Should You Leave Grow Lights On 24 Hours A Day?


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Providing providing sufficient light to your plants inside a grow space can be tricky, but using Grow lights enable your plants to thrive indoors and extend your growing season. Plant grow lights are a great tool to make your growing process more efficient, but they can also be harmful to the plant if don't follow the proper lighting schedule.

Now you might wondering — how long should you keep grow light on or if it is recommended to leave grow lights on for 24 hours?

The answer to the question is no, it is not recommended to leave your grow light 24 hours a day, because this will force plants to grow quickly. It is ideal to keep a grow light on for about 12 to 18 hours. Too much light in various growth phases can interrupt a weed plant’s natural process of growth and dormancy. Worse, light exposure for a whole day can damage and kill your plants.

In this article, I’ll explain Consequences of Leaving Lights on 24/7 and  the recommended hours to keep lights on depending on several factors.

This includes the plant type, temperature, type of light, grow size space, and a plant’s growth stage. Once you learn the proper hours for light exposure, you can increase plant yield and profit.

Does more light mean more yield?

Growers think that the longer plants get exposed to lights, the bigger their yield will be. Some even leave grow lights on 24/7 during the first weeks of the vegetative stage.

While they are correct about needing more light, the way of gaining light is incorrect. You have to consider some factors including the weed strain, type of light, and growth stage.

It is possible to leave grow lights on for 24 hours, but it is still way better to set lighting times per day. I will discuss more of this later on.

What happens to a plant that is kept under constant light?

Why High Humidity Is Bad And Causes Problems

Like humans and animals, plants need a break from direct light, too. Imagine they need to sleep as well.

When cannabis absorbs a lot of light, they will constantly engage in photosynthesis. On the bright side, this will result in the maximum growth of foliage.

Yet, there are consequences as well. If your weeds are constantly exposed to light, especially during the vegetative stage, they may experience stress.

Once they get exposed to diseases, they also become susceptible to pests, and consequently, diseases and death.

Length of the Light-Dark Cycle and Plant Growth

Plants thrive if they receive about 8 to 16 hours of light, regardless of whether it’s artificial or natural. Furthermore, growing cannabis needs at least 6 hours of darkness per day.

The same goes for any strain of weeds. They need water, carbon dioxide, and light to produce glucose, which serves as their food.

When growing indoors, it is important to determine how to control light.

Keep in mind that various kinds of plants need different light exposure and concentration.

Understand Photosynthesis And Respiration In Plants

Grow lights mimic sunlight. Good thing that plants only need light itself.

Photosynthesis lets plants absorb light and convert it into an energy source. At night, when everything goes dark, respiration turns that energy into carbohydrates for later use.

This means that exposing your plants to light 24 hours a day will prevent respiration from occurring. No respiration can impact the health of plants.

What happens when plants are exposed to light?

Plants have various growth cycles. As mentioned earlier, you may leave grow lights on for 24 hours when your plants are in the vegetative phase.

Meanwhile, plants grown for their flowers, fruits, and vegetables require a dark period to enter the flowering phase.

In other words, you must shift to the recommended exposures to light and darkness once plants transition to other stages.

Your capacity to manage the growth cycles using lights can increase a plant’s speed to production. As you can see, it is not always about turning the lights on all day to ensure growth.

What is the Ideal Amount of Light Exposure for Plants?

Fruit and vegetable bearing plants, such as marijuana, must receive 20 to 40 moles of light daily.

For example, you can generate 20 moles of light for an 18-hour vegetation stage light cycle. This is the case if you are using LED grow lights with 308 micromoles of light.

If you want to give 20 moles of light during the flowering stage, your lights must have at least a PPFD value of 462 micromoles of light to handle the 12-hour lighting cycle.

Factors to Consider: How Long Should I Leave Grow Lights On?

Here are important factors to remember when planning how long to leave grow lights on:

  • Type of Plant
  • Growth Phase
  • Type of Light
  • Temperatureand Humidity
  • Size of Grow Space

Once you have established the connection among these factors, you should be able to get an idea about how long to keep the lights on plants.

