How Long Should Grow Lights Be On for Houseplants
Plants need a balance of light and darkness to regulate cycles, impacting key processes like flowering and fruiting. For indoor gardeners, knowing the ideal grow light duration is essential for healthy growth. If you're a new plant parent unsure how long should grow light be on — don’t worry! Mars Hydro provides detailed manuals, or you can refer to this blog for expert tips.
The optimal lighting schedule depends on various factors, including your plant’s species, growth stage, Daily Light Integral (DLI), and light intensity. In this guide, we'll break down each of these factors so you can provide the best possible light conditions for your plants. Let’s dive in!
Factors Influencing Light Duration
Each species has specific needs based on its growth habits and environmental preferences. To ensure optimal growth, it's crucial to understand the factors that influence the duration of grow light exposure:
Plant Species
Plants exhibit distinct photoperiodic responses based on their species and varieties, which influence their flowering cycles. Short-day plants, such as chrysanthemums and poinsettias, require longer nights—exceeding a critical threshold—to trigger flowering. These plants typically need less than 12 hours of daylight to bloom.
On the other hand, long-day plants, like spinach and radishes, initiate flowering when nights are shorter than a certain duration. These plants generally require more than 12-16 hours of daylight to flower.
Some plants don’t depend on day length to regulate flowering. Instead, they bloom based on factors like temperature, maturity, and overall growth conditions. These are called day-neutral plants. Examples include tomatoes, cucumbers, and corn, which can bloom and produce fruit as long as they receive sufficient warmth and nutrients.
Plant Growth Stage
As plants move through their growth stages, their light requirements evolve. During the germination and early seedling stages, plants typically need 16–18 hours of light each day to support photosynthesis, which is crucial for developing strong roots and shoots.
In the vegetative stage, plants shift focus to growing leaves and stems. They still benefit from 16–18 hours of light each day, depending on the species. This extended light encourages robust foliage development, which is vital for optimizing photosynthesis and supporting the plant's overall health.
As plants enter the flowering stage, their light requirements change, typically requiring a shorter light period of around 12 hours per day. This reduction directs their energy toward reproduction, promoting strong flower and fruit development for a successful yield.
Daily Light Integral
Daily light integral(DLI) is the amount of PAR received each day as a function of light intensity (instantaneous light: μmol·m-2·s-1) and duration (day). Low-light indoor plants like peace lilies thrive with a DLI of about 1-4 mol/m²/day. In contrast, fruiting plants such as cucumbers and basil require significantly more light, typically in the range of 10-30 mol/m²/day for healthy growth.
Light Intensity
Photosynthetic Light Flux Density(PPFD) measures the light intensity reaching a specific area at a given moment, indicating the number of photons hitting a plant's surface per unit of time. Once you’ve established your plants' daily light requirement (DLI) and the ideal duration for light exposure, you can easily calculate the necessary light intensity by dividing the DLI by the duration.
For example, if your plant needs a DLI of 25 mol/m²/day and you plan to provide light for 5 hours each day, you would divide 25 by 5, which gives you a required light intensity of 5 mol/m²/hour.
General Rules
In addition to the four main factors, the hanging distance and type of grow lights also affect the optimal light exposure time. While there is no one-size-fits-all solution, experienced growers offer helpful guidelines on light duration. Generally, plants require 8-10 hours of light daily, with no more than 18 hours. It's essential to provide at least 6 hours of darkness each day to promote healthy growth and recovery.
For your convenience, we've created a chart that summarizes the light requirements for most indoor plants under grow lights. Feel free to bookmark this blog or download the chart image for easy access whenever you need it.
FAQs
Q1: Is it okay to leave a grow light on all the time?
While some high-light plants can handle continuous light, it's generally advisable to avoid leaving grow lights on 24/7 for most plants. They need a daily rest period to recover and carry out essential processes like respiration.
Q2: Is 12 hours of grow light too much?
Twelve hours of grow light is generally suitable for many plants, especially during their flowering stage. Most plants thrive with 12 to 16 hours of light daily, depending on their needs.
Q3: Can you burn a plant with grow light?
Both sunlight and grow lights can potentially burn your plants, but grow lights provide more control. By using the appropriate wattage and high-quality materials, along with carefully managing the hanging distance and the duration of exposure, you can nearly eliminate the risk of plant burns.
