Daylight shock

Should the clones I am going to plant outdoors grow under continuous fluorescent light, or should I root them under 18 hours of light? Will the shift from 24 hours of light to 14 hours light and 10 dark when I transplant them outside shock the clones of an early variety into flowering? Can I take cuttings directly to the outdoor garden?
Shelburne County, Nova Scotia

Early and mid-season varieties often have problems moving from indoor continuous light situations to the outdoor regimen that has only 13 to 15 hours of daylight and a dark period of 9 to 11 hours for two reasons.

The first is that each strain has a critical dark period. For convenience, we place all varieties under 12 hours of uninterrupted darkness to flower them indoors. However, some plants flower given fewer hours of darkness. For instance, many varieties start to flower outdoors in August when the light period is 13 to 16 hours and the dark period is 11 to 8 hours.

The number of hours of darkness required to tip the plant into flowering is the critical dark period. Plants placed outdoors that have a shorter critical dark period than the length of darkness begin to flower. The only way to prevent flowering is to interrupt the dark period with short periods of light. A bright incandescent light shining for a few seconds every hour or two during the dark period prevents most plants from triggering into flowering.

Second, plants that usually need a long period of darkness to force flowering sometimes react to the sudden change of regimen from no darkness to 9 or 10 hours of it daily, by flowering. The plant’s growth may be ambiguous, showing signs of flowering while it is still growing vegetatively, eventually going into full flowering.

Clones grown under a regimen of 18 hours of light with a 6 hour dark period are less likely to be shocked into flowering when they are placed outdoors. A plant with a known critical dark period can be grown under a slightly shorter dark period to maintain vegetative growth during rooting. When it is placed outdoors, it will not be shocked by a dramatic change in light regimen.

For instance, a plant that flowers under a dark period of 11 hours can be grown vegetatively under a period of 9 hours of darkness and 15 hours of light without fear of it flowering prematurely. It will have a seamless transition to outdoor life.

Readers with grow questions (or answers) should send them to Ed at: Ask Ed, PMB 147, 530 Divisadero St., San Francisco, California 94117, USA
You can also email Ed at [email protected], and send queries via his website at
All featured questions will be rewarded with a copy of Ed’s book, Ed’s Big Book of Buds from Quicktrading.
Sorry, Ed cannot send personal replies to your questions.