How Long Can I leave a Grow Light on? What You Should Know

Indoor growers want to see their plants increase in health, grow steadfastly and be productive. Some might wonder if allowing the grow lights to remain on for 24 hours a day is a good idea.

Grow lights should be on for no longer than 12 hrs and off for 12 hours. This allows for healthy growth. Plants require both light and darkness to develop well.

If the plants have more than 12-hour exposure to light, you put the general health of the plant at risk forcing it to re-enter its vegetative phase. If it enters this stage, the plant takes very many weeks to re-enter its flowering period. The 24 hours of light will also prevent the plant from having its respiration process. 

Risks of Leaving the Light on

If your plant is in the vegetative phase, no harm will be upon them if the lights are on for 24 hours. However, you should not leave your plants on this 24-hour light exposure. If they are in the flowering phase, the 24-hour exposure to light will be very harmful and some plants like Cannabis are susceptible to light exposure when in their flowering stage. 

Plant Lifespan Under a Grow Light

Running the grow lights on a 24-hour schedule has adverse effects on the general health of the plant. One of these effects is that the lifespan of the plants will be shortened and have signs like leggy stems, stretched and yellow leaves, very tiny stems, and leaves.

The leaves will also have spindly brown edges. If the plant is to achieve growth after three months, this period may be extended and make the plant have a delay of almost five months. If the plant is to have a lifespan of four years, the lifespan shortens to two years. 

Does the Grow light make my plants grow faster? 

Most plants need at least 12 hours of exposure to light in a day, with varying intensities. Be sure to check the guides of your plants for the specific light needs as they grow. If the plants are always getting sufficient and right amount of light, they will grow steadily. Also, if the intensity of the light is high, it will have a robust growth process. In the presence of high grow light, the leaves of the plants appear small and the inter-modal distance becomes shortened. 

How do I ensure that my plants only get enough Grow Light? 

 You can set up a timer on your Grow lights when setting the lights in your hydroponic garden. With this timer, you will not worry about forgetting to turn the lights on or off as it works automatically. The timer, therefore, ensures that the plants get a sufficient amount of light when they need to. Enough light encourages photosynthesis and other growth processes. 

How high should I hang my grow light?

How high you should hang your Grow lights depends on the age of the plants. Generally, older plants need more light than do younger plants. On the other hand, seedlings can thrive well under very low-level Grow light. Nowadays, any indoor grow light will come with different power ratings that perform well at different heights. 

If the lights are at a high level, a less amount of light will reach the plants. High wattage lights have a distance of between 16-36 inches when placed at the top of the canopy. For instance, 1000 watts High-Pressure Sodium (HPS) can be at the height of 36 inches high from your plants for blossom. 

What happens when my plants get too much Grow light?

When your plants get too much light, the standard growth processes like photosynthesis are adversely interrupted. The temperatures of the tissues increases and all the water that is usually taken up by the plant for cooling the tissues evaporate. The growth of the plant is, therefore, suppressed. In some cases, the plant may eventually die.

What happens if my plants do not get enough Grow light? 

If your plant is still a seedling, and it does not get sufficient light, the cells in the stem become elongated and tend to push the cotyledons. The cotyledon in turn, forces the first leaf up, making it appear longer than others as it tries to seek more light. 

Can Grow lights burn plants?

Yes, they can. Grow lights burn some plants, whether in cold weather or even comfortable temperatures. For instance, grow lights can give the leaves and buds of your cannabis plants some damage when they are “overexposed” to light, leading to the “light burn.”

Plants can only take in some certain amount of light after which they will begin experiencing problems like discoloration, low growth quality and sometimes, and poor productivity. Also, since most plants flourish when exposed to partial shade or lights, these plants will not only die when exposed to too much light but they can as well die. 

What are the most common mistakes made when using grow lights for plants? 

The most common mistake that you are bound to make when using the light is the inability to place them at the right distance. The wrong height means that your plants will not get enough light and will therefore, not grow to their full potential.

Placing the light too close leads to stunted growth, and positioning the light too far causes stretching. You can avoid these mistakes by carefully following the right coverage distance of the Grow light, as indicated on its Details Page. It will be helpful for optimum light coverage and penetration. 

How do you fix burnt leaves on a plant?

Soil becomes exposed during the winter season if it has thawed in spring. It will, therefore, need flushing with heavy watering. If the leaves of your plants are experiencing the light burn, water the plants slowly and regularly. The water flushes out the affected soil and prevents the tip of the leaves from burning.

Additionally, the heavy watering will leach away all the salts that have built up on the leaves. Upon noticing that the plants are showing brown burnt tips, then it possibly means that the exposure of the plants to the grow light was intense and very lengthy. 

What’s the ideal amount of light exposure for plants?

The recommended amount of light that your plants should receive is to be between 20-40 moles per day. If the plant is on an 18-hour phase light cycle, produce 20 moles of light with using your grow light with a photosynthetic photon flux density (PPFD) value of around 308 micro-moles of light.

To maximize the plant yield, produce approximately 40 moles of light daily for the plants. If the plants are at a flowering phase, your grow light needs 926 umols photosynthetic photon flux density (PPFD) value, this compensates for the short 12-hour light cycle.

Can I be flexible with the light periods and adjust the times I put on the grow lights?

Generally, plants should grow under 12 hours of light and “rest” under 12 hours of darkness. You can be flexible with the light periods of exposure that you give your plants, but this is only recommendable up to a certain extent. The vegetative phase is a very flexible stage and you may leave your light to up to 16 hours. If your light is at low-intensity, you can make it go on for 20 hours a day. 

If the plant is at the flowering phase, stick to the recommended 12-hour light cycle. If you want to be flexible, you can leave the grow lights on for an hour more or less (for 13 or 11 hours a day). It is worth noting that the amount of time the plants are under the grow light does not matter. Instead, what matters is the number of light molecules that the plant receives daily. These light molecules are what affect the vegetable, fruit and total yields of the plant. Therefore, if your grow light has a weak intensity, you can either consider purchasing another grow light or consider being flexible with the light periods. 

How well can I use the grow lights?

Pick the right type of row light and use a digital thermostat to know the correct temperatures that suit your plants. Know how far the lights should be hanged and suspend them from chains, making them easily adjustable to accommodate the growing plants. Rotate the plants and leave enough space between them. Positioning the lights well makes the plants grow well. Knowing how to use your grow light is a beneficial idea. This knowledge will help you extend the lifespan of your plant.

Are these grow lights harmful to human beings?

Generally, grow lights are not harmful to human beings. There are however, some occasions that these lights may be detrimental and could pose health hazards. Safety depends on different factors like the duration, wavelength, and intensity of the grow light. Intense rays can damage the eyes irrespective of the wavelength that is released by the grow lights. The fierce rays can also cause skin burns or inhibit your ability to see.


I enjoy all things outdoors and I love plants! I've never considered myself to be one with a green thumb, but it's my mission to learn, so I figured I would bring you along for the ride. :) Happy planting!

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