an 8 hour tea-light inside a terracotta pot makes a good heater. Invert another pot on top to make a chimney (make sure it has a drainage hole in it to act as a chimney). The candle will warm the pots, which will then radiate the heat out. If you are placing this on the glass middle staging put a saucer or insulated mat underneath the pot to protect the glass.