How Long To Keep Grow Lights On Plants?

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Have you ever wondered how long you should keep the grow lights on your plants?  If so, you’re not alone. Here’s a question we received on just that topic.

Question: When growing plants indoors with grow lights or artificial lights like fluorescent lighting:

man checking the grow lights setupPin
  • How long to leave UV light on Plants?
  • How far away should the light be from the top leaves.

Thank you – Christina, Bristol, Connecticut

Answer: Christina, most seasoned indoor growers would agree that the minimum amount of controlled light required for blooming plants is 12 to 18 hours of light a day.

If all light comes from fluorescent grow lights alone, plants should be given 14 to 18 hours of artificial light every day.

After some experimentation, you will be able to determine the timing best suited to your house plants depending on the plant variety.

For example, African Violets under grow lights and Peperomia Ruby Glow produces spectacular plants.

Avoid This Mistake I First Made!

Avoid the mistake I made when I first started growing plants under grow lights.

Noting the fine results of a 15-hour light period, I figured that if a little light is good, a lot will be even better, and so I left the lights on round the clock.

But the extra hours, I found, retarded growth on some plants instead of increasing growth.

Growth quality in most plants is dependent upon the period or hours of darkness they receive, as well as the period of light.

Now a timer switches the grow lights on at 7 A.M. and off at 10 P.M. When growing plants in a basement setup, the lights may need to stay on 2 to 3 hours per day longer.

An automatic timer switches lights on and off making it handy for:

Going on vacation

Individuals working the night shift

Those who must be away from home from early morning to late evening.

It timer costs about $15 online. More deluxe units costs about $30 on up per unit.

If you click this link and make a purchase, we earn a commission at no additional cost to you.

How Long Do Fluorescent Bulbs Last

The cost of fluorescent lighting (it varies in different localities) and the bulbs or fluorescent tubes are usually good for about 12 months.

Replacements cost about $7-$15 a tube.

A change every 6 months is advisable, as the tube’s efficiency decreases with age.

The discarded tubes will be adequate for household lighting or for nursing foliage plants which require only a minimum of light.

A 100-watt incandescent bulb has a working life of about 700 hours, but, after 500 hours, it darkens and provides fewer foot-candles.

A knowledge of foot-candles produced by fluorescent light is helpful in indoor gardening.

The table below is based on two 40-watt fluorescent tubes with the measurements taken from the center of the span of tubes and the center of the individual tube.

Each 40-watt tube, added and placed in close proximity to the other 40-watt tubes, will give the over-all area about 150 additional foot-candles.

Clear, clean glass used below fluorescents, as in terrariums, cuts down foot-candles to a slight degree – perhaps 5 to 10 foot-candles per inch of distance.

chart with lumens or foot candles on plant lighting distancePin

Over the last few years more lighting types have entered the market. LED grow lights, High Intensity lights and more.

Lighting setups may change but plants have not. They still require the right lighting and light intensity to produce great plants. Check out this article on the question – does LED light help plants grow?

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