Type of Plants

It is better if you are familiar with the kind of plant. There are three types: low-light, medium-light, and high-light plants. A seed box must show the plant type.

Low-light plants should get about 4 hours of artificial light. On the other hand, medium-light plants require about six hours of light exposure. Meanwhile, high-light plants need about 14 to 16 hours of light.

Growth Phases

Light and dark cycles depend on the growing stage. Each phase requires a specific duration of light exposure to successfully complete the growing state.

Cloning/Seedling State: Should Seedlings Get 24 Hours of Light?

Should Seedlings Get 24 Hours Of Light
No, that is a terrible idea. This is one of the most critical phases of plant growth, so growing conditions should be ideal. Seeds would find it hard to germinate when exposed to constant light.Thus, the advisable light-dark cycle for seedlings is 16 hours on and then 8 hours off.

While it is true that seedlings need lots of light to grow, they also need darkness to survive. Since seeds are still ‘baby’ plants, they require a dark time to undergo respiration.

Vegetative State

The Vegetative Phase Needs More Light To Sustain Growth. But This Also Means The Cannabis Plants Must Rest For At Least 6 Hours. Hence, The Ideal Light-Dark Cycle Must Be 18 Hours On And 6 Hours Off. The plants will not grow taller as you expect it to be from 24-hour exposure. Yet, this cycle provides you a healthier growth. 

As long as you follow suggested light-dark cycles, the plant will be able to form lots of leaves and a solid stem.

Flowering State

Flowering State

For this phase, buds will begin to form nice and stinky. The lights should help produce potent and big buds that are suitable for smoking or making edibles.

A bigger area coverage doesn’t necessarily need bigger light wattage. Regardless of the type of light, here’s a general guideline of

The flowering stage won’t focus on growing taller, but it will make sure your plants get wider.

At this state, you can’t leave the lights on for 24 hours. Otherwise, the weed plants will stay in the vegetative stage. Thus, it’s best to follow the light-dark cycle of 12 hours on and 12 hours off.

Plants have mechanisms that let them know if there’s less light. The transition from lightness to darkness enables the plants to grow full flowers.

Type of Grow Light

Since you are using artificial lights, cannabis will need longer hours of light exposure.

Although the vegetative phase benefits from 24-hour light exposure, the rest of the stages needs light and dark cycle. The type of grow light also plays a key role in this matter.

LED Grow Lights

This type of light produces certain, yet complete light spectrum wavelengths. It supports photosynthesis, as well as all growth phases.

The best part is that although LED lights generate great power, they only consume low energy.

On the downside, LEDs can be damaging to the human eye. They are also pricier than regular lights.

Regular Light Bulbs

Like LED grow lights, regular light bulbs are energy-efficient and long-lasting. They are also inexpensive.

However, regular lights have narrow spectral wavelengths and get heated easily. Sometimes, it could be challenging for them to flower a growing space.

Compact Fluorescent Lights

You can place CFLs about six to seven inches away from your plants. The downside is that unlike LED lights, CFLs are not fitted with cooling systems.

High-Intensity Discharge Lights

HIDs are the go-to lights for growers who are on a tight budget. Unfortunately, they are less powerful than LED lights.

In addition, they are more prone to heat. This is why I recommend using a mirror or foil underneath the weed plants. In this way, the light can reflect towards the foliage.

Metal Halide Lights

MH lights produce a large amount of light per watt. On the flip side, the bulbs emit a lot of heat, which may cook seedlings.

Temperature and Humidity

Lights impact temperature, and subsequently, humidity.It is ideal to keep the temperature inside a grow space between 72 to 82 degrees Fahrenheit. Meanwhile, relative humidity must be around 30% to 40%.

Temperature And Humidity

Size of Grow Space

A bigger area coverage doesn’t necessarily need bigger light wattage. Regardless of the type of light, here’s a general guideline of how many watts you need for a grow area.

