Greenhouse Heating & Lighting
Greenhouse heating is something that is often governed by economics and the gardener’s needs.
Prior to heating a greenhouse one should ask themselves;
- What do I want to heat my greenhouse for?
- Is it to extend the growing season?
- Is it to keep plants in a frost free environment during the winter months?
If it is the latter then ask yourself;
"Will it cost more to operate the system, than the value of the stock it is set up to protect? or would replacing the stock be a cheaper option"?
As an alternative, you could consider storing these plants in your house, rather than trying to maintain high temperatures in a greenhouse.
Once you have resolved these questions you can now set about creating the system best suited to your needs.
The first consideration is to decide whether to use Electric, gas, or oil (paraffin) Electric.
Electric is versatile, easy to control, and it allows the gardener to add various types of equipment, for example; a fan heater, a soil cable, plus lighting and purpose made propagators.
If going this route employ a qualified electrician to install it, and ensure that all of the equipment is waterproofed.
Piped natural gas may be difficult and expensive to install.
Liquid Gas / Oil:
Liquid gas and paraffin, are less user friendly, in so far as, they have to be lit when required, and the containers will require refilling on a regular basis.
Other factors against gas and oil is their combustible nature.
These tend to create high levels of gases and condensation which will have to be catered for by means of a good ventilation system.
This then can create the potential for loss of valuable /expensive heat when the ventilators are opened to release the gases.
Add to that the safety issue :- Failure to remove these 'toxic gases' could be harmful to seedlings and humans alike.
To sum up:
At first, electricity may seem to be the more costly option compared with oil & gas, but if we are to believe that fossil fuels are to become more
expensive, then electricity may not be as expensive an option as it first seemed.
Add to this the versatility of electric, and its relative safeness to use, this seems to suggest that this is the best way to go.
Assuming the decision has been made to use electricity then it is time to do a spot of planning!
Planning a Propagation / Heating system
It has now become law that a qualified person must be employed to carry out electrical works as described in the following suggestions.
This is to ensure that the work carried out conforms to National Regulatory Standards!
This regulatory stuff is essential, for the simple reason; should an accident occur and your wiring is not up to these standards, you may find that some insurance companies would refuse cover in the event of a claim.
The first thing to do (if it is not already in place) is to get a power supply to the greenhouse!
This can take the form of an overhead wire or an underground conduit.
These each have their advantages and disadvantages, and are often decided by their practicality so to this end we will just leave it at that.
If the electrician has fitted your wiring properly, they will have fitted an RCD (Residual-current device), or an RCBO (Residual Circuit breaker with overload protection).
These are safety device to disconnect the power from the system should the supply cable be damaged or overloaded in any way.
Residual Circuit Breaker
Once the power is in the greenhouse it is often beneficial to have a form of control panel situated somewhere in the greenhouse!
This will allow you to isolate the power in the greenhouse rather than having to go back to the main panel each time you want to switch off/on the system.
In terms of powering up individual appliances this can be done in one of two ways and these are;
Install a circuit breaker and wire each of the appliances into separate breakers.
The other, and possibly the simplest of the two, is to set up a number of water proofed fused power sockets to plug appliances into.
The next question is;
What type of heating appliance/s do you require?
For economic reasons it is worth considering three forms of heating to give various temperature levels rather than having a general temperature throughout the greenhouse, for example;
If ‘frost free’ conditions are required, a thermostatically controlled fan heater set to give an ambient temperature of 3°-5°C (35°-40°F) would fit the bill.
This same heater could double up to cool the greenhouse during the summer months by by-passing the heating element, and operating the fan only.
This would circulate fresh air throughout the greenhouse and reduce humidity.
3 kw Fan Heater
When starting your seed and cutting programme in January/February, raise the fan heater output to maintain a general air temperature circa 5°-10°C(40°-50°F)*
* This setting is dependent on the type of plants being protected
Installing a thermostatically controlled heated bench or mat will comfortably produce a temperature 10°-15°C (50°-60°F) which is a suitable temperature for rooting cuttings or germinating seed.
If you are prepared to do a bit of DIY you can construct your own to a size that suits your needs, rather than buying a purpose made one that does not fit exactly fit in with what you would like!
The following details are suited to building a hot bed that would fit into an 2.4 m (8ft.) wide / long greenhouse;
The finished size will be; 2.1m x 0.75m (7'x 2'.6")
- The box sides are formed with 100 x 25mm (4"x1") treated timber as is the base where laths are spaced out with a gap of 38-50mm (1½-2") between each board.
