Heating and cooling

Temperature, the No 1 key to your success!

The most important factor to maximize the results from the lighting good quality and the appropriate watering with a well balanced nutrient solution is the temperature of your indoor garden. This is the key!

Use a good temperature thermostat

The horticultural thermostats are precious and precise tools for indoor gardening. The temperature control of your indoor garden is the most important factor to be successful in the process of growing plants. Plants have specific comfort requirements that we need to respect to maintain their stomas opened. (Figure 1a and 1b) They need it to maximize the photosynthesis and to absorb fully the nutrient elements. Plants are "moody" and "picky" living organisms: if it is too warm, or to cold, they close their stomas. The closing of their stomas is enough "to stop the photosynthesis". The plant will stop the absorption of nutrients; will also stop to absorb water and carbon dioxide, stopping to convert the light, nutrients and carbon into cells, leaves and flowers... Plants close their stomas to protect itself of extreme conditions, and it means in fact that "the shop is close!" Without a good control of temperature, you may have to choose to stop gardening because the plants do not give good results considering the money involved, the care given, the equipment used and the energy injected in your gardening.

Figure 1 : a) Each leave is made of thousands of stomas located on top of leaves and sometimes, under the leaves. b) Diagram of an open stoma c) diagram of a close stoma.

Each plant has specific comfort requirements

Each plant species has specific temperature requirements. There is always an ideal temperature during the lighted period (during the photosynthesis) and also another ideal temperature during the "sleeping" period in the dark while the plants reject useless by-products. The specific comfort range of your plant, when lighted or in the dark, is normally 2 degrees Celcius (4 degrees Farenheit) around what is considered the ideal temperature. A good thermostat used in indoor gardening should maintain the temperature within + or- 1 degree Celcius upon given setting. The standard thermostats used for heating and air conditioning (figure 2a and 2b) are not adapted for gardening and to possible temperature variations of the selected temperature needed with those thermostats could reach + or - 3 degrees Celsius (3 to 5 degrees Fahrenheit: figure 2c) Every time the thermostat is on standby, the heating or the ventilation or the air conditioning being 1 or 2 degrees out of the comfort zone, your plant will close stomas, "closing the shop", until the temperature gets back the ideal zone. A good electronic heating thermostat costs the same price as a horticultural thermostat which gives full comfort to your plants. The gardener will obtain, in good articles, in any good magazines or any good gardening book, the ideal lighted and darkness temperature settings for to the needs of the growing plants. The advice of an experienced gardener can also be very useful

Figure2 a) Bimetallic heating thermostat b) Electrical plinth thermostat c) Temperature variation of the greenhouse (yellow line) representing + or - 3° C with a bimetallic heating thermostat or with an electrical mural plinth thermostat. The blue line on the graph shows a variation + or- 0,5° C with an electronic quality thermostat. That will cost around $100.00 more. (Source Hydro-Québec )

Verify the measures of the control system

It is always useful to validate the adjustments of the settings entered in your thermostats (figure 3). We also say to "validate the calibration". That operation is done using a precise and reliable reference thermometer. One has to note the actual temperature when the heating system is starting and the level of the temperature when the system is turning off. This check tells if your thermostat is well adjusted. The difference between the temperature set by the gardener, (called set point) and the temperature when the heating system is starting is called the "differential". Also, there is a differential between the set point and the temperature when the heating system is turning off. A good horticultural thermostat has very small differential variations and so, can maintain your plants in the right selected temperature zone. Many models of horticultural thermostats have adjustable differentials. To verify if a thermostat is measuring correctly your heating level or your cooling system level, it is recommended to use an accurate thermometer.

Figure 3 Four instruments measured the temperature in the same location. You read 22 Celsius, 22,8 Celsius, 23,4 Celsius , 22,1 Celsius , so our best estimate resulting of it is 22,5 Celsius. Four instruments including one with 2 readings do not give the same result, they shows 5 different measures of the same temperature. Which one is right? Notice that the third thermometer has an internal sensor and an external probe located very close and they shows a difference of 1,3 Celsius . There is no other explanation than a flagrant inaccuracy in terms of measurement.

Temperature controllers with an adjustable differential

If a gardener has an horticultural thermostat with an adjustable differential (figure 4), it is always tempting to save energy by increasing the differential of the set point. Doing so, one wants to reduce the number of minutes the fans or the heating equipment works. Be careful with the differential's adjustment. The plant's comfort zone is very limited: sometimes +or- 2 degrees C and in sometimes +or- 1 degree C for demanding plants. One should not widen the differential too much as to reduce fan noise, energy or anything else because plants will run the risk of falling out of their specific comfort zone. The gardener's experience and the results of the crop represent the best guides and the best references to select an acceptable and appropriate differential.

Adjust and adapt for sensor location

Some models of horticultural thermostat allow adapting for sensor location. Ideally with plants, temperature should be measured in the top third of the foliage. It is not always easy! It is not always possible to suspend the thermostat at that level. The thermostat can also be affected by the heat resulting from the artificial lighting. A simple solution is to put your controller on a wall, in a convenient location, to adjust and to plug the heating equipment or the cooling system and then to correct the reading with an offset. Doing so, you can adjust the temperature considering the real temperature measured among your plants. Again, the gardener has to use a precise thermometer located among the leaves and well protected against the direct lighting system. It is then possible to know the difference between the real temperature among the plants and the temperature measured by the controller. From there, one can adapt the measure of the temperature controller by inserting an offset to correct the reading. In a small room, in a vertical garden, or in a greenhouse, the difference of temperatures measured by the controller and the thermometer located among the foliage is assumed to always be constant.

Selecting a temperature controller

Considering the needs of the garden, the technology and the budget available, the gardener may use only one temperature controller to deal with heating, during winter time and darkness, and rearrange equipments in warmer conditions to deal with the cooling needs (ventilation or air conditioning). A better way is to use two horticultural thermostats, one for heating and another one for cooling. The integrated controller is ideal because it manages the dead zone between the heating and the cooling. It is more difficult to manage those two different operations with two separate thermostats. With two specific separate controls to manage the temperature, you can face two opposite actions: for example, the heating is functioning at the same time than the fan and energy is wasted.

Figure 6 a) A precise low price specific horticultural temperature controller needing careful adjustment in reference to a reliable thermometer b) more precise specific horticultural temperature controller with a measure display graduation c) Integrated temperature and relative humidity controller with numeric display and factory calibrated digital sensor accurate at - or + 0.5° C.

Test and try to learn and succeed!

Many gardeners wish to test different temperature combinations during lighted and darkness periods to find and identify the best results. How to know which combination gives the best result? SIMPLY BY MEASURING AND TAKING NOTES FOR EACH CULTURE! It is useful to log the temperature combinations selected for each growing stage (planting, vegetative growing, flourishing and ripening of fruits) and also important to measure the size of the crop for each culture in the garden. It is easy. The gardener accounts the number of leaves and fruits, measure their size and weighs the harvest. Doing so, the garden's production is measured and can be compared. If the gardener grows lettuce, he can weigh the amount of leaves harvested. If he grows strawberries, he weighs all fruits harvested. The harvest will vary from one crop to the next. For example, in the same garden, a strawberry plant will produce 1 kilo of fruits (about 2 lbs) during the first crop and deliver 2 kilos (about 4 lbs) during the next. It means a better production for the same work, with the same lighting system, the same seeding, potentially with the same food.

What in the world...?

The gardener will find most answers in his notes considering the existing temperature conditions used during each week for each culture, the duration of lighting, temperature variations, relative humidity variations and carbon dioxide availability.