  • 1 x 1: 32 watts
  • 2 x 1: 64 watts
  • 2 x 2: 128 watts
  • 3 x 3: 288 watts
  • 3 x 4: 384 watts
  • 4 x 4: 512 watts
  • 4 x 5: 640 watts
  • 5 x 5: 800 watts
  • 5 x 6: 960 watts
  • 6 x 6: 1152 watts

How Far Should Grow Lights Be from the Plants? 

How Far Should Grow Lights Be From The Plants
​A light fixture’s distance will depend on the type of bulb and growth stage. Depending on the bulb type, the distance can range from three to 60 inches away from the canopy. In effect, you prevent the lights from damaging the foliage.

Moreover, note that the higher the wattage, the How High Should Your Grow Lights Be Above Your Plants.    

Type of Grow Light

Distance From the plant

LED Grow Lights

12 to 24 inches

Regular Light Bulbs

12 to 19 inches

Compact Fluorescent Lights

3 to 12 inches

High-Intensity Discharge Lights

24 to 60 inches

Metal Halide Lights

24 to 30 inches

Signs & Symptoms of Insufficient Light Exposure on Plants

Although I have provided a guideline, sometimes a bit of experimenting can change this. Also, watch out for obvious signs that your cannabis plants need light.

The most common sign is that the plant is growing visibly taller towards the light. It means the plant is reaching or begging for more exposure. In some cases, lack of light would result in stunted growth. Similarly, it could be that new leaves are not vivid in color.

How Not to Overexpose Your Plants to Light

We have seen how grow lights serve as a crucial tool in growing marijuana indoors. But overexposing also poses some risks.

Once you see the leaves start to curl, you will know that there’s too much heat. The branches may also appear to be leggy.

Think of it this way, humans have a routine in eating to gain energy and sleeping to take a rest.

The same goes for growing cannabis. They require a routine for getting light exposure and dark time for resting.

  • You can go old school and use an alarm to signal when to turn on and off the grow lights.
  • The cheapest way to remember the duration is by setting an alarm. But if you don’t mind spending some bucks, you can purchase a light timer. A timer will automatically turn the lights on and off according to your preferred schedule.  
  • Aside from that, put a thermometer to monitor the grow room’s temperature. You may also opt for a controller, which lets you manage the grow lights, temperature, and humidity altogether. 

Furthermore, check whether you have too many lights inside the grow space. It could also be that the grow lights are too close to the canopy.

Another way is to reduce light exposure or gradually turn off the lights. This will let the plants rest in a dark space to consumer energy.

What If 18 Hours of Light Is Not Working for My Plants?

Don’t push the lights to run for 24 hours. Although plants can accept lots of light, the actual heat might be harmful. If this is the case, you should change to a stronger grow light. A stronger grow light can run for fewer hours. For example, some regular light bulbs do not provide sufficient light exposure for your plants even after 18 hours. In this situation, you may want to consider changing to full spectrum LED lights that mimic sunlight.

By doing so, there’s a guarantee that even if you are not leaving grow lights on for 18 hours, your plants have received the necessary red and blue lights to sustain growth.

I suggest using LED lights with wavelengths of 630 to 660 nanometers. This range enables the blue and red lights to boost plant growth.

Another way is to rotate your plants every week. If your grow lights tend to focus its intensity at the center, your foliage won’t get full coverage.

Plan Your Light Cycles

How long you will leave grow lights on would depend on factors such as the type of plant, growth stage, temperature, and grow space.

If you want to become a successful indoor grower, you have to invest in learning how to maximize equipment such as grow lights.

I have experienced how difficult it could be to set up a schedule for lights. But with the information from this post, you can do it faster and easier!

To sum it up, leaving grow lights on for 24 hours could impact a plant’s health. Like humans, plants need to rest as well.

Remember, only the vegetative state can tolerate 24 hours of light exposure. But all phases have recommended on-off cycles to let weed plants undergo photosynthesis and respiration.

Likewise, turning off grow lights for a few hours lessens operational costs, but without sacrificing plant yield.

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