Growing in hot conditions

In this article I explain how you can manage climate and temperature when growing plants in hot conditions. The following advices are general guidelines and plant behavior is always subject to local circumstances. Growers should always follow up on local legislation and consult a professional agronomist when in doubt. Syngenta does not accept any liability in any business damage resulting from the advice.
Author: Ben Geijtenbeek – Senior Crop Technical Specialist – Syngenta Flowers
Ben Geijtenbeek has forty years of experience as a technical expert in floriculture. He is specialized in educating growers in various cultures with new techniques and technologies. This way he revives the potential of Syngenta’s genetics, from the corporate philosophy “Bringing plant potential to life”.

How to grow in the heat?

The question on how to grow plants in warm to hot conditions is not that difficult to answer as you might think. At many places on earth; from subtropical to tropical, plants are growing abundantly. As long as we understand the involved processes, we can copy these environmental circumstances to our greenhouses and the sites where our products are produced, sold and appreciated.
The first step in helping growers to overcome heat stress is made by breeders like Syngenta Flowers on the day they started introducing heat tolerant flowers like Pansy Colossus, Pentas BeeBright, Zinnia Magellan, Mandevilla Rio and many others
Flower bed

Flower bed with Zinnia Magellan


One of the most important actions is to balance the key growing factors as temperature, light level, water availability, climate (air humidity) and fertilization. Make sure all of these are known and under control as much as possible. This means that a change in one factor by natural circumstances, let’s say temperature, needs to be followed up by an action from you as grower. Increase also the light intensity, the water supply and the fertilization level, because the biological machine – the plant – is running at a higher pace.


All biological processes in the plant are influenced by temperature. With a higher temperature, these processes generally go quicker. This means that when you want to slow down the growing speed, you need to ventilate to the maximum, day and night. You might want to do this for instance because you want to keep the growth limited. Generally speaking, speedy growth gives weakness, stretching and less sturdiness in the plants.  Especially when the light level is too low.

Air humidity

The air humidity influences the evaporation speed of the plant. Too much humidity blocks the evaporation speed. This has a direct effect on the uptake of water and fertilizer at root level. 
Humid environments also increase the risk on condensation on the plant during night and early morning. A wet plant promotes fungi spores development and damage as a result.   
However, too low humidity forces a plant to close their stomata to avoid drying out. That leaves us with insufficient evaporation as well, and therefore no cooling down effect, which in turn means more risk on sunburn. This can result in a delay of the growth or a complete stop of plant growth. 

Out of balance

When we are no longer able to keep the balance between the growing factors in our greenhouse, we need to help our crops by controlling with more or less water, more or less light level, more or less humidity and more or less temperature. Because these unbalances in growing factors will harm the plant and creates plant damage somewhere.

How do we know all this?

Your plants give you a clear message when their leaf tissue becomes floppy while the soil is wet enough to promote normal growth. This means that there is an unbalance between the uptake of water and the evaporation speed. The biggest mistake you can make at that moment is watering more. Excessive watering removes all the oxygen from the root system, which promotes dying of roots at the exact moment when the plant needs them the most to survive. You need to apply screens to lowers the direct sun radiation on the leaves and give your plants room to recover their inside water balance. As you see, the best method is to use sunscreens that can be opened or closed rather simple.To keep them closed when the sunlight is too much, and put them open again on cloudy, or rainy days to maximize the light level for the plants. The fixed screens (green of black even) which we sometimes see in greenhouses are not the best for controlling light level.  Nor for temperature control. And they have a negative impact on air flow in the greenhouse, which especial in dark periods contributes to a healthy and active plant. To control the air temperature in Summer, outside screens or whitewash on the tunnel has a far better impact.
Sun screen indian flowers grower
My aim with this story is to explain to you that the plant can handle high temperatures or other unfavorable growing conditions to quite some extent, as long as the other basics are at the plants’ disposal. 

Main tool

The main tool to lower the leaf tissue temperature is evaporation by ventilation.
What to do to keep this process running?
  • Open windows as early as possible to get outside and inside temperature equal.
  • Lower the humidity in humid climates to make evaporating easier. But avoid too low RH levels which makes plants closing their stomata. Because this stops the evaporating process, stops the growth, and gives a big change of burning. A good RH is somewhere between 40-80%.
  • Protect the plants from direct radiation by shading.
  • Paint the greenhouse roof, use humidifiers or a pat and fan system.
  • Give plants enough space. This promotes the circulation of air.
  • Try to do plant maintenance before the hottest time of the day. Watering, spacing, spraying and so on.
  • Use the dark period each day to give the plant the opportunity to relax.


You know what a good dinner and a cool glass of fresh juice can do for you during a nice summer evening in your garden after a busy day in the company. Try to treat your plants the same way.  
The more you make them happy, the more they will make you happy by a top quality.
But be aware: a wrong temperature can be experienced by us as well. However, a wrong humidity, or wrong light level is something we as humans find hard to experience. So, you need to be keen on this. The best tool to control climatic factors is the use of an automated temperature control system, or even better, a specific climate computer. 
Finally. Know the risks, know your plants, be in control 24/7, and take immediate action when some elements break the rules. 
Insufficient level of fertilizer
This picture shows an insufficient level of fertilizer, which has damaged the lowest leaves. A too dense production blocks the evaporation, and finally the botrytis finishes the job.
The not yet visible impact is the weakened plant, which has to overcome the potting stress.  Result will be an uneven re-growth after potting, and possible losses in the coming weeks. 
Young Plant Production
This is a picture that shows you a young plant production on tables, with a sprinkler system, and on top of that a watering boom. This enables the grower to be very specific with watering the different crops.
The picture also shows a movable sunscreen in the right top of the picture, a ventilator, and two big ventilators as part of a pat and fan cooling and humidifying system.


Ben Geijtenbeek
Syngenta Flowers