What Do Plants Do In The Dark?

When the topic of horticultural lighting comes up, the conversation typically revolves around intensity, color temperature and electrical usage.  A lot of attention is given to making sure plants receive enough good quality light to perform photosynthesis, but what happens when the lights go off?

Photosynthesis and Respiration

Well, first, let’s start with a reminder of what happens when plants receive light from the sun or an artificial source.  They perform photosynthesis by using that light along with carbon dioxide and water to produce carbohydrates, the energy required for them to live.  At the same time, a byproduct of this process, oxygen, is released into the air.  That’s why deforestation and destruction of the rainforest is such a big deal – less plants to convert the carbon dioxide we breathe-out into oxygen we breathe-in.

But there’s another process going on in plants that happens 24/7, regardless of whether the sun is out or not – respiration.  Just like people, plants must breathe to survive.  And just like people, they breathe in oxygen and breathe out carbon dioxide.  This is called respiration.  The oxygen they breathe-in is used to assist in biological processes that also include the carbohydrates created during photosynthesis, allowing the plant to grow.

During the day, or when a light source is present, plants will perform photosynthesis and respiration at the same time.  At night, or when the lights go out, plants focus more of their energy on respiration and stop performing photosynthesis.  Because respiration is always taking place, that DOES make it possible to keep some plants on a 24-hour light cycle. 

Do Plants Need Rest?

Probably.  At least, we tend to think that adequate periods of darkness make for happy healthy plants.  While it may be tempting to artificially extend days in order to give your plants more opportunity to create carbohydrates through photosynthesis, you’re also robbing them of their period of focused rest, recovery and growth.  Not all plants respond well to this.

Plants tend to adopt a circadian rhythm based on light cycles that create patterns in their biological activities, very much like humans and other animals.  It’s also likely that there are other processes going on when the lights are out that we may not yet fully understand, much like our own need for sleep.

Longer Days and Higher Yields

If your goal is to grow the biggest plants possible, AND the variety you are growing can handle it, 24 hours of light during certain periods of the plant’s lifecycle WILL make a positive impact.  The question you would need to then answer is whether the additional cost of electricity and other inputs is worth the additional amount of growth.