How Long do you Leave
Grow Lights On?

by Rita Rudy
(St. Paul, MN)

We use grow lights, but are not sure if they should be left on 24/7 or turned off in the evening. Please let us know.

Answer: Rita- the answer to your question depends partly on what you are growing, and partly on what you want from your garden. Let me give you a few examples and hope it will clarify things. Any time you intend to grow plants in the vegetative stage, they should receive at least 18 hours of light per day. Plants grown vegetatively do not require a dark period, and therefore you could give them as much as 24 hours of light per day. These plants would grow bigger faster, and there is no negative affect to giving your plants 24 hours of light per day. Plants such as lettuce, spinach, and herbs all produce edible parts when grown vegetatively.

Plants grown for their fruits, vegetables, or flowers often require a dark period in order to force them into their flowering stage. Detailed information on how to do this can be found on my flower forcing page. If you are trying to grow peppers indoors, for example, many varieties require a 12 hour dark period. The plants need to receive this dark period day after day, with the lights going on at the same time and off at the same time each day. Also, it is important that NO LIGHT should interrupt the dark period.

Given the controlled temperature in an indoor climate, plants treated in this way often produce peppers for many more months than they normally would outdoors. Other pepper varieties will produce peppers without a dark period at all! In general, all of the produce you would normally harvest at the end of September/beginning of October is dependent upon a dark period to flower.

The above examples are likely to fit the bill for 99% of the gardening you want to do. The most important thing is to know in advance weather or not your plants require a dark period to fruit or flower. I sure hope this helps you out Rita, and Happy Growing!