Frost Protection or a Warmer Greenhouse? Choose Your Target
GREENHOUSE HEATING AND WINTER ENERGY
For Malta’s cooler months, decide what winter protection means before planning a heater. Keeping plants above a damaging minimum is not the same job as maintaining conditions for active growth. Those aims call for different targets, monitoring and heating plans. A greenhouse warms in sunlight and loses heat after dark, so an afternoon reading or general forecast cannot tell you the minimum your plants will experience at their own position.
01
Set the winter target before choosing a heater
Begin by naming the outcome, not choosing a number. Frost protection means limiting the chance that selected plants fall below a damaging minimum; it does not promise warmth for new leaves, flowers or fruit. An active-growing target is a different brief: conditions must suit continued development, and the appropriate range varies by species and growth stage. A plant expected to rest is not necessarily better off simply because the greenhouse is warmer. Use plant-specific guidance for both a lowest safe condition and a preferred growing range. A daytime outdoor reading or the word winter is not a greenhouse setpoint.
Write down which plants will stay, whether they must merely survive or keep growing, and whether the target applies to the whole greenhouse or a defined group. Make the intended outcome specific: keeping an established plant through winter is different from expecting it to produce new growth during that period. This distinction helps you decide which plants actually need the warmer conditions you are considering. If a mixed collection has different minimum temperatures, heating everything to suit the tenderest plant is a choice, not an automatic benefit. The Wall Greenhouse options are listed from 3.69 to 14.34 m²; comparing footprints can help you plan the plants, benches and access you want. Base that layout on the collection you intend to keep rather than assuming every available space needs the same temperature. Check the exact option and its documented specifications when you move from a layout plan to an equipment decision.
A survival minimum is not a growing target
Treat a plant’s lowest stated temperature as a limit to understand, not automatically as a sensible thermostat setting. Check what the figure describes: brief exposure or sustained conditions, air around the foliage or the root area. The duration and repetition of cold periods can matter, as can how the plant has acclimatised. In a pot, roots are exposed to different conditions from those of a plant growing in the ground. Keep any qualifications alongside the temperature in your notes. A figure described as a short-term minimum should not become an overnight operating target simply because it is the only number available. Likewise, guidance for established plants should not automatically determine the target for plants at another growth stage. General labels such as hardy do not give you enough detail to plan a precise greenhouse target.
Once you understand the plant guidance, choose a thermostat setting with a practical margin rather than asking it to switch at the damage boundary. The margin should reflect sensor accuracy, its position and the cold spots you find in the greenhouse; avoid borrowing a fixed figure from a different crop or structure. If the source gives only a minimum, look for clarification about duration and measurement point. When several plant groups need separate consideration, the Limited Edition Aluminium Greenhouses range, listed from 3.77 to 28.08 m², provides footprint options to compare as you plan their placement.
02
Active growth needs more than warm air
A warmer setting does not create growing conditions by itself. Plants need suitable light as well as warmth, and winter light may be limiting even when the greenhouse receives direct sun for part of the day. If the air is kept warm while light is weak, growth may not develop as expected. Check guidance for the particular plant or crop, including its temperature and light requirements, and distinguish a minimum intended to prevent damage from the range recommended for continued growth. The target for resting plants may differ substantially from the target for seedlings or a crop being brought to maturity.
Some plants have different recommended ranges for day and night, while a basic controller may offer only one setting. Before choosing equipment, decide whether your growing aim requires those separate settings or whether a single setting meets the relevant plant guidance. Do not describe a plan as suitable for active growth if it addresses only the overnight minimum and leaves the daytime requirements unresolved. Sun can also lift the temperature above a heater’s setting without ensuring that nights remain warm. Warmer growing conditions can change how quickly plants develop and use water, so observe the plants rather than assuming a winter watering routine still fits. Review progress against the growth you actually intended, not just whether the heater reached its setting. The T-Model Orangery Greenhouse is listed from 16.52 to 30 m²; use the selected footprint when sketching benches and growing space, then check the documented details for the actual greenhouse.
Measure the coldest part of the plant space
Measure where the plants are, not simply where the thermometer is easiest to reach. Place a reliable sensor at a representative height for the plants being protected, shaded from direct sunlight and away from a heater outlet or a door draught. Do not let it touch the frame or glazing, since that reading may not represent the surrounding air. A single sensor in the centre can miss a colder position near an entrance or at one end. If plants occupy different heights or areas, check more than one position before deciding where the controller’s sensor should sit.
