Choosing a grow light
Picking a grow light means matching the bulb type, its light color, and how long it runs to your plants' needs. Current grow light options include HID (high intensity discharge) bulbs, compact fluorescent and T5 tubes, and LED units. Plain incandescent lamps are not a good choice, since their light sits high in red and orange but gives off too much heat. Blue and red light have the strongest effect on growth, because plants absorb these colors the most. Most indoor plants need 12 to 16 hours of light a day, with some houseplants needing up to 16 to 18 hours. Too little light leaves stems thin and leggy, and plants may drop their older leaves. Too much direct light can scorch or bleach the leaves, sometimes killing them.
When to do it
Natural light reaches its highest level in summer and drops to its lowest level in winter. It works much better to grow seedlings under fluorescent or LED light than to depend only on natural light. Short-day plants, including chrysanthemum, poinsettia and Christmas cactus, flower only when days run shorter than about 12 hours. Long-day plants flower only once days run longer than 12 hours, while day-neutral plants flower no matter the day length. Many indoor plants that get little or no artificial light enter a resting stage, and they should not be fed during that time. If temperatures run high while days are long, cool-season crops such as spinach will bolt and flower early.
What you need
You need a light fixture sized for the bulb you choose, whether that is an HID, compact fluorescent, T5, or LED unit. Mount tubes in fixtures fitted with reflectors, and make sure every fixture is wired with a plug before you use it. A power strip lets you plug in multiple ballasts and bulbs at once. Bulbs are sold by light output, from about 10 to 15 watts for low-light plants up to more than 20 watts for high-light plants. Reflective materials or a white background around your plants increase the light that reaches them. A simple timer can turn the lights on and off automatically each day. An inexpensive light meter lets you check how many foot-candles your plants are getting. A small fan placed near seedlings gives them a gentle sway once they grow two true leaves.
Step by step
- Step 1
Check how much natural light your plants get through the seasons
Natural light peaks in summer and drops to its lowest point in winter. Surround your plants with reflective materials, a white background, or a supplemental light to raise the light level.
- Step 2
Start seedlings under fluorescent or LED light rather than relying only on a window
Low intensity light leaves seedlings pale, thin and spindly.
- Step 3
Pick a bulb type from HID, compact fluorescent, T5 or LED lights
Skip plain incandescent lamps, since their light runs high in red and orange but gives off too much heat.
- Step 4
Weigh each bulb's running cost and heat before you buy: LED lights are very energy efficient and run cool, though they cost more upfront
Fluorescent lights are moderately efficient and cheaper upfront, but they don't last as long as LEDs.
- Step 5
Match the light's color to the growth stage you want
Blue and red light have the greatest effect on growth, since plants absorb these colors the most. Blue light drives mostly leafy, vegetative growth. Red light, combined with blue, encourages flowering instead.
- Step 6
Choose cool white fluorescent tubes for a spectrum high in blue light
Fluorescent grow lights mix red and blue wavelengths to imitate sunlight.
- Step 7
Set up the fixture: make sure it is wired and has a plug, wiring it yourself if it does not
Mount fluorescent tubes in fixtures fitted with reflectors. Insert the bulbs into the ballasts and plug them into a power strip.
- Step 8
Position the lights above your seedlings 2 to 4 inches away, raising them as the seedlings grow
Others say to keep lights no more than 4 inches above the tops of seedlings, with 2 inches as the ideal. Still others recommend keeping tubes at least 2 inches above the seedlings.
- Step 9
Hang two 40-watt fluorescent tubes 12 to 15 inches above full-grown plants
Raise the light's height a little each day as the plants grow taller.
- Step 10
Set the daily light duration to match your plant type: most plants need 12 to 16 hours of light a day
For compact, sturdy growth, some growers provide 14 to 16 hours a day instead. Indoor herbs need only about 12 hours of artificial light a day. Sixteen hours of light with eight hours of darkness suits most other plants.
