Can You Leave A Grow Light On All The Time?


Many gardeners and growers raise plants indoors for at least part of the year.  If you are one of them, you may be wondering how long you should leave grow lights on.

So, can you leave a grow light on all the time?  No, you should not leave a grow light on all the time.  Some plants are sensitive to day length, and will not produce flowers if they do not get any darkness.  Some grow lights can also generate heat, which can harm plants.  Of course, it can also be expensive to keep your grow lights on all the time.

Of course, there are many factors to consider when using grow lights:

  • the type of plant you are growing
  • the plant’s stage of development
  • the type of lights you are using
  • the duration of light and darkness

Let’s take a closer look at why you should not leave grow lights on all the time.  Then we’ll answer some common questions about plants and light.

Can You Leave A Grow Light On All The Time?

No, you should not leave a grow light on all the time.  The primary reason is that some plants are sensitive to the amount of light and darkness they get each day (that is, day length).

Day length tells these plants what season it is, which tells them when to produce flowers.  For example, when the days are short (9 to 10 hours of daylight per day), these plants know that it is the middle of winter, so it is not a good time for them to flower.

onion plants
Some onion varieties are sensitive to day length. Short-day onions grow best with less than 12 hours of sunlight per day.

Plants are separated into three categories, depending on their sensitivity to day length:

  • Short day plants need less than 12 hours of daily sunlight to flower (to say it another way, short day plants need more than 12 hours of darkness each day to flower).  Some examples of short-day plants are blueberries, raspberries, sweet potatoes, and some onion varieties.
  • Long day plants need more than 12 hours of daily sunlight to flower (to say it another way, long day plants need less than 12 hours of darkness each day to flower).  Some examples of long-day plants are beets, carrots, lettuce, spinach, and some onion varieties.
  • Day neutral plants do not depend on the amount of daylight to flower.  Some examples of day-neutral plants are apples, corn, cucumber, tomatoes, and some onion varieties.

For more information, check out my article on how day length affects plant growth.

As you can see, leaving a grow light on all the time will confuse your plants about what season it is.  Another good reason to turn them off for a while is excessive heat.

Some lights, such as incandescent or high-intensity discharge lights, generate a lot of heat.  If you leave them on all the time, your plants may get too hot or dry.

Finally, leaving grow lights on all the time is expensive, and so it is a good idea to turn your lights off for at least a few hours per day.

Of course, plants do need some light in order to produce energy and grow.  For example, red and blue light are the most useful types of light for plants to use for photosynthesis (the process where they convert sunlight, water, and carbon dioxide into oxygen and energy in the form of sugar & starches.

Generally speaking, plants will need more hours of exposure from artificial light than they would need from sunlight.  This is because it is difficult to get the exact ratio of light types that is found in sunlight.

sunlight through forest
Natural sunlight is the best for plants. If growing indoors, you will probably need more hours of artificial light for plants than you would need actual sunlight.

Of course, if your plants need more light, you can use higher-intensity grow lights instead of leaving them on all day long.  You can also try reducing the distance from the light to your plants (generally, the tips of plants should be 6 to 12 inches (15 to 31 centimeters) from the grow light.

What Happens When Plants Get Too Much Light?

In addition to becoming confused about day length and the time of year (seasons), plants that get too much light may also experience other problems.

For example, according to the University of Minnesota Extension, “Plants exposed to too much light may become scorched, bleached and limp.”

If you notice this happening to your plants, then increase the distance from your grow light to your plants.  You may also need to switch to lower-intensity lights, or decrease the amount of light exposure that your plants receive each day.

How Many Hours a Day Should Grow Lights Be On?

Most plants need at least 8 hours of sunlight per day.  This translates to at least 8 hours of artificial light from grow lights per day.

As mentioned earlier, the exact length of light exposure depends on the plant and its stage of development.

Short-day plants will need less than 12 hours of light (more than 12 hours of darkness) per day in order to start flowering.  If you want to delay flowering of short-day plants, give them more than 12 hours of light per day.

carrots
Carrots are long-day plants and need more than 12 hours of light per day.

Long day-plants will need more than 12 hours of light per day to start flowering.  You can delay their flowering by providing less than 12 hours of sunlight.

It can be easy to forget to turn your grow lights on or off.  If this happens, your plants will get too much or too little light.

Incorrect light exposure can cause slow plant growth.  It can also cause delayed or premature flowering of your plants.

To avoid these problems, use a timer to automate when the lights go on and off.  As the days get longer in spring, gradually increase the amount of light per day to mimic what is going on outdoors.

What Type of Grow Light to Use?

There are four options to consider when deciding which type of grow light to use: incandescent, fluorescent, high intensity discharge, and light emitting diode (LED). Let’s start with incandescent lights.