Use a minimum-and-maximum thermometer or a logger that records overnight lows; a daytime snapshot cannot confirm that a protection target was maintained. Note the minimum alongside weather, vent positions and heater behaviour so you can interpret the result. For a minimum-and-maximum thermometer, follow the instructions for resetting the stored readings and record the period they cover. Otherwise, an earlier low could be mistaken for the result of the latest night. If possible, compare the controller reading with an independent sensor near the crop: one shows what the control responds to, the other what the plants experience. Label the measurement positions in your notes, especially if you move an instrument, so that comparisons remain meaningful. Follow the instrument instructions for checking accuracy and batteries. If you use a low-temperature alert, test that it works and reaches someone able to respond.
03
How the greenhouse affects heating demand
The heat needed to hold a target depends both on the conditions you want inside and on how quickly the greenhouse loses heat. A larger difference between the desired indoor temperature and the outdoor temperature means more heat is needed to maintain the same conditions. The exposed area, height and shape, covering, joints, openings, ventilation and insulation all affect heat loss. Wind exposure and changes in weather matter too, so a mild, calm night is not a reliable stand-in for every cold night. A protection target may call for less heating than active growth, but there is no single saving figure that applies to every structure.
Record the selected greenhouse, its size, the area you intend to heat and which openings need to remain usable. Seek documented details for the exact configuration instead of inferring heat retention from appearance. The Pyramid Greenhouse range is listed from 5.53 to 18.9 m²; its area range helps compare footprints, but floor area alone does not establish enclosed volume or heat loss. Use this information to identify the assumptions behind your heating plan, not to calculate heater size from floor area alone. Any assessment of equipment capacity needs the actual greenhouse details and realistic outdoor conditions.
Balance insulation with daylight and ventilation
An insulating layer may reduce heat loss, but it can also reduce the light reaching plants, change airflow and contribute to condensation. Before fitting a temporary lining or thermal screen, check its compatibility with the glazing, frame and ventilation arrangements, and follow the manufacturer’s guidance on fixing and removal. Do not cover vents, drainage routes, doors or access needed for normal use. The useful question is not only whether a material is described as insulating, but what effect it has in the exact arrangement and whether plants still receive appropriate light and air.
Keep ventilation in the plan even when the night-time target is low. Sunlight can raise the temperature during the day, and closing every opening to hold warmth may leave excess heat or moisture with no managed route out. Decide how you will respond to daytime conditions without losing sight of the night target, and check how the chosen greenhouse’s vents are operated. The Limited Edition greenhouses with storage are listed from 5.52 to 28.08 m²; if storage is part of your plan, decide whether that space belongs inside the area you intend to heat.
04
Match the controller to the chosen target
Choose the controls to suit the goal. For frost protection, the priority is maintaining the chosen minimum at a representative plant position. For active growth, the aim may be to maintain a suitable range over longer periods, possibly with different day and night targets. Check the heater’s documented thermostat range, sensor arrangement and instructions against the target you have selected. Ask how you select the target, where the temperature is measured and which reading the display shows. A selected temperature and a measured temperature serve different purposes; make sure you can distinguish them when checking operation. If you need separate day and night settings, confirm that the proposed controls provide them rather than assuming every thermostat does. A controller only responds to its sensor: it cannot compensate for a misleading reading, poor airflow or a heater unable to supply enough heat for the conditions.
Assess the exact greenhouse configuration and the heater’s documented output and control specification together; selecting a target is not the same as assessing heater capacity. The Prestige Greenhouse range is listed from 3.77 to 28.08 m², so the precise size matters when the enclosure is assessed. Do not take a model name or floor area as proof of insulation, ventilation, heat retention or heater compatibility. If specialist installation is needed, ask a qualified professional to confirm that the proposed equipment is suitable and follow the manufacturer’s instructions.
Plan for interruptions and cold nights
Plan for a cold night when normal operation is interrupted, not only for an ordinary one. A power cut, depleted fuel supply, failed thermostat or open door can leave plants below the intended target; which risks apply depends on the equipment you choose. Decide how conditions will be checked, who will receive a low-temperature alert and which plants take priority if heat cannot be restored. Do not assume a small portable heater can protect the whole greenhouse, or use equipment outside the conditions stated by its manufacturer.
