The Effect of Light and Temperature on Crops
Strong sunlight and temperature are related.
Introduction
Different crops grow best in their specific optimal temperature range. A thermometer shows the temperature in absolute terms. The optimal temperature range is influenced by other factors, like humidity, air circulation and light level. Understanding the underlying causes of temperature and fluctuations is essential for effective greenhouse management. To a significant extent, these changes are driven by strong sunlight.
Sunlight comprises a spectrum of different radiation waves, visible to us as light in a range of colours (the rainbow). For cultivation purposes, we categorise light into three main groups:
Ultra Violet (UV): This light, with wavelengths below 400 nanometers (nm), is not highly utilised by most plants. However, the blue portion of the spectrum is important for promoting shorter plant stature and increasing Brix levels (dissolved solids) in the plants. A negative impact of UV radiation is its degradation effect on materials such as plastic films covering the greenhouses, trays, crates, and pots.
Photosynthetic Active Radiation (PAR): This range of light, with wavelengths between 400 and 700 nm, is used by plants for photosynthesis. The PAR spectrum encompasses blue, green, yellow, and red light, which is crucial for plant growth.
Infra Red (IR): This light has wavelengths above 700 nm and is the radiation primarily responsible for increasing temperatures.
The UV and IR light have less direct involvement in photosynthesis. Most crops in greenhouses need protection from excessive IR radiation to prevent dehydration and scorching of leaves and the blackening of red rose petals. Diffused greenhouse plastic helps mitigate these issues by scattering direct radiation. This improves light quality and allows for deeper penetration into the plant canopy, reaching lower leaves. Certain greenhouse plastics can also block a portion of IR radiation, which helps to lower greenhouse temperatures during hot periods. Conversely, in cold weather, allowing IR radiation into the greenhouse can be beneficial for warming.
While LED lighting offers the option to deliver specific light colours (wavelengths) during different growth stages, this remains an area of ongoing research. Furthermore, light is not an isolated factor; it links with temperature, day length, and other environmental conditions. In natural conditions, the full spectrum of sunlight generally supports balanced plant growth. Notably, morning light tends to be richer in blue wavelengths, while evening light has a higher proportion of red wavelengths.
Blocking direct sunlight
Generally, reducing sunlight from entering the greenhouse will moderate the increase of temperature. Though it’s crucial to remember that light is the primary driver of plant growth.
Shading can be provided using shade nets, which can be deployed or retracted as needed. Fixed shade nets will create a shortage of light in the morning and evening and on cloudy days. These nets can block light at varying percentages, ranging from 30% to 80%. Shade nets with a cover of aluminium strips on the upper side reflect sunlight and moderate temperature increase. Ideally, placing shade nets above the greenhouse structure would be optimal, but challenges exist related to wind damage and the mechanics of opening and closing the nets. Painting the greenhouse rooftop is another common practice. Companies like Mardenkro B.V. (https:// www.redusystems.com) specialise in greenhouse roof paints. Some paints are designed to wash off with rain over time, while others are more permanent. Certain formulations even become transparent during rainfall, allowing more light to enter during dark weather.
Strategic shading can be particularly beneficial by midday when sunlight intensity is highest and temperature increases sharply. However, to maximise growth, full sunlight is generally required, especially during the crucial two hours around sunrise and sunset. Be aware that in the sub-tropics, the day length is say 12 hours; hence every hour is 8.3% of growth potential. Missing an hour of light in the morning and in the evening adds up to 16.6% of growth potential.
For cultivation in the open field, shade roofs and sides can be installed. This creates, moreover, a microclimate, providing shading and wind protection, as well as some protection against predators. Planting of shade trees protects crops underneath (e.g. coffee plantation) and creates a bit of microclimate. In all cases, keep in mind that air circulation is also important.
Radiation and Convection
Blocking sunlight by shading reduces heating through radiation, the direct transfer of heat via electromagnetic waves, sunlight.
Another mechanism of warming is convection, which involves the movement of air that has absorbed heat, often from the soil and structures warmed up by radiation.
Effective temperature management requires differentiating between these two modes of heating. As discussed, radiation can be managed by reducing direct sunlight entering the greenhouse. Controlling heat buildup through convection involves air exchange or air cooling. Natural cooling occurs when cooler outside air enters through lower vents, displacing warmer air that is pushed up by cooler air, and exits through top vents. However, this natural ventilation can be imperfect due to turbulence and airflow patterns within the greenhouse. Additionally, during hot summer middays, the outside air itself may be warm and have low humidity. In such cases, providing sufficient water through misting, fogging, and maintaining adequate topsoil moisture is crucial. The relatively drier air will enhance evaporation from the soil, misters, and the plants themselves (evapotranspiration), which has a cooling effect.
Evaporative Cooling
The pad-and-fan system is an active cooling method. It works by drawing outside air through a continuously wetted pad (with other vents closed). As this water-saturated air is pulled through the greenhouse by the exhaust fans on the opposite side of the pads, the water evaporates, cooling the air and increasing humidity. However, the cooling efficiency of this system is significantly reduced under conditions of high ambient humidity, such as during the monsoon season.
A static evaporative system has mist nozzles in the top of the greenhouse. Misters and foggers on floor level maintain a moist soil from where water evaporates.
A significant natural cooling factor is the plants themselves. An empty and dry greenhouse exposed to full sun can experience extremely high temperatures (over 60°C), primarily due to infrared radiation. In contrast, a greenhouse with a dense crop canopy shades the soil, cools the air through evapotranspiration, and helps maintain humidity levels. Therefore, substantial plant biomass during the summer can significantly contribute to maintaining reasonable temperatures and humidity. In rose cultivation, building biomass over time involves harvesting somewhat shorter stems and employing green pinching techniques. In other crops, pruning and leaf removal are carefully managed to balance vegetative growth with production goals.
Notes on Temperature and Daylength
Bear in mind that when talking about the daily temperature, we often choose the maximum temperature. The 24-hour average temperature, however, is more important for many crops. Say, 35C or 36C for 2 or 3 hours around midday and early morning temperatures of say 20C, gives an average of 28C. The plant is able to function normally during 9 hours of daylight. It is important to open all vents of the greenhouse just before sunrise and for some hours thereafter. That is currently the lowest temperature of the day.
Important Note: Our subjective feelings of temperature and humidity can be misleading. We may perceive drier air as cooler due to increased evaporation from our skin, even if the actual temperature difference with more humid air is minimal. Therefore, relying on a thermometer is essential for accurate measurements. Traditional mercury thermometers (sometimes replaced by alcohol thermometers) are common. Wet and dry bulb thermometers are useful for determining humidity levels, and minimum/maximum thermometers are valuable for record-keeping. While a wide range of digital thermometers is available, more reliable models tend to be more expensive.
Conclusion
Light is the source of growth for plants (photosynthesis), but strong sunlight is also the reason air heats up, especially in greenhouses; the hot air may accumulate. Reducing radiation by shading and exchange of air may ameliorate this effect. A balanced shading strategy is important, because during midday hours, light intensity may be excessive (in relation to available CO2), but at the beginning and end of the day all available light shall be allowed to reach the crop. Different forms of evaporative cooling can be used; the crop itself is able to moderate temperature increase through evapotranspiration.
It is important to realise that the factors, light, temperature, CO2, air humidity, air circulation and soil humidity all come together in managing the conditions for the plants