The climate under glass

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

Sunlight can raise greenhouse air well above outdoor temperature within a short time. Plants also release water into the air through transpiration. Towards evening, glass, aluminium profiles and leaves cool down, relative humidity rises and some surfaces can reach dew point. South Africa spans very different climates, from coastal areas to Highveld gardens and hot inland regions: cool nights can matter in spring, while removing solar heat promptly is often crucial in summer.

Good greenhouse management therefore means managing relationships rather than just 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 boundary layer around leaves, irrigation changes moisture, and shading changes leaf and air temperature.

For home gardeners, the practical aim is a stable, responsive microclimate suited to the crop actually being grown. Lettuce and seedlings need different conditions from tomatoes, cucumbers or peppers. Warm lowland gardens, coastal sites, high-altitude areas and exposed gardens can also differ greatly.

When planning a new greenhouse, include climate control in the design from the start. In the Bloomcabin greenhouse range for South Africa look beyond appearance and consider usable vents, height, air volume and glazing.

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

For many heat-loving vegetables, a comfortable daytime range is roughly 20–27 °C, while nights may be somewhat cooler. This is not a fixed target. Tomatoes can set fruit poorly during prolonged heat; peppers may shed flowers at unsuitable day or night temperatures; lettuce bolts sooner in excessive warmth.

The most useful rule is to define the climate around your crop mix. A house for leafy vegetables can run cooler. A house with tomatoes, peppers and basil needs more warmth. With mixed crops, zoning, generous spacing and effective ventilation help.

During the day, light drives photosynthesis and leaves can become warmer than the surrounding air in direct sun. 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 the plant cannot recover.

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

In cooler areas, buffering night cold is a major greenhouse benefit. In very warm locations, however, a greenhouse must be able to release heat at night in summer. Larger air volumes respond more slowly; models such as the Bloomcabin Classic therefore have different climate dynamics from a very small enclosed unit.

A min-max thermometer or data logger is one of the most useful tools. A single midday 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 values are especially misleading. Relative humidity depends on temperature: warm air can hold more water vapour than cold air. If the absolute amount of water stays the same while temperature falls, relative humidity rises. A greenhouse that seems unremarkable in the afternoon may be close to saturation a few hours later.

The highest humidity is often not by the door but within the crop canopy. Leaves continually release water there, while dense foliage reduces air movement. A hygrometer at the entrance may show acceptable values although the climate between tomato leaves is already much damper.

For a mixed home greenhouse, a moderate working range is more useful than a magical percentage. Very dry air increases evaporative demand and can lead to wilting. Very humid air slows drying, can impair pollination in some crops and raises the risk of condensation and fungal disease.

The main practical rule is not to leave the greenhouse near saturation for long periods, especially overnight. Dry leaf surfaces and moving air usually prevent disease more effectively 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 describes evaporative demand on the leaf.

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

Home gardeners do not need to become greenhouse engineers. A sensor measuring temperature and RH, and calculating dew point or VPD, is enough to reveal useful patterns. Above all, remember that 70 % RH at 12 °C means something very 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.

That is why good climate control needs more than one number.

The dew point is the temperature at which air can no longer retain all its water vapour as a gas. If glass, metal, pipes or leaves cool below it, water condenses. The coldest surfaces become wet first. With further cooling, the crop itself can carry condensation.

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

The answer is not to leave the greenhouse wide open all night. A better approach combines suitable heating, short humidity-venting periods, air circulation, morning irrigation, clean ground surfaces and adequate plant spacing.

A common misconception

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

It can. Cold saturated outdoor air often contains less absolute moisture than warm greenhouse air. Once brought inside and warmed, its relative humidity drops and it can absorb additional water.

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

For a home greenhouse, roof vents, doors or lower openings and internal air movement usually work better together than one small window. Automatic vent openers are especially valuable when a sunny morning begins while nobody is at home.

Where climate control is a priority, compare structures with usable openings and enough air volume, such as the Prestige range 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 zones.

Polytunnels: often depend strongly on passive ventilation, so generous openings are particularly 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 are highly effective. During extended heat, shade netting or solar-control glazing can be particularly useful for seedlings, lettuce and fruit vulnerable to sunscald.

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. That is why air volume matters as much as the model. For greater thermal stability and working space, you could compare the Bloomcabin Orangerie .

Cooling sequence

1. Open roof vents and doors early.

2. Increase air movement through the crop.

3. Add shading if solar input remains excessive.

4. Stabilise moisture and temperature around roots.

5. Move plants if the same area overheats repeatedly.

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

Avoid standing water. Every puddle becomes an additional source of humidity.

Keep adequate spacing. Dense crops create humid microclimates exactly where disease starts.

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

Vent humidity even in cool weather. Brief air changes can replace warm humid indoor air with outdoor air that is drier in absolute terms.

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

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

Remove unnecessary plant material. Old leaves, weeds and overcrowded growth increase transpiration and block airflow.

11 • Measurement

Where should temperature and humidity sensors hang?

The main sensor belongs roughly at crop-canopy height, where leaves actually experience the climate. Avoid direct sun unless it has a radiation shield. Positions directly 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 have several climate zones: sunny side, shaded side, centre, edges and dense foliage can all differ. If a crop struggles only in one corner, place a second data logger there.

A generous structure such as the Bloomcabin Orangerie benefits from several measuring points that reveal differences between the sunny end, shaded zone, central path and crop canopy.

A practical sensor set shows current temperature, RH, daily minimum and maximum, plus dew point or VPD where possible. Data logging is especially valuable because it reveals how quickly the climate changes after sunrise, irrigation or opening vents.

How hot is too hot in a greenhouse?

It depends on the crop. In a mixed vegetable greenhouse, heat-stress risk rises sharply when temperatures remain around 30–35 °C or higher for extended periods. 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 humidity. Cool saturated outdoor air can contain less absolute moisture than warm indoor air. Once warmed in the greenhouse, its relative humidity falls and it can take up water.

What humidity is too high?

The percentage is not the only factor. High RH becomes critical when temperature approaches dew point and leaves remain wet for long periods. A short peak is less troublesome than an almost saturated whole night.

Do I need a fan?

Not every small greenhouse needs mechanical circulation. As the crop becomes denser, it becomes more valuable. Fans reduce stagnant zones, but do not replace roof vents, doors or real outdoor air exchange.

Is a larger greenhouse easier to control?

Greater air volume usually changes temperature more slowly than a mini structure. Vent area, glazing, wind, orientation and crop density still matter. Even a large house with openings that are too small can overheat.

Which greenhouse suits targeted climate control?

Look for usable ventilation, adequate headroom, air volume and good plant spacing. Compare, for example, Classic, Prestige and larger Orangerieformats to suit the 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 two sensors are available, compare aisle and canopy humidity.

3. Check glazing and leaves for morning condensation.

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

5. Check root-zone moisture before changing irrigation.

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

Use the local Bloomcabin product pages as planning tools too: greenhouse category and Classic greenhouse.