Do You Know How Long To Leave The Lights On Indoors?

Question: When growing plants indoors under grow lights or artificial lights like fluorescent lighting, how long should the lights stay on and at how far away should the light be from the top leaves. Christina, Bristol, Connecticut

Answer: Christina, most authorities would agree that the minimum amount of controlled light required for plant bloom is 12 to 18 hours 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 variety.

Avoid This Mistake

Avoid the mistake I made! 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 promoting it. Growth quality in most plants is dependent upon the period of darkness they receive, as well as the period of light. Now I have a timer that switches the lights on at 7 A.M. and off at 10 P.M. When I grew plants in a basement setup, I often left lights on 2 to 3 hours longer.

reflected light by CD from fluorescent tube lamp showing individual bands of color

An automatic timer will switch lights on and off making it handy for going on vacation, ndividuals working the night shift, or those who must be away from home from early morning to late evening. It costs about $15 online. More deluxe units costs about $30 on up per unit.

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 14 months. Replacements cost about $3-5 a tube. A change every 6 months is advisable, however, since a 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 following table, 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.

Distance in Inches
Foot-Candles
1
1,000
2
950
3
750
4
650
5
560
6
460
7
430
8
370
9
360
10
350
11
300
12
250

Tips To Improve Your Plant Care
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