Growing plants indoors under lights is becoming increasingly popular. Whether you want to start your own indoor veggie garden or grow houseplants year-round, using grow lights allows you to successfully grow plants even without access to sufficient natural light. But how many grow lights do you really need? The number of lights required depends on several factors.
What Type of Plants Are You Growing?
The light requirements for plants vary greatly depending on what you are trying to grow. Here are some general guidelines for how much light different types of plants need:
- Low light plants: Most common houseplants like pothos, philodendrons, and dracaenas need fairly low light. A couple of grow lights in a small area is usually sufficient.
- Medium light plants: Things like herbs, lettuces, and other leafy greens need a moderate amount of light. You’ll need more grow lights to cover a larger growing area.
- High light plants: Fruiting plants like tomatoes, peppers, and cucumbers are light hogs. They require intense light, so you need a high concentration of bright grow lights.
As a general rule of thumb, fruiting plants need 2-3 times more light than leafy greens. Make sure you research how much light your specific plants need before installing your grow light setup.
How Large is Your Growing Area?
The size of the area you want to illuminate is another key factor. A single grow light can only cover a limited space. Here are some guidelines for grow light coverage areas:
- Small growing areas (2-4 sq ft): One or two grow lights is usually sufficient.
- Medium growing areas (4-9 sq ft): You’ll need 3-6 grow lights.
- Large growing areas (10+ sq ft): Use at least 8 lights, up to 12 or more.
Measure the length and width of your shelving or growing area to determine the square footage. More space means you’ll need more grow lights for full coverage.
What Type of Grow Lights?
There are several different types of grow lights to choose from. The most common options include:
- LED grow lights – LEDs are very energy efficient. They don’t put out as much heat and last longer than other types of grow lights.
- Fluorescent grow lights – Fluorescent tubes are relatively inexpensive. They come in different light spectrums for growing plants.
- High intensity discharge (HID) lights – Metal halide and high pressure sodium lights put out intense light. They are better suited for larger grow spaces.
- Incandescent grow lights – Standard incandescent light bulbs can be used but are very energy inefficient.
LED and fluorescent lights are the most popular options for home growers. HID lights put out so much heat and use so much electricity they are better for commercial growing operations.
Light Intensity and Height
The intensity of light decreases exponentially the further away the source is from the plants. Most grow lights need to be kept 12-24 inches above the top of the plants for optimal light levels. As a general rule of thumb:
- LED grow lights should be kept 18-24 inches above the plants.
- Fluorescent grow lights should be kept 6-12 inches above the plants.
- HID grow lights should be kept 24-36 inches above the plants.
You may need more grow lights if you have high ceilings or need to cover very large growing areas. The lights need to be close enough for the plants to receive sufficient intensity.
Daily Light Integral
The daily light integral (DLI) is a measurement of the amount of photosynthetically active light plants receive in a 24 hour period. It is measured in moles of light per square meter per day. Most plants need a DLI of at least 10-20 moles/m2/day. Here are the DLI requirements of some common indoor plants:
| Plant | Daily Light Integral |
|---|---|
| Lettuce | 10-20 moles/m2/day |
| Tomatoes | 25-35 moles/m2/day |
| Peppers | 25-40 moles/m2/day |
| Cannabis | 35-50 moles/m2/day |
| Cucumbers | 40-50 moles/m2/day |
| Succulents | 50+ moles/m2/day |
You need a higher concentration of grow lights to achieve the DLI requirements of plants that need more light. Use a DLI calculator to determine how many lights you need.
Supplemental Natural Lighting
If your indoor garden gets any natural sunlight from windows or skylights, you may be able to reduce the number of grow lights needed. However, keep in mind that natural light levels change throughout the day and the seasons. The grow lights will provide consistent light levels when the sun isn’t shining.
South or west facing windows provide the most light. North facing windows likely won’t provide sufficient light for growing plants. East facing windows get light only in the morning. Measure the natural light levels with a light meter to determine if any supplemental lighting is needed.