- Step 11
Give African violets their own light and dark schedule
Run fluorescent light for African violets about 16 hours a day. They need eight hours of darkness every night, and 8 to 12 hours, up to 16 hours, of light to bloom well. Bright light of about 1,000 foot-candles, with no direct sun, gives African violets their best growth and flowers.
- Step 12
Automate the on/off cycle with a simple timer
A timer can be part of the setup so lights switch off and on automatically, or it can simply turn them off each evening. Be sure not to run grow lights for more than 16 hours a day.
- Step 13
Protect the uninterrupted dark period for flowering plants
It is not the length of the light period but the length of unbroken darkness that drives flowering. If a long dark period is interrupted by even a short burst of light, it loses its effect on flowering.
- Step 14
Check your plant's day-length category before you set a schedule
Short-day plants form flowers only when days run shorter than about 12 hours. Long-day plants form flowers only once days run longer than 12 hours. Day-neutral plants form flowers no matter the day length.
- Step 15
Force chrysanthemums into bloom out of season by covering them with a cloth that fully blocks light for 12 hours a day
Give a Christmas cactus more than 12 hours of darkness a day at 50 to 55°F (10 to 13°C) until flower buds form.
- Step 16
Check your light levels in foot-candles: about 100 ft-c for 12 hours a day is just enough to maintain plant quality for a year
Foliage plants need at least 200 ft-c for 12 hours a day to show any benefit from fertilizing. Plants grown entirely under fluorescent light should get about 600 foot-candles.
- Step 17
Place supplemental lights at a distance that matches your plant type
For seedlings, that means 4 to 6 inches, moved up as they grow. For hydroponic lettuce and herbs, use 6 to 12 inches for 12 to 14 hours a day. For foliage houseplants, use 12 to 24 inches for 12 to 14 hours a day. For flowering houseplants, use 6 to 12 inches for 14 to 16 hours a day.
- Step 18
Adjust fertilizer to the light level your plant receives
Indoor plants in the low light of most interiors have reduced fertilizer needs. Many plants that get little or no artificial light enter a resting stage, and you should not fertilize them during that time. During the growing season, fertilize at the label rate every two or three months. Indoors, the best N:K fertilizer ratio is 1:1 unless light is extremely high.
- Step 19
Watch for signs of too little or too much light
Too little light causes elongated, skinny stems and thin, spindly growth. Too much direct light bleaches or scalds the leaves, sometimes killing them. Make any change in light intensity gradual rather than sudden.
- Step 20
Match your plant's light category to a window exposure
A low-light plant suits a north window or a fairly dark corner. A medium-light plant suits east-facing windows, or a spot near a west-facing window out of direct light. A high-light plant suits brightly lit spots such as south- or southwest-facing windows.
- Step 21
Manage temperature alongside your lighting setup
If temperatures run high while days are long, cool-season crops such as spinach will bolt and flower early. Temperatures higher than needed raise respiration, sometimes above the rate of photosynthesis. Daytime temperatures that are too low often produce poor growth by slowing photosynthesis. Aim for a daytime temperature about 10 to 15 degrees higher than the nighttime temperature. Most seeds germinate best between 75°F and 80°F (24°C and 27°C), while most root cuttings prefer 55°F to 65°F (13°C to 18°C).
Mistakes to avoid
Leaving grow lights on around the clock is a frequent mistake, because many plants need a dark period each night to grow properly. For most houseplants, that means keeping the lights running no more than 16 hours a day. For a flowering plant, even a short burst of light during a long dark period can cancel out its effect on blooming. Too little light shows up as leggy stems that stretch toward the light, dropped leaves, and flowering plants that fail to bud. Leaves can also fade from pale green to yellow to white. Seedlings grown in weak light often turn out spindly and pale. Too much direct light does the opposite damage, scorching or bleaching leaves and sometimes killing them. Changing light levels suddenly is also a mistake; any shift in intensity should happen gradually instead. Under low light, high ammonium in fertilizer can cause leaves to curl. Many plants that get little or no artificial light enter a resting stage, and feeding them during that stage is a mistake too.
How this page was made
- Written with AI assistance from the sources listed on this page. AI is never the source of a fact.