1/ IS HEATING NECESSARY?
It is not possible to make a cold Greenhouse completely safe from freezing if outside temperatures full substantially below 0 degrees centigrade – tender plants will be at risk. As a rule of thumb, you can expect the difference between the minimum outdoor temperature and that within an average cold greenhouse to be approximately 2ºC. Each degree of temperature near freezing is critical for plants, but good management will help and it needn’t be costly – to you or the environment.
2/ SIMPLE IDEAS
Keeping potted plants drier will protect roots, enhancing winter hardiness. Select plants for over wintering carefully, growing only (or grouping) those of similar temperature requirements. At the very least, invest in a propagator with electric bottom heat if you don’t have one already (those with thermostatic settings will give better temperature control.) Even if you provide no other heat source, this will help keep vulnerable plants frost-free.
You can reduce heat-loss by installing winter insulation over glazed areas. The most effective method is to use bubble wrap polythene, cut to size and fixed on the inside of the Greenhouse, close to the glass. This is a fiddly task, since the material has to be cut to fit, and it may take more than one person to install the insulation. Furthermore, the insulation is best occasionally removed during the warmer months to admit more light and to facilitate cleaning. Suppliers of greenhouse accessories will stock clips designed to fix the insulating material to the glazing bars. You’ll need a pair of large of sharp scissors to cut the polythene to shape.
Partial insulation is also possible. If you wish to protect plants in part of the house, but don’t want the arduous task of cladding the whole interior with bubble-wrap, section off part of the structure. You can separate it from the remainder with a polythene sheet, or a bubble-wrap curtain, and put vulnerable treasures inside the enclosure.
4/ REDUCING THE HEATING YOU USE…
Greenhouse heating systems range from providing background warmth against frost to constant heating. Remember that each degree rise in ambient temperature carries additional cost and with global concern about carbon footprint it makes sense to use as little artificial heat as possible. Here are some ways you can keep your heating use to a minimum:
- TAKE THERMOMETER READINGS
In a Hartley Botanic Wisley 8, six pane greenhouse, a 2.5 kilowatt electric fan heater can be set low, so that heat comes on only when the temperature falls dangerously low. You will need to take a few days to work out the ideal setting on the dial of the heater and to match this with overnight minimum temperature. To do this you will need a reliable ‘max and min’ thermometer placed among plants on the staging or fixed about a metre above the ground, well away from the heater.
- COMFORTABLE MINIMUM
A night minimum of 4º or 5ºC is a safe temperature to maintain if you want to be safely buffered against severe snap frosts. If you want to run your heating on a tight budget, have horticultural fleece available to spread over vulnerable plants during excessively cold spells.
- SOIL-WARMING CABLES
One of the most economical heating systems is simply to warm soil, either in a bed, or propagation bench. Soil-warming cable provides heat where it is most needed, at the roots, so even if top-growth is cold-damaged, the roots will survive. Cable is safe and easy to install and, with a thermostat, lets you control soil temperature more accurately. It is particularly useful for protecting young plants in cold times of the year. Its performance depends on the cable type, prevailing temperatures and how the system is installed, e.g. a 6m cable consumes 75 watts and correctly installed will sustain temperatures between 15°C and 25°C per square metre of covered bench. Cover plants with fleece or a canopy of translucent sheeting to retain heat.