Reflective Surfaces
Lining your growing area with reflective surfaces will maximize light efficiency. White paint, mylar, and aluminum foil can reflect light back towards the plants. This allows you to get away with fewer grow lights since the reflective surfaces bounce light around the space.
A light shelf placed above the plants along the wall of a room with a window can reflect natural sunlight back towards the plants. Installing reflective walls or light shelves allows you to supplement natural light with fewer artificial grow lights.
Lighting Schedule
Plants need light during their photosynthesis periods. Most edible plants utilize light best when given 16-20 hours of light per day. For flowering plants, you can increase yields by switching to 12 hours on /12 hours off when they start to bloom. Here are some typical lighting schedules:
- Seedlings – 18 hours on / 6 hours off
- Leafy greens – 16-20 hours on / 4-8 hours off
- Fruiting plants – Max 16 hours on / Min 8 hours off
- Flowering plants – 12 hours on / 12 hours off
You can use timers to control your grow lights and make sure plants get enough darkness. During their dark periods plants engage in crucial activities like respiration and root growth.
Signs Your Plants Need More Light
It’s important to monitor your plants and watch for signs they may not be getting sufficient light. Here’s what to look for:
- Spindly, leggy growth – Plants stretch towards light sources when desperate for more illumination.
- Smaller leaves – Inadequate light results in stunted growth.
- Leaf yellowing – Lower leaves often turn pale or yellow when lacking light.
- Flower or fruit drop – Insufficient light leads to poor flowering and reduced yields.
- Algae growth – If algae starts growing in your hydroponic reservoir, it’s stealing light from your plants.
Boost the lighting if you notice any of these warning signs. It’s easier to prevent issues by providing ample light from the start.
Combining Cool and Warm White LEDs
Most LED grow lights come in cool white or warm white. Cool white LEDs look bluish and provide more light in the blue spectrum. Warm white LEDs look more yellow/orange and have more red spectrum light. Combining both cool and warm white LED grow lights provides full spectrum light that plants can use most efficiently for photosynthesis.
Here are a couple options for mixing cool and warm white LEDs:
1. Buy a pre-packaged full spectrum LED grow light – Many pre-assembled LED grow light panels now include a mix of cool white, warm white, and sometimes additional red and blue LEDs to provide full spectrum illumination out of the box.
2. DIY mix and match LED strips or bulbs – You can buy individual cool and warm white LED strips or bulbs and arrange them in whatever ratio you prefer to create balanced full spectrum coverage. Some common combinations are 60% warm white with 40% cool white or 50/50 warm and cool LEDs.
Shoot for at least 3000K-5000K color temperature from the warm white LEDs and 5000K-7000K from the cool white LEDs. Mixing the light color temperatures allows you to achieve ideal 4000K-5000K full spectrum light for indoor plants.
Balancing Light Intensity and Coverage
In addition to providing the right color spectrum, LED grow lights also differ in terms of light intensity and coverage area. Light intensity is measured in micromoles while coverage is measured in square feet or meters. When installing LED grow lights, you need to find the right balance between intensity and coverage for your space.
Here are a few key factors to consider:
- Higher power LEDs produce greater light intensity from each diode, but have smaller coverage areas.
- Lower power LEDs have wider coverage, but less intensity in the center below the lights.
- Fruiting plants need higher intensity lights while leafy greens need broader coverage.
- Spread out lower intensity LEDs to cover larger spaces more evenly.
- Combine intense and wide coverage LEDs to get the best of both.
Refer to the manufacturer specs for intensity and coverage details. Arrange LEDs to create uniform light distribution without dim spots or intense hot spots. Blending LEDs with different light dispersal is key for even grow light coverage across your canopy.
Conclusion
Determining how many grow lights you need comes down to factoring in your plant types, growing area size, light types, mounting heights, and lighting schedules. Start by researching how much light your specific plants need, then use grow light coverage information and DLI calculators to figure out the number and wattage of lights required to meet your plants’ needs based on your unique setup. Observe your plants and adjust your lighting as needed to keep them happy and thriving!
