Temperature

Your plants are constantly processing energy 24 hours a day. In the light cycle they make and store sugars, during the dark cycle these sugars are transported throughout your plants. Your plants need a continuous energy supply in order to grow and mature. The speed at which your plants process energy is directly related to temperature.

Ideal Temperatures

If you want your plants to grow quickly and stay healthy, it's important that you keep them within their 'comfort zone'. This is between 22-28°C. Most plants are very comfortable up to 28°C, but some are sensitive to higher temperatures.

You should use a good quality thermometer to measure the temperature of your growroom. Ideally, choose a thermometer that records the maximum and minimum temperature. This will allow you to monitor the hottest and coolest temperatures in your grow room over a given period. Make sure you place your thermometer in a well-ventilated location, out of direct light.

Transpiration and Temperature

Small pores on the leaves (stomata) allow water vapour and gasses to leave the leaf and keep the plant cool (transpiration). If your plants are within their temperature 'comfort zone' (22-28°C) and are transpiring healthily, they'll keep themselves up to 2°C cooler than the ambient air temperature.

As a defence mechanism to conserve water, your plants' stomata will start to close when the air temperature exceeds 30°C. With the stomata shut, your plants will not be able to transpire to keep themselves cool. If the temperature remains too high, the water trapped in your plants' leaves can overheat, causing internal cell and enzyme damage. The result is poor growth and unhealthy plants.

Consequences of High Temperature

If the temperature is too high, your plants will respond in a few ways. They'll appear stretched as inter-nodal distance increases with a high temperature. This results in very a 'leggy' plant with poor flowers or fruit quality. During periods of high temperature, your plants will have difficultly laying down dense material because their energy is being used so fast that they cut back on the energy used to make flowers and fruit.

High temperatures are often associated with low relative humidity. As a result, the roots of plants take more water than nutrients to compensate for the loss of water from the leaf due to high transpiration. This increases the nutrient strength in your reservoir or growing media which if left unchecked can lead to high nutrient concentrations, this can cause over-fertilisation.

Controlling Temperature

To keep the growing environment within your plants' 'comfort zone' (22-28°C), you must remove the hot air from your growing area. Using an inline extractor fan, you can take hot air and duct it out. Due to heat rising, you should place your extractor fan at the top of your room.

The lights in your room are the main heat generators, so the more lights you have, the larger your fan will have to be.

As well as removing the hot air, you also need to supply the room with fresh cooler air. This can be done by using another inline fan to pump fresh, C02 rich air into your room. Relying on one extractor to take the wasted air out and draw the air in through vents is not as effective in bringing temperatures down.

Heat can often build up in your growing area under the lights. It's important that you have good air exchange and keep the air well mixed to avoid heat build-up.

Hint - An efficient way of removing heat from HID lights is to use air cooled reflectors.

The time of day your lights come on can also have a big effect on your growroom temperature. In the hotter months, it's advisable that you start the light cycle in the evening, running the lights through the night. This allows you to bring in cooler night time air into your room during the light cycle and avoids extreme mid-day temperatures. Running your lights through the night is great in winter also, counteracting extreme low temperatures outside at night.

If your growroom temperature is constantly high and growth is affected, you may want to invest in an air conditioner. Air conditioners are an effective way to reduce high temperatures in the hotter months. However, they remove water from the air, thus lowering humidity, so a humidifier may be needed when using air conditioning units.

If your temperature is high and humidity is low, the use of a humidifier may be enough to increase humidity and lower temperatures. Increasing the humidity has a cooling effect and can bring the temperature down significantly.

The last resort for combating high temperatures is to simply cut back on the number of lights in your room. If you have two lights and are finding that your day time temperature is constantly above 30°C and your plants are struggling, you'll get a poor yield and poor quality produce. By cutting back to one light the temperature will drop to a manageable level and your plants will remain healthy. Obviously, this will drop yield, but it will allow you to grow better quality produce.

Roots and temperature

If roots are to function efficiently and take up nutrients, they need to be kept at a good temperature 24 hours a day. For established plants with a good root mass, the temperature around the root zone (rhizosphere) should be 20-24°C. High root temperatures can lower the dissolved oxygen content of the nutrient solution which will increase the risk of root diseases. Roots with temperature below 15°C will be unable to take up nutrients efficiently.

Day Time and Night Time Temperatures

Plants are very sensitive to differences in temperature between the light cycle and dark cycle. If you want fast vegetative growth, you need to try and keep your night and day temperatures as close as possible. However, to promote flowering and fruiting, you need to increase the difference between night and day by 4 - 8°C.

If your temperature differential is bigger than 10°C, you'll put your plant under stress. The sugars your plants have made during the light cycle will be poorly transported in the dark cycle, thus lowering growth rates.

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