Climate under glass

A greenhouse is not simply warmer than outdoors. It is a small climate system that changes continuously through the day.

Sunlight can lift greenhouse air far above the outdoor temperature within a short time. Plants also release water through transpiration. Towards evening, glass, aluminium profiles and leaves cool, relative humidity rises, and surfaces may reach dew point. In Malta, bright sun, warm spells and exposed coastal gardens make early heat removal especially important, while cooler winter nights still matter.

Good greenhouse management therefore means managing relationships, not merely one thermostat setting. Temperature affects photosynthesis, respiration, flowering and fruit set. Humidity affects transpiration, nutrient movement, pollination, disease pressure and condensation. Air movement changes the leaf boundary layer, watering changes moisture, and shading changes leaf and air temperatures.

For home growers, the practical aim is a stable, responsive microclimate suited to the crop. Lettuce and young plants need different conditions from tomatoes, cucumbers or peppers. Sheltered urban gardens, exposed rooftops, coastal sites and warmer inland locations can also behave very differently.

When planning a new greenhouse, consider climate control from the outset. In the Bloomcabin greenhouse range for Malta look beyond appearance and consider usable vents, height, air volume and glazing.

There is no universal ideal temperature. In a mixed home greenhouse, distinguish cool-season crops from heat-loving ones. Lettuce, spinach and many young plants grow well at much lower temperatures than tomatoes, cucumbers, aubergines, basil, peppers or chillies. Growth stage and available light also shift the best range.

For many warmth-loving vegetables, a comfortable daytime range is roughly 20–27 °C, with slightly cooler nights. This is not a fixed target. Tomatoes can set fruit poorly during persistent heat; peppers may drop flowers in unsuitable day or night temperatures; lettuce bolts faster when it is too warm.

Set the climate according to your crop mix. A house for leafy vegetables can run cooler. A house with tomatoes, peppers and basil needs more warmth. With mixed planting, zoning, generous spacing and effective ventilation all help.

During the day, light drives photosynthesis and leaves can become warmer than surrounding air. At night, photosynthesis stops while respiration continues. A moderate day-to-night difference is normal. Problems arise when nights are cold enough to slow metabolism, or so warm that plants do not receive a recovery period.

On clear nights, leaves and glazing can cool below the measured air temperature through radiation. That surface temperature determines condensation. A leaf may reach dew point even while the air sensor still reads several degrees higher.

In cooler weather, buffering night cold is a valuable greenhouse benefit. During Maltese summers, however, the structure must also release heat after dark. Greater air volume responds more slowly; models such as the Bloomcabin Classic therefore have different climate behaviour from a very small enclosed unit.

A min-max thermometer or data logger is one of the most useful tools. One lunchtime reading often hides the two figures that explain plant responses: yesterday’s maximum and the overnight minimum.

03 • Humidity

What is the ideal greenhouse humidity?

Simple blanket humidity targets can mislead. Relative humidity depends on temperature: warm air can hold more water vapour than cool air. If the amount of water stays the same while temperature falls, relative humidity rises. A greenhouse that seems fine in the afternoon may be close to saturation a few hours later.

The highest humidity is often in the crop canopy rather than at the entrance. Leaves constantly release water while dense foliage reduces air movement. A hygrometer by the door may look acceptable although the microclimate between tomato leaves is already much damper.

For a mixed hobby greenhouse, a moderate working range is more useful than a magic percentage. Very dry air increases evaporative demand and can cause wilting. Very humid air slows drying, can impair pollination in some crops, and raises condensation and fungal-disease risk.

The key rule is not to leave the greenhouse near saturation for long, especially overnight. Dry leaf surfaces and moving air are usually more helpful for disease prevention than chasing an exact RH reading every hour.

Vapour pressure deficit, or VPD, describes the difference between the water vapour air could hold at a given temperature and the moisture actually present. It is often more informative for plants than relative humidity alone because it better reflects the drying demand on a leaf.

With very low VPD, air is very moist relative to capacity. Transpiration slows and leaf surfaces dry poorly. With very high VPD, air is strongly drying; water leaves the plant quickly and roots must replace it. The favourable range depends on crop and growth stage.

Home growers do not need to become greenhouse engineers. A sensor measuring temperature and RH and calculating dew point or VPD is enough to reveal patterns. Most importantly, 70 % RH at 12 °C means something different to a plant than 70 % RH at 30 °C.

Simply put

RH describes the air. VPD better describes how strongly air can draw water from a plant.

Good climate management needs more than one figure.

Dew point is the temperature at which air can no longer hold all its water vapour as gas. When glass, metal, pipes or leaves cool below it, water condenses. The coldest surfaces become wet first. With further cooling, condensation can settle across the crop.

This moisture is more than unattractive. Drips from glazing can splash soil and plant debris. Wet leaves create favourable conditions for Botrytis and other pathogens. Sealing a greenhouse completely every evening to “keep all the heat in” can therefore create a humidity problem.

The answer is not to leave everything wide open all night. Better results come from suitable heating, brief moisture-clearing ventilation, air circulation, morning watering, clean ground and adequate plant spacing.

A common misconception

“It is rainy outside, so ventilation cannot dry the greenhouse.”

