Grow lights can be important supplements to the health of your houseplants, and are integral to growing food and other plants from seed to maturity indoors. The distance of the light from your plants makes a difference in how effective, ineffective, or harmful it may be to your plant.
When light sources are further away, they’re less intense than when they’re close up, meaning less light energy is being issued directly to the plants.
It also means that lamps, when they get close, are much more intense, and too much light at once can damage the plants’ leaves and stems.
Lots of variables go into what the right distance is for your situation, and keeping an eye on the plant to monitor its condition and responsiveness to the light is the best way to inform which, if any, adjustments need to be made.
grow light placement
Since so many plants can be grown indoors in homes and greenhouses, there are lots of different lighting setups. Generally, grow lights are suspended above the plant, integrated into a tray shelving unit, or in a free-standing lamp. While these setups may be different for your purpose and situation, if a lamp is too close to a plant, it can burn or bleach the leaves, and if the light bulb is too far, it won’t emit enough light energy and result in small or few flowers or fruits, leggy stems, small and sparse leaves, and slow plant growth in general.
light bulb intensity
Like regular light bulbs, grow lights come in different lumens, the measure of radiant light output and intensity, which makes a difference in how effective they will be at different distances from the plants. Most manufacturers provide a light distance chart that shows the intensity and area coverage at certain suspended heights. The intensity of a light bulb can be affected by moving it closer or further from the plant, by using a dimmer, or by using (or not using) a shade or cover to focus the light’s dispersal.
The wattage of the bulb is the amount of energy needed to emit light, but the actual lumens issued by the bulb will vary by the way it produces light. This means wattages between lightbulb types, like incandescent and LED, don’t translate to one another in terms of light intensity.
This is the classic light bulb that contains a filament that heats up until it glows. It’s the most energy intensive type of light bulb, and while it can be used as a grow light, it doesn’t produce the full spectrum of light needed for plants to survive on. These traditional lights give off mostly red light wavelengths, and can be most helpful with fruiting and flowering plants. These also give off the most heat, so they should be kept at a distance from any foliage.
An LED (light emitting diodes) bulb has a microchip system that emits visible light, which can be programmed to different colors or the full spectrum. This makes them an ideal choice for any indoor growing need, and they are much more efficient in their power usage: up to 90% less power is used by these bulbs. Their technology makes LED grow lights more expensive than other bulbs, but they last much longer than other ones, too. These produce some heat energy, but not as much as incandescent ones.
fluorescent grow lights
Fluorescent, or compact fluorescent (CFL) lights are another popular, energy efficient choice for home growers. These don’t use a filament like incandescent bulbs, but they are filled with argon gas, a little mercury, and phosphorus-coated glass, that illuminates with an electric current. They use about 75% less energy than incandescent bulbs, producing more lumens per watt (although fewer than LED bulbs). These light bulbs give off a low level of heat and can be found in red, blue, or full spectrum options.
HID, or high intensity discharge, lights, are similar to fluorescent ones in that they are made with elemental gasses that illuminate with an electric current, giving off a full spectrum of light. These high intensity lights give off little heat and are very energy efficient, but they are expensive and normally used by commercial growers.
The way the light is dispersed is another factor in deciding how close or how far a light bulb should be positioned from your indoor plants. When lights are focused, or unfocused, they disperse light differently. A bulb with a hood or shade blocks light from being emitted in some directions and focuses the spread of it into a concentrated flow for efficient use by the plants underneath or nearby. A light’s spread is broken down in three ways:
- Wide spread – above 120 degrees
- Medium spread – 100-120 degrees
- Narrow spread – 90 degrees or below
Most grow lights will have a wide spread so they can cast as much light below them as possible, whereas narrow spread grow lights focus the light spread and intensity like a spotlight. A wide spread can usually be closer to a plant than one with a narrow spread, which might focus too much energy to the leaves. The lumens (total light radiation) of light emitted won’t be affected by the spread, but the lux (the area coverage of the light) may be adjusted by widening or narrowing the light dispersal.
where should grow lights be located?
In general, grow lights for mature plants should be 1-3 feet away, although seedlings and sprouts can tolerate low-intensity lighting within a few inches in their initial periods of rapid growth.
If your plant is in a reflective grow space, like in an indoor grow tent, then higher is better for the light to be better reflected by the walls to reach the lower stems and leaves. A narrowly focused light may need to be set further away than a wide spread light to make sure to not burn the leaves. An LED or CFL light can be closer than an incandescent light. Always consult the directions provided by the manufacturer should give you the specifics for the intensity and model of light you’ve chosen, keep an eye on the plant(s), and adjust as necessary.
signs a grow light is too close to a plant
If your grow lights are too close to your plants, they will show some telltale signs of overexposure, much the same as they would if placed in direct sunlight for too long. These include:
- Browning edges or burnt spots
- Curling or wrinkling
- Drying out
- Pale or whitish coloration
signs a grow light is too far from a plant
When plants aren’t getting enough light, either from too little sunlight or grow lamps that are too far away or too weak, they’ll show the following signs:
- Long, leggy stems
- Small and sparse leaf growth
- Pale or whitish coloration
monitoring your grow light’s distance and effectiveness
While these rules of thumb and the manufacturer’s directions are important guidelines for positioning your plants and using grow lights, regular monitoring of the plants’ conditions will be your best way of knowing if the plant is getting enough light. Since plants have different lighting needs by species, the right setup may depend on the plant itself as much as the position of the light source. What’s too close or too far for some may be the right setting for others. Knowing your plant’s origins and lighting needs in the first place will be another important guideline in figuring out how much light your plant needs, in terms of time and intensity.
How many lights do I need for my indoor plants?
The number of light bulbs will affect the intensity of the light your plants are getting: too many bulbs too close will damage the plants, while any number of lights too far may not be enough. Large growing operations will use many light fixtures at specific distances to make sure the large number of plants in various growing stages are receiving the amount of light each individual needs without any light burn, while your houseplants at home may be happy with one or two extra lights for those darker days and evenings.
How do I know how much light my plants are getting?
The best way to know if a light is too close or too far is to measure the intensity with a PAR meter. PAR stands for photosynthetically active radiation, referring to the amount of light that a plant can use for photosynthesis. A PAR meter is used to know how much light is reaching a certain spot. This is useful since lumens measure the amount of light issued, but not how much is being used by the plant, while lux refers to the total amount of space covered by a light. The PAR measurement, in micromoles, lets you know how much light is reaching a given location, like a leaf. PAR meters display in the 400-700 visible light range, and some extend to a wider 340-1040 light range, which includes infrared and other invisible light radiation.
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