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I started my first hydroponic garden indoors in the middle of winter. Not only that but it was during the global energy crisis in 2022 (so I didn’t have the heating on much!). I can recall how excited I was to get started. But I quickly learned the hard way that air temperature was a really important factor, not only in seed germination but also in enabling my hydroponic crop to even grow at all.
Air temperature is a key factor in hydroponic plant growth, affecting processes like photosynthesis, transpiration, respiration, germination and even fruiting and flowering. As a general rule, as temperature rises, so does the rate of each of these processes. But if temperatures are too low, these processes are slowed or halted, resulting in poor plant growth.
In this article, I’ll explore in detail how air temperature is a critical factor to consider for any hydroponic grower and the different steps you can take to create an optimal growing environment for your plants.
How Air Temperature Affects Hydroponic Growth
The role of air temperature in hydroponics
Temperature is just as important in hydroponics as it is for other types of gardening. It can influence almost all of a plant’s vital processes needed for successful growth and ultimately make the difference between the success or failure of your crop.
So how exactly does air temperature affect plant growth in your hydroponics system?
1. Germination of seeds
I found out the hard way just how important temperature was when it came to germinating seeds. My first attempt failed completely because it was the middle of winter and (because of the 2022 energy crisis caused by the war in Ukraine), I barely had my indoor heating on.
Different seeds require different temperatures to germinate (generally ranging between 55°F – 80°F. If your grow room was below that, or fluctuating too much, then it’s unlikely your seeds will germinate.
I’ve noticed much better results since purchasing a seedling heat pad, which helps to maintain the correct temperature in the water, grow medium and supports more consistent air temperature as well.
2. Flowering
Temperature is one of the leading influences on flowering. Different plants have different flowering thresholds and most will not flower until that threshold has been met. This temperature threshold can often be higher than the plant’s ambient temperature, so if you’re growing for flowers you may need to increase the air temperature to manipulate the flowering process.
3. Fruits and vegetables
Just like flowering, many plants will not produce fruits or vegetables until a certain temperature threshold has been met. This threshold is different for every plant and generally varies between 40° and 85°F. As a rule of thumb, if temperatures are warmer fruits and vegetables will generally form larger (but not necessarily better quality!).
4. Crop quality
Crop quality is also influenced by the air temperature. Lower temperatures can force plants to reduce the amount of energy they use and instead store it as sugar. This can have the effect of improving the sweetness and flavour profile of some fruits and vegetables. Conversely, higher temperatures can reduce sugar storage in some plants and create poorer-quality crops. For example, high temperatures often create bitter-tasting lettuce.
5. Photosynthesis
Photosynthesis, the process by which plants absorb energy from the sun to produce oxygen and sugar, is one of a plant’s most important. If the temperature is too cold, the rate of photosynthesis can be reduced which can adversely affect plant growth.
Plants tend to photosynthesize best when the air temperature is 10 to 15°F higher during the day than at night, which allows them to focus on energy production during the day and to respire (rest) at night.
So, it’s important to mirror these natural fluctuations in temperature where possible within your own grow room rather than maintain a consistent temperature 24/7.
6. Respiration
Higher temperatures can increase the rate of respiration faster than the plant can photosynthesize. For a plant to grow effectively, photosynthesis (which produces energy) occurs at a faster rate than respiration (which burns energy). It’s important to ensure that your hydroponic grow room isn’t too warm to avoid this process. Check the needs of each plant you are trying to grow.
What is the ideal air temperature for hydroponic growth?
The ideal air temperature for growing plants hydroponically is generally between 65-75°F (18.3-23.8°C). This range is considered to be the ‘comfort zone’ for the majority of popular plant types.
