There are several debates about whether it’s okay or not okay to leave your grow lights on for 24 hours a day. Most hydroponic growers keep asking the question – can you leave grow lights on 24 hours a day? The fact is while there isn’t any sure answer to this, it is more likely that leaving grow lights on 24 hours a day may be detrimental to some plants depending on the growth stage of the plants and other factors.

However, different people have tested this out with varying results, with some good results and some bad results. It is obvious that the results you get may depend on other factors like the type and species of plant, the growth stage of the plant, quality and quantity of light exposure, etc.   

Grow lights are useful in augmenting the amount of light that plants get especially if plants are not receiving the adequate supply of sunlight needed for their growth and development.

The Two Phases of Photosynthesis

On the other hand, plants also do need some period of darkness to grow and develop properly and exposure to grow lights for 24 hours may become potentially harmful for the plants.

Plants produce their food in the presence of light during a process known as photosynthesis. This process occurs in two phases – the light phase and the dark phase. Water reacts with carbon dioxide in the presence of light to form carbohydrates during the light phase. The carbohydrate is then metabolized for growth and development needs or stored during the dark phase. The excess by-products of this process (free radicals) can be harmful to the plant if the plant is exposed to excessive light.

Type of Plants Based on Light Requirements

One reason why people often get mixed results from leaving grow lights on for 24 hours may be as a result of the different light requirements for different plants. Some plants need very minimal amounts of lights to do well and giving them more light than they need may become harmful for them.

Other plants need just enough light, not too much and not too little. Another category of plants needs high amounts of light to grow and develop very well. Thus, it becomes difficult to know how much light exposure will be appropriate for plants if you don’t the light requirements for that plant already.

Plants are typically classified into three categories based on their light requirements. These are:

• Low Light Plants: These plants require little or no direct sunlight. Light from a single fluorescent bulb source may be all that they need. Examples include Chinese evergreen (Aglaonema), Cast iron plant (Aspidistra), Ponytail palm (Beaucarnea recurvata), Parlor palm (Chaemadora), Dumb cane (Diffenbachia), etc.

• Medium Light Plants: These plants need just enough light to get by and do not require too much or high amounts of light exposure to do well. Lights from two fluorescent bulbs may be sufficient for these plants to grow and thrive. Examples include Amaryllis, Norfolk Island pine (Araucaria), Asparagus fern (Asparagus), Ferns such as Boston fern (Nephrolepis), and bird’s nest fern (Asplenium), Rubber plant, etc.

• High Light Plants: These are plants that require high amounts of light to grow and develop well. They need all the lights they can get and these may be the plants that are likely to do well when exposed to grow lights or any light source for 24 hours. Examples of high light plants include most varieties of tomatoes and peppers, Cacti and succulents, Hibiscus does best in high-light conditions, Citrus such as calamondin orange, kumquat, Meyer lemon, Hibiscus, Culinary herbs such as basil (Ocimum), thyme (Thymus), lavender (Lavandula), various orchids, Ti plant (Cordyline), etc.

Light Requirements Based on Growth Phases of Plants

Light needs of plants also differ based on their growth phase and their response to 24 hours of light exposure may significantly differ depending on the stage in their growth and development.

Some plants may need and better respond to long durations of light exposure in seedling stages and flowering stages than during growth stages. Others may respond better to long durations of light exposure during growth stages than flowering stages or during the early growth stages as seedlings.

For instance, exposure to red light wavelength is known to help plants grow flowers, seeds, and fruits easily at the right time, while exposure to blue light wavelength is known to bring about stem growth in plants.

It is important to test and understand how your plants will respond to light before giving them long hours of light exposure.

Disadvantages of Leaving Growth Lights on 24 Hours a Day for Vegetables

While there are advantages of exposing plants to light for 24 hours, the disadvantages may outweigh the advantages. For instance, light exposure may help seedlings to grow and establish a good root system but receiving light for more than 18 hours may not provide any additional benefit especially when plants outgrow the seedling stage or early phase of life.

Some of the disadvantages include:

• Risk of Poor Growth: The risk of poor growth as a result of detrimental effects of long exposures of plants to light can often be serious and it may significantly affect plant yields and general outcomes. 
• Cost of Electricity: The cost of electricity for keeping lights on for 24 hours a day can quickly add up the cost of running your garden or farm. This may not make any logical sense especially if you can still get the same results without keeping lights on all day and all night.
• Risk of Damage: There is also a significant risk that the light bulbs may fail or burn out quickly especially since most manufacturers recommend that these grow lights to be turned off for at least 15 minutes per day.

Conclusion

Hope this provides a good explanation to the question – can you leave grow lights on 24 hours a day? Light exposure is very important and necessary for most plants to grow, thrive, and produce good yields, but leaving grow lights on and exposing plants to grow lights for 24 hours may not necessarily be beneficial as the disadvantages of doing this may be more than the advantages.