Incandescent Grow Lights

Incandescent light bulbs are the ones that most people are familiar with.  They emit lots of red light, but very little blue light.  This means that they are not a balanced choice for growing plants indoors.

light bulb
Incandescent light bulbs give off lots of red light and heat, but very little blue light. They only last about 1000 hours.

Standard incandescent light bulbs are not as energy efficient as the other grow lights available, since they produce lots of heat.  Furthermore, incandescent light bulbs have a lifespan of only 1000 hours.

The only possible benefit is that they are cheaper than other grow lights.  However, this initial cost savings will soon be offset by increased electricity costs and more frequent bulb-buying.

Fluorescent Grow Lights

Fluorescent light bulbs are often seen in office buildings.  They usually come shaped as tubes in lengths of 2, 4, or 8 feet.

fluorescent light bulbs
Fluorescent light bulbs give off a good balance of red and blue light, but not much heat. They last about 10,000 hours.

Fluorescent light bulbs give off mainly red and blue light, making them a great choice for grow lights.  They also produce much less heat than incandescent bulbs.

This means there is less chance of overheating your plants, and also less wasted energy.  In fact, fluorescent light bulbs are 2.5 times as efficient as incandescent light bulbs, meaning you will save on energy costs.

Fluorescent light bulbs have a lifespan of 10,000 hours, which is 10 times as long as an incandescent bulb.  If you choose fluorescent lights, look for ones developed specifically for growing plants.

High Intensity Discharge Grow Lights

High intensity discharge (gas) lights are based on sodium vapor or metal halides.  These light bulbs are often used in greenhouses.

high intensity discharge lights
High-intensity discharge lights are way more efficient than incandescent bulbs, and they last a long time. However, they do give off lots of heat – this can actually be useful in a greenhouse if you need to warm it up.

High Intensity Discharge lights are ten times as efficient as incandescent bulbs, and they last a long time.

One potential drawback is that these bulbs give off lots of heat.  However, this could be an advantage if you want to heat your greenhouse while also providing light for your plants.

Light Emitting Diode Grow Lights

Light Emitting Diodes (LEDs) are extremely bright light bulbs that give off very little heat.  They are also energy efficient and long-lasting.

LED
LEDs (light emitting diodes) are very bright, last a long time, and do not give off much heat. However, they are expensive.

LEDs can even be customized to produce the exact wavelengths of light that you want.  For example, you could get an LED that produces only red and blue light, which is optimal for plant growth.

The only drawback of LEDs is that they are expensive.  However, this higher initial cost could be offset by lower energy costs and replacement bulb costs over time.

For more information on the types of grow lights, check out this article on grow lights from the University of Missouri Extension.

How Much Light Should Plants Receive?

The amount of light that plants should receive will depend on the type of plant and its preference for light intensity.  We can measure light intensity using a unit called a foot-candle.

According to the University of Missouri Extension, “A foot-candle is a measure of light intensity, or brightness. It is defined as the amount of light received by a 1-square-foot surface that is 1 foot away from a light source equivalent to one candle of a certain type.”

Plants can be separated into three categories based on their light-intensity preferences:

  • Low-Light Intensity Plants prefer light intensity of 50 to 250 foot-candles.  This would be equivalent to 1 fluorescent light bulb that is a 2-foot long tube.  Some examples of low-light intensity houseplants are philodendron, snake plant, and Chinese Evergreen.
  • Medium-Light Intensity Plants prefer light intensity of 250 to 1000 foot-candles.  This would be equivalent to 2 fluorescent light bulbs that are each a 2-foot long tubes.  Some examples of medium-light intensity houseplants are jade plant, Schefflera, and Boston fern.
  • High-Light Intensity Plants prefer light intensity of over 1000 foot-candles.  This would be equivalent to 4 fluorescent light bulbs that are each a 2-foot long tube.  Some examples of high-light intensity houseplants are cacti, succulents, and citrus such as oranges and lemons.

For more information and more plants of each light-intensity type, check out this article on light requirements from the University of Minnesota Extension.

If you want to optimize the amount of light your plants get without changing light bulbs, consider using reflectors to maximize available light.  You can also arrange plants so that they are not shading each other from the grow lights.

Conclusion

By now, you know that you should not leave a grow light on all the time.  In addition to confusing plants, it may generate too much heat, and could cost a lot of electricity.

I hope you found this article helpful – if so, please share it with someone who can use the information.  If you have any questions about grow lights, please leave a comment below.

jonathon.david.madore

Hi, I'm Jon. Let's solve your gardening problems, spend more time growing, and get the best harvest every year!

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