This allows any surplus water to escape and also lightens the weight of the box
- Next place 25mm (1") thick polystyrene sheets on the laths over the whole area of the box.
- Place 25mm of river sand on top of the polystyrene sheets.
- Drill a hole through the edge board then push the rod of the Rodstat through it into the sand.
- Spread out a 25 metre (80ft) 300 watt soil warming cable* on top of the sand.
- Cover the cable and rodstat by adding more river sand up to the top of the box.
- Thoroughly wet the sand, connect the cable and rodstat to the power supply, and the job is complete!
- An optional extra which prevents the sand drying out too quickly is to cover the whole bed with a sheet of polythene.
* With larger / smaller beds use a warming cable to suit the dimensions of the hotbed!
Warming cable and and Rodstat
The polythene prevents the sand becoming contaminated with spilled compost and or seeds, meaning the same sand can be used for a number of years!
High level heating.
When temperatures of 15°C (65°F) and above are required, use a purpose made thermostatically controlled propagator, manually adjusted to the desired temperature.
In the interest of personal safety ensure that propagators have a 'waterproofed' plugs fitted.
Table / Bench top Thermostatic Propagator
Glass is a poor conductor of heat and as such, much of the heat you have generated with your newly installed heating system will be lost through the glass.
Installing a lining of either bubble plastic (bubbles to the glass) or cellophane sheeting to the inside of the roof and walls of the greenhouse will help to reduce this heat loss.
If your greenhouse is made of alumnium you may find that you have to purchase fixing clips especially designed for this task, whereas with a timber framed greenhouse; drawing pins will normally suffice!
For general lighting one can use fluorescent lights designed for potentially damp conditions!
For plants that require light to germinate one can buy purpose made Growlights.
A cheaper option is to fit time controlled fluorescent light/s approximately 300-400mm (12"-16") above the hotbed to supplement the poor natural light of the winter months, thus speeding up growing/rooting times.
Another good reason for having supplementary lights is to counteract the amount of natural light light lost by fitting the insulation sheeting mentioned above.
The timing unit can be a simple adaptor that fits into a wall socket and set to give the requisite On / Off time settings.
A useful time setting is for the lights to come on roughly two hours before sunrise and then switch off two hours after sunset
Overhead fluorescent lighting
A few Do's and Don'ts:
Keep the greenhouse as draughtproof as possible by replacing broken panes of glass when necessary.
Keep watering to a minimum.
Although the ambient temperature in the greenhouse might be quite low the root system in relatively dry plant pots is a few degrees warmer, the reverse is the case in over-wet plants.
When outside weather conditions allow, ventilate the greenhouse to reduce the chances of moulds and fungi developing.
Leaving the fan heater running on a cold or cool setting for a few hours will do the same.
An alternative to using a Greenhouse.
This is highly dependent on what type and number of plants being grown.
If the intention is only to sow a few plants, then you could consider an electrical heated propagator that is designed to sit on a window sill.
These come in various lengths e.g. the might hold three quarter sized rays or as many as seven.
Ideally the model you choose should be thermostatically controlled in order to keep the temperature at a desired level.
Window sill Propagator
A window sill propagator can become quite hot during the day as the sun shines through the window on to it, or too cold after the sun goes down, a thermostat will help to alleviate this problem, and also save on power usage!
Manufacturers may suggest that the propagator should not to be placed in a sunny windows as direct sunlight may cook the seedlings when they appear.
This is a valid point, but seedlings need good light as well as a consistent temperature so this can put the grower in the difficult position of whether to place the propagator on a sunny window sill or not!
Sadly there is not an answer that will cover every eventuality, meaning; it will be up to the grower to use his / her discretion.
Here are a couple of suggestions;
- If the electric supply flex is long enough, move the propagator off the window sill on to a table that is set back from the window, or on the floor directly under the window sill during very sunny situations, then return it when the light intensity drops.
- If a very sunny window sill is the only position available, then one can shade the propagator by placing a piece of agricultural fleece over the propagator.
A similar situation can be used in reverse!
- When light levels are quite low, position a board covered in kitchen tinfoil in a position to reflect light back to the window sill rather than into the house.
This practice is also very useful if growing potted seedlings on a window sill, as it helps to keep the seedlings upright, rather than them naturally bending over towards the light source.