How Long Should a Grow Light Be On (For Best Plant Growth)
If you grow plants indoors, then you’ve likely grappled with questions about proper grow light schedules.
But with confusing misinformation out there, it’s tough to know what durations to aim for.
The truth is that ideal grow light hours depend on many factors — plant varieties, growth stages, seasons, and even ambient temperatures.
Excessive brightness can stress plants, while too little light hinders growth. So we need to tailor schedules carefully.
In this post, I’ll cover the optimal daily duration for vegetative vs. flowering stage plants. You’ll learn how elements like crop types and goals influence ideal run times, too.
I’ll even share specific hour recommendations to properly customize schedules for herbs, vegetables, and ornamental plants.
Let’s begin.
TL;DR Summary
Vegetative Stage:
Flowering/Fruiting Stage:
Additionally:
Optimal Daily Light Duration
When growing plants indoors, one of the most common questions is: how long should you run grow lights each day?
The optimal daily duration depends largely on whether plants are in the active vegetative growth stage or the flowering/fruiting stage.
A. Vegetative Stage Plants
| Plant | Optimal Daily Light Duration* |
| Tomatoes | 16 hours |
| Peppers | 16–18 hours |
| Lettuce | 12–16 hours |
| Herbs (e.g., basil, parsley) | 12–16 hours (14-18 for some varieties) |
| Houseplants (e.g., pothos, peace lily) | 12–16 hours |
| African violets | 8–16 hours |
| Orchids | 12–16 hours |
| Succulents | 10–14 hours |
| Cacti | 12–16 hours |
*The above numbers are researched and inspired by various trusted sources, and they are estimated. The actual number may vary based on your unique nuances and circumstances.
For vegetables, herbs, and other plants still actively growing foliage and stems, most recommendations are 14–18 hours of light exposure per day.
This allows for continuous, robust plant development.
Providing 18–24 hours can extend time to maturity in some species, but risks stressing plants. A minimum of 4–8 hours of darkness is still needed for proper functioning.
Some key research findings on optimal vegetative stage photoperiods:
- Lettuce grown under 29 mol·m−2·d−1 DLI (16 on/8 off) had 1.4-1.5x more biomass than under 19 mol·m−2·d−1 DLI (source).
- One study found that Basil receiving 18 hours of light per day gave the highest dry mass, while 20–24 hours gave stunted growth.
- Too much light (20+ hours) causes lettuce to rapidly elongate and bolt early.
KEY TAKEAWAY: For compact, high-yielding vegetative growth, aim for 14-18 hour photoperiods with adequate dark periods overnight.
B. Flowering/Fruiting Stage Plants
| Plant | Optimal Daily Light Duration* |
| Tomatoes | 12–14 hours |
| Peppers | 12–16 hours |
| Lettuce | 12–16 hours |
| Herbs (e.g., basil, parsley) | 12–16 hours |
| Houseplants (e.g., pothos, peace lily) | 12–16 hours |
| African Violets | 10–14 hours |
| Orchids | 12–16 hours |
| Succulents | 10–12 hours |
| Cacti | 10–14 hours |
*The above numbers are researched and inspired by various trusted sources, and they are estimated. The actual number may vary based on your unique nuances and circumstances.
Once plants transition to reproductive stages, the optimal light exposure changes.
12–16 hours daily light is required to initiate flowering in some, but not all, flowering plants.
For example, phalaenopsis orchids require 12+ hours of darkness to initiate flowering. Tomatoes are day-neutral and do not require a particular photoperiod to flower.
Additionally, Chrysanthemums require short days (less than 12 hours of light) to flower.
However, some long-day plants like spinach may need over 12 hours of light to flower.
Factors Influencing Optimal Schedules
When figuring out the best lighting durations for your indoor plants, there are a few key factors that impact how long grow lights should stay on each day.
By understanding what these elements are and how they affect ideal schedules, you can customize photoperiods tailored to your plants.
A. Plant Types
Different plant varieties have their own specific needs when it comes to daily light integral (DLI) and hours of illumination.
For example, leafy greens typically need 12–18 hours, with 14-16 being optimal, for robust growth. Tomatoes and peppers prefer photoperiods around 14–16 hours in the vegetative stage.
Meanwhile, succulents are sensitive to overexposure—stretching can occur above 14 hours daily. So they do best at 8–12 hours under intense light.
Paying attention to optimal durations for your particular crops’ types allows you to provide ideal light integral levels without risk of light stress or etiolation.
B. Growth Stages
In addition to plant variety, the stage of growth also impacts ideal illuminance schedules.
Seedlings and young plants usually require 18–24 hours of light at first to establish healthy early development.
Then as plants mature, the optimal duration shifts—for example, tomatoes need 16 hours of light in vegetative growth but only 12–14 hours once flowering begins.
Since needs change over time, it’s important to adjust light schedules based on phenological stages to balance vigor and yields with reproductive development based on your goals.
C. Cultivation Goals
Finally, the specific purpose you have for growing plants indoors affects suitable light durations.
If prioritizing lush greens and higher productivity from culinary herbs, then longer 14-18 hour photoperiods are preferable.
NOTE: Sun-loving herbs need longer photoperiods, not all culinary herbs.
But for flowering houseplants mainly grown for their blossoms, 10–12 hours of alignment with seasonal daylight may suffice.
So be sure your schedules align with the traits you want to encourage.
Role of DLI
When creating tailored lighting schedules, an essential metric to consider is Daily Light Integral or DLI.
DLI refers to the total daily dose of photosynthetically active radiation (PAR) that plants receive, measured in mol∙mˆ-2∙dˆ-1.
It indicates the amount of light plants obtain to drive growth over 24 hours.
Research shows that matching target DLI ranges through customized photoperiods boosts productivity for indoor crops.
Vegetables may need 12-25 mol∙mˆ-2∙dˆ-1 while young seedlings only require ~5 mol∙mˆ-2∙dˆ-1 (source, source).
Rather than just guesswork on duration, using DLI allows you to optimize light delivery specific to plant species and growth phases.
For instance, extending time under lower-intensity fixtures can achieve the same DLI as shorter high-intensity lighting. This energy-saving approach still meets plants’ requirements.
Furthermore, as needs change over time, DLI targets can adjust—for example, orchids need higher integrals in flowering vs. propagation.
So whether growing herbs or orchids, relying on DLI metrics to inform schedules promotes vigorous, high-quality plants.
PRO TIP: This free DLI calculator by Omni Calculator easily allows you to accurately measure DLI for your specific needs.
Conclusion
As we wrap up, the key takeaway is that finding the optimal light duration for your indoor plants is, wait for it, all about balance.
Hopefully, now you feel empowered to create informed lighting plans.
Keep fine-tuning based on observations for thriving indoor harvests.
FURTHER READING: Looking for actionable advice on ideal grow light distance? Read our comprehensive guide that tells you exactly how far you should keep a grow light from your indoor plants.
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Dhruvir Zala