Most indoor plants thrive with 12 to 14 hours of artificial light per day, but the hours alone don’t tell the full story. What matters equally is how intense that light is, and intensity requirements vary dramatically depending on whether you’re keeping a snake plant alive on a shelf or growing tomatoes to harvest. Getting both duration and intensity right is the key to healthy growth under grow lights.
Why Hours and Intensity Both Matter
Plant scientists use a measurement called Daily Light Integral (DLI) to describe the total amount of usable light a plant receives over 24 hours. Think of it like rainfall: DLI is the total amount collected in a bucket by the end of the day, while intensity is how hard it’s raining at any given moment. A plant can reach its daily light target through moderate intensity over many hours or high intensity over fewer hours.
This is why simply leaving a dim light on for 16 hours won’t satisfy a tomato plant, and blasting a fern with intense light for 6 hours can cause damage. You need to match both the brightness level and the duration to what your specific plant requires.
How Many Hours Per Day
For most indoor growing situations, 12 to 14 hours of light per day is the standard recommendation. You can go as low as 10 hours or as high as 16, but staying in that middle range works for the widest variety of plants. A simple plug-in timer eliminates the guesswork.
One important wrinkle: some plants are sensitive to day length when it comes to flowering. Short-day plants like poinsettias and Christmas cacti need long, uninterrupted dark periods to bloom. Long-day plants like African violets and tuberous begonias flower when they get more light hours than dark hours, typically 14 to 16 hours. If you’re growing something specifically for its flowers, the photoperiod you choose can make or break blooming. For foliage plants and most vegetables grown for leaves, this isn’t a concern.
Light Intensity by Plant Type
Intensity is measured in PPFD, which stands for the number of light particles useful for photosynthesis hitting a square meter every second. Grow light packaging increasingly lists this number, and inexpensive light meters or smartphone apps can measure it at the leaf surface. Here’s what different plant categories need:
Low-Light Houseplants
Snake plants, pothos, and similar low-light species prefer 75 to 200 foot-candles of light, which translates to roughly 100 to 150 PPFD. These plants survive at even lower levels (down to 25 foot-candles), but they won’t actively grow. A small LED panel placed 12 to 18 inches away is usually sufficient. These are the easiest plants to light artificially because even modest setups exceed their needs.
Medium-Light Houseplants
Plants like monstera, most philodendrons, and begonias prefer 200 to 500 foot-candles, or roughly 150 to 250 PPFD. A 100 to 200 watt LED fixture positioned 12 to 24 inches from the leaves handles this range well. These plants benefit from the full 12 to 14 hours, and you’ll notice faster, fuller growth compared to relying on a north-facing window.
Herbs and Leafy Greens
Lettuce, spinach, parsley, and basil need noticeably more light than decorative houseplants. Lettuce does well with a DLI of 12 to 17, while basil is hungrier at 15 to 25. In practical terms, aim for 100 to 250 PPFD at the canopy and run your lights for 12 to 16 hours. A sweet spot for lettuce and most herbs is around 170 PPFD maintained consistently from seedling through harvest. Basil, which is more light-demanding, benefits from the higher end of that range.
Fruiting Vegetables
Tomatoes, cucumbers, zucchini, and peppers are the most demanding indoor crops. Their recommended DLI is 20 to 30, and they need 500 to 600 PPFD during flowering and fruiting stages. Seedlings of these plants start at a gentle 100 to 200 PPFD, then you increase intensity as the plant matures by lowering the light or increasing its power. Growing fruiting crops indoors requires a serious lighting setup, typically 300 watts or more of quality LED power, and 14 to 16 hours of daily runtime.
How Light Color Affects Growth
Plants primarily use red and blue wavelengths for photosynthesis. Chlorophyll absorbs these two ranges most efficiently, which is why many older grow lights glow purple (a mix of red and blue LEDs). Modern full-spectrum white LEDs contain a balanced blend of wavelengths, including red and blue, and work just as well while being easier on your eyes.
Blue light promotes compact, sturdy growth and helps with leaf development. Red light drives flowering and fruit production. Green light, despite common belief, isn’t completely wasted. It penetrates deeper into leaf tissue and reaches lower leaves that red and blue light might not. Plants grown under higher light intensity also develop thicker leaves, which increases their capacity to convert light into energy. For most home growers, any full-spectrum LED marketed for plants will provide an adequate color mix without needing to think about specific wavelengths.
Placing Your Grow Light
The distance between your light and the top of your plant (the canopy) is one of the most important variables you can control. Too close and you risk light burn: bleached, curling, or crispy leaf tips. Too far and the plant stretches toward the light, becoming leggy and weak.
For low-wattage LEDs (100 to 300 watts), the general guidelines are:
- Seedlings: 18 to 24 inches from the canopy
- Vegetative growth: 12 to 18 inches
- Flowering and fruiting: 8 to 12 inches
For medium-wattage LEDs (300 to 500 watts), add about 6 inches to each of those ranges. For high-power fixtures above 600 watts, seedlings should start at 30 to 36 inches, with the light moving closer as the plant matures. These are starting points. Watch your plants closely during the first week after any adjustment. If new leaves look pale or develop brown edges, raise the light. If stems are stretching and leaves are spaced far apart, lower it.
Choosing an Efficient LED
LED grow lights have become dramatically more efficient in recent years. The key spec to compare is photosynthetic photon efficacy (PPE), which measures how many usable light particles the fixture produces per watt of electricity. A good target for 2025 is 2.8 or higher. Top-tier professional fixtures now exceed 3.0, meaning they deliver the most light for the least power. Cheaper panels often fall below 2.0, which means you’re paying significantly more in electricity to achieve the same light levels.
When shopping, look for fixtures that list their PPFD at specific distances. This tells you exactly how bright the light will be at your canopy, which is far more useful than wattage alone. A well-designed 150-watt LED can outperform a poorly designed 300-watt one if it has better optics and higher efficiency.
Estimating Your Electricity Cost
The formula is straightforward: divide your light’s wattage by 1,000, multiply by the hours you run it per day, then multiply by your electricity rate per kilowatt-hour. For a 200-watt LED running 14 hours a day at the U.S. average of about $0.16 per kWh, that works out to roughly $1.34 per month for a single fixture. A more powerful 600-watt setup running the same schedule costs around $4.03 per month.
For a small herb garden or a shelf of houseplants, electricity costs are minimal. Indoor vegetable gardens with multiple high-powered lights can add up, but modern LEDs have cut those costs substantially compared to the older fluorescent and HID fixtures that dominated a decade ago. If you’re running a larger setup, choosing a high-efficiency fixture with a PPE above 2.8 can save meaningful money over a growing season.