It can. Cool saturated outdoor air often contains less absolute moisture than warm greenhouse air. Once brought in and warmed, its relative humidity falls and it can take up additional water.

Ventilation removes heat and water vapour, brings in fresh CO₂ and reduces temperature differences. With natural ventilation, warm air rises towards roof vents while cooler outdoor air enters through doors or lower openings. Fans can support air exchange or internal circulation.

For a hobby greenhouse, roof vents, doors or lower openings, plus internal air movement, are usually more effective than one small window. Automatic vent openers are particularly valuable when a sunny morning begins while nobody is home.

If climate control is a priority, compare structures with practical openings and adequate air volume, such as the Prestige series or a lean-to greenhouse from Bloomcabin.

Roof vents, side openings or doors?

Roof vents: ideal for releasing rising hot air.

Doors / lower openings: supply replacement air and create stronger cross-ventilation.

Circulation fans: do not replace outdoor air exchange, but reduce hot, cold or humid pockets.

Polytunnels: often rely heavily on passive ventilation, so large openings are especially important. An alternative growing format is the Bloomcabin polytunnel greenhouse.

Ventilation comes first. Shading does not remove heat already stored inside; it reduces incoming solar energy. Together, the two measures work very well. During prolonged hot periods, shade netting or solar-control glazing can help young plants, lettuce and sunburn-prone fruit.

Do not forget the root zone. Black pots on dark surfaces can become much hotter than the air. Larger containers, mulch, lighter surfaces and consistent watering reduce stress. Avoid heavy leaf removal immediately before a heatwave.

Climate changes very quickly in small structures. Air volume therefore matters as much as the model. For more thermal stability and working room, compare the Bloomcabin Orangerie .

Cooling order

1. Open roof vents and doors early.

2. Increase air movement through the crop.

3. Add shading if solar input remains too strong.

4. Keep root-zone moisture and temperature stable.

5. Move plants if the same area overheats repeatedly.

Water early. Morning watering gives leaves, paths and surfaces time to dry.

Avoid standing water. Every puddle becomes an additional moisture source.

Keep spacing. Dense crops create humid microclimates exactly where disease begins.

Move air. Even circulation reduces temperature differences and speeds drying.

Clear humidity in cool weather too. Brief air changes can replace warm humid indoor air with absolutely drier outdoor air.

Keep surfaces above dew point where possible. Root heat, insulation and air circulation reduce cold surfaces.

Reduce late watering. Plants should not enter the coolest night period with wet leaves.

Remove excess plant matter. Old leaves, weeds and overcrowded growth increase transpiration and block air.

11 • Measurement

Where should greenhouse temperature and humidity sensors be placed?

The main sensor belongs roughly at plant-canopy height, where leaves experience the climate. Avoid direct sun unless it has a radiation shield. Positions immediately beside a heater, door, fan outlet or roof vent are also unsuitable.

One sensor is often enough for a very small greenhouse. Larger houses commonly contain several climate zones: sunny side, shaded side, centre, edges and dense canopy can differ. If a crop has trouble only in one corner, add a second data logger there.

In a generous structure such as the Bloomcabin Orangerie several measuring points help reveal differences between sunny end, shaded zone, central path and crop canopy.

A practical sensor set shows current temperature, RH, daily minimum and maximum, and preferably dew point or VPD. Data logging is especially useful because it shows how quickly climate changes after sunrise, watering or opening a vent.

How hot is too hot in a greenhouse?

It depends on the crop. In a mixed vegetable greenhouse, heat-stress risk rises clearly when temperatures remain around 30–35 °C or higher. Flowers and fruit set often react before mature leaves do.

Should you ventilate when it rains?

Often yes, especially in a heated greenhouse holding trapped moisture. Cool saturated outdoor air can contain less absolute moisture than warm indoor air. When it warms inside, its relative humidity falls and it can absorb water.

What humidity is too high?

The percentage alone does not decide it. High RH becomes critical especially when temperature moves towards dew point and leaves remain wet for a long time. A brief peak is less problematic than an almost saturated whole night.

Do I need a fan?

Not every small greenhouse needs mechanical circulation. It becomes more valuable as the crop thickens. Fans reduce stagnant zones, but do not replace roof vents, doors or genuine outdoor air exchange.

Is a larger greenhouse easier to control?

Greater air volume usually changes temperature more slowly than a miniature structure. Vent area, glazing, wind, orientation and planting density still matter. Even a large house can overheat if its openings are too small.

Which greenhouse suits targeted climate control?

Look for usable ventilation, sufficient working height, air volume and good plant spacing. Compare Classic, Prestige and larger Orangerieformats to suit your site and crop plan.

A practical weekly routine

Five minutes of climate observation can prevent weeks of problems

1. Record the previous night’s minimum and the previous day’s maximum.

2. If you have two sensors, compare aisle and crop-canopy humidity.

3. Check glazing and leaves for condensation each morning.

4. Check that automatic vents opened and closed at the right time.

5. Check root-zone moisture before changing your watering routine.

6. Watch plant responses: flower drop, soft growth, leaf rolling, wilting, spots or unusually slow ripening.

Also use local Bloomcabin product pages for planning: greenhouse category and Classic greenhouse.