But the ideal air temperature, of course, is ultimately dependent on the type of plant(s) you are trying to grow. To try and help, I’ve listed some common plants I like to grow hydroponically, alongside their ideal air temperate range in the table below.
| Plant Type | Ideal Temperature Range |
|---|---|
| Leafy Greens (Lettuce) | 60-65°F (15.5-18.3°C) |
| Chillies | 60-75°F (15.5-23.8°C) |
| Herbs (Basil, mint, cilantro etc.) | 65-70°F (18.3-21°C) |
| Tomatoes | 70-75°F (21-23.8°C) |
| Beans/Runners | 60-70°F (15.5-21°C) |
| Citrus Fruit (Limes, lemons) | 75-100°F (23.8-37°C) |
| Strawberries | 60-80°F (15.5-26.6°C) |
| Peppers | 70-80°F (21-26.6°C) |
Benefits of maintaining the correct air temperature
Air temperature influences most plant processes from germination to photosynthesis. Controlling the temperature of your grow room to meet the exact needs of the plants you are growing is key for optimal plant growth. Some key benefits of getting air temperature right are:
- Faster germination rates
- Faster, stronger plant growth
- Higher-quality crops with stronger flavours and/or greater sweetness
- Prevention of disease and illness
- Control of the fruiting and flowering process
The cost of getting the temperature wrong
If the temperature is too hot, or too cold, you may find it difficult to achieve the results you’re after from your hydroponics system. Whether you’ve set up in a grow tent, or in your living room, or even outside on your purchase, it’s still important to get the temperature right. If you don’t, you risk the following:
- Stunted plant growth
- Greater risk of illness and disease
- Lacklustre fruiting (if any at all)
- Poor quality crops (bitterness)
- Death of your plants
Methods for controlling air temperature in your grow room
So we’ve established that air temperature is just as important in hydroponics as with traditional gardening. So how can you best control it for optimal results? Fortunately, there are a number of controls you can add to your growing set-up that can help.
Heating
Here are a few things you can add to increase the temperature around your hydroponics system:
1. Turn on your central heating
It may seem obvious, but in the middle of an energy crisis, to me, it didn’t seem like a good idea. However, turning on your central heating is a great way to maintain a consistent temperature if you growing indoors (i.e. in your living room).
2. Greenhouse Heaters
Another method is to purchase a good old-fashioned greenhouse heater to raise the temperature of your grow room. I’d mainly recommend this if you were growing in your basement or garage. They can be quite powerful, however, so you should monitor the temperature closely and don’t overdo it.
3. Tubular Grow Room Heaters
Specifically designed for hydroponics, these tubular grow room heaters are great for maintaining your grow room temperature within the optimal growing range.
Cooling
Here are a few things you can do to decrease the temperature around your hydroponics system:
1. Add a fan
Free-standing oscillating fans are relatively cheap and can help increase airflow around your plants (a benefit in its own right) as well as reduce the surrounding air temperature. Fans don’t use much energy either so you won’t be paying too much extra on your bills.
2. Install air conditioning
Depending on the size and scope of your hydroponics endeavour you may want to consider installing an air conditioning unit to help you maintain cooler temperatures. Air conditioning units are more expensive to install than a simple oscillating fan, but they are a great, permanent solution for controlling air temperature. Remember though, air conditioning units tend to use a lot of energy and may increase your monthly electric bill.
3. Keep your hydroponics system in the shade
If your hydroponics system is outside, consider constructing an awning or keeping the system in the shade to avoid too much direct sunlight. Sunlight is a great source of energy for your hydroponic plants but too much of it can be damaging or even deadly. Consider carefully the needs of the plants you’re trying to grow. If they don’t do well in full sun all day, consider creating some shade.
Humidity is important too
It’s important to consider humidity as well as air temperature. Humidity plays an important role in your plant’s development and, like temperature, can make or break your crop. Too much, and your plants and system can become gummed up with mould and disease.
I’ve written an informative guide about dealing with mould in hydroponics which I recommend you read.
Too little humidity and your plants may struggle to reach maturity. It’s therefore important to keep humidity at moderate levels (with a target of between 40-60% relative humidity).
Is water temperature important for hydroponics too?
The temperature of the water used in your hydroponics system is just as important as the surrounding air temperature. The temperature of your nutrient solution mainly affects a plant’s metabolic processes as well as oxygen solubility. Incorrect temperature, particularly when your water is too cold, can negatively impact a plant’s ability to grow and produce flowers and/or fruit.
Final considerations
Air temperature influences the majority of key processes important to your plant’s development, like photosynthesis and respiration. It is a key factor in determining how successful you’ll be able to grow plants hydroponically. Just like in any other type of gardening, getting it right is essential.
Too hot or too cold, and seeds will not germinate, plants become stunted and in the worst cases, they can die altogether. Fortunately, controlling the temperature around your hydroponics system straight forward by installing heaters, air conditioning or fans.