How Long Do You Leave LED Grow Lights On?

LED grow lights have transitioned from laboratories to ordinary households, becoming the secret weapon of modern gardening enthusiasts.


They not only simulate full-spectrum light ideal for plant growth but also allow precise control over light duration and intensity. This helps various plants efficiently undergo photosynthesis even in less-than-ideal environments, promoting rapid growth and high-quality yields.

However, just like watering and fertilizing plants require moderation, using LED grow lights isn’t a matter of keeping them on continuously. This leads us to the question we’ll delve into today: How long should LED grow lights be on?

This post aims to explore this question, helping you understand the mysteries of the light cycle, grasp the light requirements of different plants at various growth stages, and adjust the duration of LED light usage based on different environmental conditions.

Table of Contents

Basis of Photoperiod and Plant Requirements

When it comes to the light cycle, it’s like a biological clock bestowed upon plants by nature. It determines when plants grow when they rest, and even when they enter the flowering stage.

The photoperiod theory is essentially about how sensitive plants are to the length of daylight in a day, acting as a calendar for plants to judge when to enter the next growth stage.

Each plant has its daily routine. Some plants prefer staying up late, known as long-day plants, as they only start flowering when the days are long enough. Common examples include spinach and rapeseed, which require sufficient sunlight during the day to happily produce flowers.

On the contrary, there are also early-to-bed, early-to-rise plants, known as short-day plants, such as some ornamental flowers. They only sense the arrival of spring when the days are short, prompting them to start nurturing flowers.

When it comes to setting specific lighting schedules, it depends on the type of plants we are nurturing. For example, if we are growing vegetables, we need to determine whether they are “night owls” or “early birds.”

During their growth stage, plants may require longer periods of light to accelerate leaf growth and nutrient accumulation. However, during the reproductive stage, we need to adjust the lighting according to their preferred photoperiod to ensure timely flowering and fruiting.

In summary, different plants have different lighting needs at different stages of growth, much like how we have varying food preferences.

Seeds in their infancy may only require faint light to awaken their dormant vitality, while mature plants may need ample sunlight to ensure healthy growth.

We must learn to observe and understand the habits of our plant babies and cleverly use LED grow lights to simulate appropriate day lengths, helping them thrive happily in any environment.

Recommended Duration for Using LED Grow Lights

Case Analysis

Herbs: If we are cultivating lavender, mint, and basil. Lavender prefers long daylight hours, requiring at least 14-16 hours of light exposure per day, so LED lights should be kept on for this duration.

Mint is less demanding, requiring around 12 hours of light, while basil falls somewhere in between, with approximately 14 hours of optimal light exposure.

Vegetables: Let’s take tomatoes, lettuce, and peppers for example. Tomatoes are similar to lavender in their preference for long daylight hours. They require approximately 14-16 hours of light exposure per day to grow vigorously and produce abundant fruit.

Lettuce, as a leafy vegetable, benefits from 12-16 hours of light exposure for optimal growth. Peppers also require ample light, but not excessively so. Around 12-14 hours of light exposure is suitable for peppers.

Flowers: Take roses, chrysanthemums, and lilies for example, each with their preferences. Roses require 12-16 hours of light exposure during their growing period to promote bud formation.

Chrysanthemums prefer slightly shorter daylight hours, around 10-14 hours, while lilies have moderate daylight requirements, typically 12-16 hours. Light requirements during the flowering period may vary and need to be adjusted according to specific varieties.

Succulents: Succulent plants typically enjoy ample sunlight but dislike prolonged direct exposure. When using LED lights, it’s advisable to provide gentle illumination for 4-6 hours in the morning and evening to avoid scorching from intense midday sunlight.

Cannabis Cultivation: For certain legally cultivated cannabis strains, the duration of light exposure can affect gender differentiation and growth rate. Generally, many cannabis growers adopt a lighting cycle of 18 hours of light followed by 6 hours of darkness, which helps promote rapid growth and flowering of the plants.

Correspondence Between Growth Stage and Light Duration

Seed Germination Stage: During the germination stage, when seeds have just sprouted, they don’t require much light. Providing a gentle light source for 2-4 hours per day can help stimulate germination.

Vegetative Growth Stage: Once seedlings have developed true leaves, gradually increase the duration of light exposure to fit the optimal range for the specific plant species. This helps promote sturdy stems and lush foliage.

Flowering & Fruit-bearing Stage: During this phase, plants enter a crucial stage of their lifecycle. Light exposure duration may need to be adjusted to trigger or maintain the flowering and fruiting processes.

Environmental Factors and Lighting Adjustment

Seasonal Changes: During winter when natural daylight hours are shorter, it’s essential to maintain the total light exposure duration close to the ideal requirement for plants.

In contrast, during summer, particularly in hot and intense sunlight conditions, reducing the use of LED lights is advisable to prevent plants from experiencing excessive stress.

Natural Light Intensity: When natural sunlight is strong, it may be appropriate to decrease the supplemental lighting provided by LED lights.

Conversely, on overcast days or when natural light is insufficient, increasing the duration of LED light usage is necessary to maintain a relatively stable total light exposure. The key is to observe plant conditions and adjust the lighting schedule accordingly to ensure both energy efficiency and meet the plants’ growth requirements.

Precautions and Misconceptions

However, don’t assume that more light is always better. Just like humans need rest after work, plants also require appropriate resting periods.

Continuous exposure to excessive light can exhaust plants! Over-illumination may lead to leaf burn, and yellowing, and disrupt the plant’s normal physiological rhythms, hindering its growth. Just as you feel weary after staying up all night, plants can become weak due to excessive light exposure.

The solution is simple: adhere to each species’ light requirements, and timely turn off LED lights to give plants a chance to rest. This allows them to absorb nutrients better and grow healthily.

You can do this by mimicking the time the sun rises to turn on the LED lights and turning them off after sunset.

For example, in spring and summer, the sun typically starts “working” around five or six in the morning and “finishes” around seven or eight in the evening. During this period, provide light for the plants. In winter, when daylight hours are shorter, adjust the LED light switch time according to the actual sunrise and sunset times in your local area.

Remember, try to align the lighting cycle with the natural rhythms that the plants are accustomed to. This not only helps them grow and develop normally but also contributes to improving yield and quality.

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