Ask the heater maker or installer what the documented equipment can maintain in the actual conditions and what contingency is compatible with it; avoid improvising an additional heat source. The Spacious Greenhouse range is listed from 47 to 70 m². For any chosen size, include the full area covered by the heating plan when discussing capacity, rather than relying on a product category or a warm reading beside the heater. A tested alert and a clear response plan are more useful than raising the target everywhere without checking what the equipment can do.
05
Give mixed plant groups separate targets
Where plants have different needs, separate them by winter purpose before deciding to heat the whole greenhouse. Group plants with similar documented minimums and growth aims, and avoid making cooler-tolerant plants part of a warmer plan without a reason. A smaller protected zone may be an option if it can be arranged without blocking circulation or access. A local cover or enclosed shelf can create different conditions from the wider greenhouse, so measure around the plants rather than assuming that a boundary holds the temperature they need.
Sketch benches, paths, doors, vents and storage before assigning zones. The heater’s airflow and the controller sensor should suit the layout, while plants should remain easy to reach for watering and checks. The Aluminium Classic Greenhouse range is listed from 3.77 to 28.08 m²; compare the footprint of the exact option with your plan rather than judging usable growing space from the range name. Revisit the arrangement when you move plants or benches, since the coldest monitored position may change.
Use temperature records to review the plan
Keep a winter record that connects your target to what happened. Note overnight minima at plant level, the conditions outside, which plants were in each zone, the thermostat setting, ventilation changes and whether the heater ran or an alert sounded. Keep the selected setting separate from the measured minimum so you can see whether the intended conditions were maintained. These details can help reveal whether a sensor is poorly positioned, a thermostat setting is too close to the plant limit or ventilation changes are affecting conditions. When you alter the plan, record what changed and why. Compare the results with the previous arrangement while allowing for differences in weather, rather than attributing every improvement to the latest adjustment. This makes the record useful for decisions, not just a list of temperatures. Plant appearance is useful to observe, but it is not a precise or immediate thermometer; consider it alongside instrument readings and plant-specific guidance.
Review the goal before a cold spell and when your plant collection changes. If the aim is to protect resting plants, there may be no reason to sustain a growth target; if growth is the aim, check that light, water and equipment specifications support it rather than simply raising the thermostat. For a running-cost estimate, use the heater’s actual rated input, realistic operating time and your own energy tariff. Weather and thermostat cycling affect use, so a catalogue figure or guess is not a reliable bill estimate. Keep protection and active growth as separate planning scenarios and choose the one that fits your plants.
Frequently asked questions
Should frost protection be set to the freezing point?
There is no universal temperature that protects every plant. Different plants, roots and leaves can have different limits, and a published minimum may describe only brief exposure. Use guidance for the plants you are keeping and allow for the accuracy and position of your sensor.
What is the right winter greenhouse temperature in Malta?
It depends on the plants and the conditions at your greenhouse, so there is no single Malta-wide setpoint. Decide whether you are protecting plants or encouraging growth, then use plant-specific temperature guidance and check overnight readings at the plants’ position.
Can a greenhouse stay warm overnight without a heater?
Sunlight may warm a greenhouse during the day, but the temperature can fall after dark as heat escapes. The amount of change depends on the structure and conditions. Log overnight minima rather than assuming a sunny afternoon will protect plants through the night.
Should I heat the whole greenhouse for one sensitive plant?
Not necessarily. If practical, keep plants with similar temperature needs together and assess whether a smaller protected zone is possible. Measure the air around that group, and make sure the arrangement does not block access or ventilation.
Does a thermostat setting tell me what temperature my plants receive?
Not by itself. The reading depends on where the thermostat sensor is, and the result also depends on the heater and airflow. Compare the controller reading with an independent thermometer placed near the plants.
The useful winter target is the one tied to the plants’ real purpose: protection from damaging cold, or conditions for continued growth. Once that distinction is clear, plant-specific guidance, overnight measurements and documented greenhouse and heater details can shape a practical plan. In Malta’s cooler months, a measured target is more dependable than assuming that one setting will suit every plant or every night.
